Mrs Claire Romanzin

Keywords: CRESU, laboratory astrophysics, reactive collisions, ionic processes, chemistry CRESU, astrophysique de laboratoire, collisions réactives, processus ioniques, chimie du milieu interstellaire, atmosphère de Titan

Understanding the mechanisms governing the formation of complex organic molecules in the interstellar medium and cold planetary atmospheres relies in particular on the exploration of ionic processes under extreme conditions. The CRESU-SIS experimental setup, developed to study ion-neutral reactions in the gas phase at low temperatures, is able to meet this challenge. This unique instrument, which combines a selective ion source (SIS) with the CRESU reactor which generates a uniform supersonic flow at very low temperature, makes it possible to measure the rate coefficients of the reactions between ions and neutral molecules and to determine the branching ratios between the different exit channels. The reactivity of N2 + , Ar + , and CH2CN + ions with hydrocarbons (e.g CH4, C2H4 ...) has been studied experimentally and analyzed using quantum calculations. The reaction between N2 + and the linear isomers of C3H4 (allene and propyne) has been included into a photochemical model for the atmosphere of Titan.

Chapter 1:

Ion chemistry in space

Introduction

Astrochemistry, also known as molecular astrophysics, is 'the study of the formation, destruction and excitation of molecules in astronomical environments and their influence on the structure, dynamics and evolution of astronomical objects' as stated by the pioneer of this field, Alexander Dalgarno in 2008. 1 Due to a complex mixture of gravity, turbulence, rotation, radiation, thermodynamics, and magnetic fields, the matter in the universe is not static, but rather in continuous evolution through what is known as the cosmic matter cycle schematically depicted in the figure 1.1. As the star nuclear fuel is depleted, the star becomes unstable and loses its mass by ejecting material into the interstellar medium, which accumulates to form an interstellar cloud of gas and dust at very low density. When the mass of the interstellar cloud becomes sufficient to cause contraction by self-gravitation, a protostellar system is formed. A protostellar system undergoes a contraction, leading to the formation of a central protostar and rotating disk of gas and dust that accretes more material to form more molecules (accretion disk). In the outer region of the accretion disk, planets and comets will form. When the central temperature and the density increase, a thermonuclear reaction is ignited in the central star and a newborn star appears. Planets, comets, and interplanetary material remain in orbit around the star. Three main categories have drawn the attention of astrochemical studies: the interstellar medium (ISM), the circumstellar environments and planetary atmospheres. Interstellar medium. The interstellar medium is a very dilute gas that fills the space between stars, it is composed by gas and dust: by mass 99 % of gaseous species which include atoms, molecules, radicals and ions, and 1% of cosmic dust. The temperature in the ISM ranges from more than 10 6 K down to 10 K and densities from as low as 10 -4 to more than 10 8 molecules cm -3 .

Several environments with different physical and chemical characteristics are found in the ISM: the interstellar clouds, the intercloud gas, and the diffuse nebula (HII regions). The interstellar clouds correspond to the colder and denser concentration of the gas. The intercloud gas is divided in two categories based on temperature: the hot coronal gas which is a product of hot gases ejected from stellar explosion and winds, and the warm intercloud medium which is the intergalactic vast region. HII regions are composed by ionic gases due to UV, X-Rays, and cosmic rays. The H + ion is the main compound in these regions (with temperatures around 10000 K).

The most famous example of HII regions is the Orion Nebula.

Circumstellar environments.

In their late stage of evolution, stars eject considerable amounts of material into space (up to 10 -5 of the mass of the star per year), generating a huge layer of gas and dust. 2 The temperature and density gradient found in the circumstellar envelope give rise to a complex chemistry in these regions (Figure 1.2) attested by the detection of numerous species. A well-known example is the circumstellar envelope of the carbon-rich star IRC+10216. Planetary atmospheres refer to the envelope of gases that surrounds any of the planets within or outside the solar system. In our solar system: Jupiter, Saturn, Uranus, Neptune, Earth, Titan (a Saturn's moon), Venus, Mars, and Triton (a Neptune's moon) have atmospheres.

Similarly to the interstellar medium and the circumstellar envelopes, ions and neutral molecules are observed, and play a crucial role in the chemistry of the atmospheres of rocky planets, gaseous planets and their satellites, for example: Titan's atmosphere. 3 Until recently, the positive species were the only ions detected in space; however, this situation has changed with the discovery of various molecular anions in the interstellar medium and Titan's atmosphere. 4

Observation of ions in space environments

Ions in astrophysical environments are sparse, this is a reason that makes their detection not simple and requires methods that are highly sensitive and unambiguous. Radioastronomy has been proved very successful in detecting molecular ions and neutrals in space. Most of emission lines for specific rotational transitions of various simple molecules or molecular ions (∼80%) are detected by radiotelescopes in the MHz or GHz frequency range. These spectral features can be exceptionally sharp, due to the very low temperature and (by terrestrial standards) low pressure of the interstellar clouds containing these molecules. For example, recently, the Yebes 40 m and IRAM radiotelescopes were used in the detection of the rotational emission lines of the CH3CO + ion in the TMC-1. 5 The detection of the HC3S + ion in the TMC-1 was also performed by the Yebes 40 m observatory. 6 Another example is the detection of the presence of the deuterated ammonium ion, NH3D + in Orion and in the cold prestellar core B1-bS using the IRAM 30 m radiotelescope (which is the most sensitive single antenna radiotelescope in the world). 7 ' 26.7") at the expected frequency of NH3D + JK=10-00 transition. The upper panel contains the raw data resulting from a total integration time of 51 hours with a spectral resolution of 48.8KHz (0.054 km s -1 ). The bottom panel shows the same data smoothed to 98 KHz. For a LSR velocity of the source of 6.5 km s -1 the derived line frequency for the observed featureis 262816.73±0.1 MHz. Adapted from J. Cernicharo et al. 2016 7 Infrared spectroscopy is also used to detect molecules in astrophysical environments, particularly those that are highly symmetric or with non-zero permanent dipole moments. This technique is useful for the detection of transitions that occur between vibrational states. For example, T. R.

Geballe detected the IR spectrum of the H3 + ion in dark and diffuse clouds, and in the Galactic Center source GCS 3-2. 8 This ion play an important role in interstellar chemistry, by initiating a chain of reactions that lead to the production of many of the complex molecular species observed in the interstellar medium. A substantial benefit of IR spectroscopy is the ability to observe molecules condensed into interstellar ices. While condensed-phase molecules cannot undergo free rotation, their vibrational transitions remain accessible. As of September 2021, about 250 molecules from different families have been detected in the interstellar medium or circumstellar shells. The list of these molecules is presented in the tables 1.1 and 1.2. In the interstellar medium, 25 cations and 7 anions were observed, while the first cation, CH + was detected over eight decades ago. 9 In tables 1.1 and 1.2: * indicates molecules that have been detected by their rotation-vibration spectrum, ** those detected by electronic spectroscopy only.

In addition, ions and neutrals in astrophysical environments can also be discovered by space probes that aim to discover the planets and its systems. The Cassini-Huygens mission -also called Cassini-is an example (Figure 1.5) 10 . This spacecraft was launched on October 15, 1997 by a collaboration between NASA, the European Space Agency (ESA), and the Italian Space Agency (ASI) to explore Saturn, its rings and natural satellites. atmosphere. Thousands of neutral and ionic molecules were detected. 3,11 The analysis of the Cassini orbiter measurements have revealed a surprising abundance of protonated nitriles in Titan's ionosphere (Vuitton et al 2006) 12 , suggesting a more evolved nitrogen chemistry than was previously believed to exist. 11 Molecules in the interstellar medium, circumstellar envelops, and planetary atmospheres are formed via several physical-chemical processes. Table 1.3 presents the list of the main chemical processes involving ions of interest for astrophysical environments. These reactions correspond to the main pathways for the formation, growth and destruction of ions in space. They are important for the construction of photochemical models with the goal of reproduction of relative abundances of observed species. In the next sections, we will detail the pathways leading to the formation, growth and destruction of cations and anions in astrophysical environments.

Ion-molecule reactions

Formation of cations in space environments

Various processes are held responsible for the production of cations in space. Cosmic radiation, UV rays, x-rays, and shock waves all contribute, albeit they are clearly not of equal importance in the various astronomical environments populating the Universe.

Cosmic rays.

The cosmic radiation was discovered in 1912 by the Austrian physicist Hess, who used the ionization chambers to determine the radiative ionization rate at various altitudes during a balloon flight. The cosmic rays (CR) that move in space at nearly the speed of light, are high energy particles composed primarily of protons and atomic nuclei species, electrons and even antiparticles, with an additional contribution of gamma photons, neutrinos and neutrons. 13 Generally, CR constitute an important source of ionization in many astronomical environments and planetary atmospheres.

Stellar flares, coronal mass ejections, explosions of supernovae and active galactic nuclei, acceleration by pulsars and jets from black holes could be the origins of cosmic rays in our galaxy. The energy of these cosmic rays usually ranges from 100 to 10 GeV, the radiation intensity strongly decreasing at energies above 1 GeV.

Similarly, the Sun also acts as a sporadic source of cosmic rays through shockwaves and solar flares. Referring to Mewaldt et al. 1996, 14 the maximum energy of solar cosmic-ray particles lies at approximately 10-100 MeV, but, very rarely, higher energy particles are observed arising from the Sun. In our solar system, in addition to the lower energy solar cosmic rays emitted by the Sun, galactic cosmic rays (GCR) which are high energy particles (energy ⁓ 10 17 eV) originating from the extrasolar system, and extragalactic cosmic rays (very-high-energy particles, up to 10 20 eV) are present. 15,16 According to Dalgarno et al. 1996, 17 the CR ionization rate in the interstellar gas depends on the relative amount of H, H2, and He. . In molecular clouds, a large majority of CR-H2 impacts leads to the formation of H2 + with a small amount of H + ions.

In star-forming regions, the contribution of the UV radiation in the production of ionized species is much less important than cosmic-rays since UV photons are blocked by a thin layer of gas and dust. Their high penetration power makes cosmic rays as a principal source of ionization in dark clouds and dark cores as well.

We will present some examples showing the role of cosmic rays in the ionization of the atmospheres of planets and their satellites. The first example is in the Venusian atmosphere (below 70 km), where the O2 + , CO2 + and CO + ions are mainly produced by the cosmic-ray ionization. 18 The second example is Mars's atmosphere: cosmic rays contribute in the ionization of neutral species especially at altitudes below 80 km. Ar + , H2 + , H2O + , O3 + and NO + ions are produced by CR ionization, consequently, their abundance is very low above this altitude. In addition, in Titan's atmosphere, at altitudes below 100 km, where the moon's haze blocks the UV radiation and the magnetospheric electrons contribute little in the ionization of neutral species, the cosmic-ray ionization is one of the most crucial pathways for ion formation. 19 According to G.J. Molina-Cuberosa et al., 20 cosmic rays penetrate deeply into the atmosphere of Titan generating an electron density peak around 90 km, similar in magnitude to the ionospheric peak produced by solar radiation in the upper atmosphere.

Photoionization

Photoionization refers to a process in which highly energetic radiation with wavelengths of < 100 nm is absorbed by removing electrons from atoms or molecules, resulting into electrically charged particles. But not every interaction between a photon and an atom, or molecule, will result in photoionization. If the radiation energy is below the ionization threshold, the photoionization cross section is near zero and photodissociation processes take over. In our solar system, the ionization potentials of neutral atmospheric species are close to the UV photons and X-rays energies, 21 making the UV radiation and X-rays the most considerable photoionizing sources.

An example for the contribution of photoionization of molecules in planetary atmospheres is the ionosphere of Mars, 22 in which the UV dissociative ionization of CO2 , i.e. the main constituent of the atmosphere, is the dominant source of the O + and CO + fragments. 23

UV radiation:

When a molecule absorbs UV radiation, to two distinct chemical processes can occur, the photodissociation and the photoionization. UV radiation can also heats molecules present in the H-dominated and less dense part of the PDR (photon dominated region) where the UV radiation drives mainly the chemistry of the medium facing the UV source. 24 In our solar system, the sun is the main source of the UV radiation.

Another example for the contribution of UV radiation in the ionization of molecules in astrophysical environments is the upper layer of Titan's atmosphere primarily constituted of N2 (>95%) and a few percent of CH4, especially above an altitude of 700 km, where it is an efficient source in the production of N2 + (the main product), N + , CH4 + , and CH3 + ions. 19 (Figure 1.8).

X-rays:

Absorption of X-rays radiation by neutral molecules in astrophysical environments can lead to the their ionization. 25 Unlike FUV photons (6 < hν < 13.6 eV), referring to Stäuber et al, 26 Xrays can also ionize atoms and molecules with an ionization threshold above 13.6 eV (e.g. H, He, N and O) and can even doubly ionize many species. For example, in the AFGL 2591 region (a star forming region), X-rays lead to the formation of many ions (such as H + , C + , N + , O + , HCO + , HOC + , N2H + , SO + …), and SH + and HSO + are found to be excellent X-ray tracers. They are 100-10 000 times more abundant in models with X-rays than models which consider only far UV ionization. 26 In addition, the interaction of X-rays with planetary atmospheres could create an ionospheric layer below those created by the UV photons, since they can penetrate further than the UV radiation. According to M. Banaszkiewicz et al., 27 solar flare X-rays interact with the atmosphere of Titan, leading to the photoionization of some neutral molecules, and so contribute to the increase of the electron and ion density at altitudes of 400-900 km.

Shock waves

When a material moving faster than the speed of sound (supersonic velocity, Mach number is greater than 1) encounters another medium, a hydrodynamic process occurs with a sudden jump in the density, pressure, temperature, and velocity of the gas, what is called the shock wave. 28 When the shock hits, the kinetic energy of the gas in converted into thermal energy, and the medium undergoes a compression and heating at temperatures higher than those encountered in photoionized ions. In the interstellar medium, the regions ionized by shock waves can be distinguishable to the photoionized regions by using temperature-sensitive lines of ions. 29 Referring to Allen et al., 30 cloud-cloud collisions, the expansion of HII regions into the surrounding interstellar medium, outflows from young stellar objects, supernova blast waves, outflows from active and starburst galaxies, and collisions between galaxies are the primary sources of shock waves in space. According to Tielens, 31 because of shock particles move at high velocities, the reactions with energy barrier can proceed in this environment, leading to a chemistry different from that occurring in dark clouds.

The Solar wind

The solar wind is a flow of charged particles that are ejected from the sun. The solar wind which is mainly composed of electrons, protons and He2 + ions is an important source of the ionization of atmospheric species. The neutrals such as C, O, Ne, Si, S, Ar and Fe, as well as traces of Kr and Xe, can also be minor constituents of the solar wind. The solar wind (then called solar corpuscular radiation) has two components: fast and slow. The fast wind is associated with coronal holes and has the characteristics of being uniform and stable. The slow wind is quite variable in terms of temperature, composition, magnetic field strength…and in contrast with fast wind, is not in equilibrium with the corona and transition layers at its base. 32 In the upper layers of planetary atmospheres, the solar wind contributes in the ionization of neutral species by electron impact, charge exchange and ion sputtering. The latter is a process by which an energetic ion deposits its energy in a material initiating a cascade of events which eventually leads to the ejection of atoms or molecules from the surface of the material. Ions lose their energy by direct knock-on (momentum transfer) collisions with an atom in the material and by electronically exciting the atoms and molecules. For example, in the Martian atmosphere, a part of the neutral atmosphere is ionized by the solar EUV and the photo-electrons from the sun.

These newly ionized particles are 'picked-up' by the magnetic field lines initially frozen in the solar wind but distorted near Mars, and they are accelerated along gyroradial trajectories in the Martian tail direction. 33 In our solar system, according to Roberston and Carvens, 34 the charge exchange reactions between highly energetic solar wind ions and atmospheric neutrals molecules lead to the formation of highly excited ions. Resulting ions cool down by emitting soft X-rays (with energies below 5 KeV) and EUV photons.

Magnetospheric ions and electrons.

The giant planets have extended magnetospheres. The rings and satellites forming the systems of these planets are directly embedded into magnetospheric plasma, which is sustained by the rotating magnetic field of the planet. The bombardment of magnetospheric ions and electrons can be a source of ionization in these regions. 35 Figure 1.9 shows the contribution of the fluxes of magnetospheric electrons and UV radiation in the ionization of water vapor molecules the H2O-dominated atmosphere of icy moons of the giant planets. Other primary components such as O2 and H2 can be ionized by the same processes. In this scheme, the H + , H2 + , O + , OH + , and H2O + can then be efficiently converted into the hydroxonium ion H3O + through reaction with molecular hydrogen. 36

Important anion formation processes

For decades, ion chemistry in astrophysical environments was focused almost entirely on positive ions. Recently, the detection of negative ions drew attention to their contribution in astrochemistry. The dissociative attachment and radiative attachment are two major pathways for anions production (Table 1.3). Radiative association and ion-pair (corresponding to production of cation and anion pair species) formation have also a contribution in the formation of anions in space.

The first observed interstellar negative ion in the gas phase was C6H -and was discovered in the 38 . This observation was rapidly followed by the detection of C4H -by Cernicharo et al. 2007. 39 The abundance between the two ions, and CN -43 anions were also discovered in IRC+10216 through radio observations. 44 The derived abundances of the C8H -, C6H -, C4H -anions compared to C8H, C6H, C4H neutrals are 5%, 1.6% and 0.014% respectively. 40 The presence of numerous negative ions with mass-to-charge ratios of up to∼10 000 has also been revealed in Titan's upper atmosphere by the Electron Spectrometer sensor of the Cassini Plasma Spectrometer (CAPS-ELS) on-board the Cassini spacecraft during high altitude flybys. 11,45 According to Vuitton et al. , 46 and despite the low resolution of the spectrum (Δm/m ∼ 17%), two negative ion peaks centered at 22 ± 4 and 44 ± 8 u/q were clearly identified and assigned to CN -and C3N -ions. A third small peak at 82±14 u/q may be present as well (Figure 1.11). In astrophysical environments where the electrons can be present with more than 1.3 eV energy such as Titan's upper atmosphere, the C3N -anion could be formed by dissociative attachment of HC3N (endoergic pathway). This also applies for the CN -anion that can be produced by dissociative electron capture on various nitrile species 46 . The presence of HCN, C2N2, CH3CN, C2H3CN and HC3N has been inferred from analysis of INMS spectra with mixing ratios ranging from 2×10 -4 to 2×10 -6 . According to Vuitton et al., C2H -, C4H -, C6H -carbonaceous anions can as well be generated in Titan's upper atmosphere by the same mechanism, i.e. electronic dissociative attachment on C2H2, C4H2, C6H2 respectively.

Radiative electron attachment

Radiative attachment of thermal electrons to atoms or radicals is a process that can lead to the formation of negative ions in various astrophysical environments:

A+ e -→ (A -)*→ A -+ hν
The radiative electronic attachment onto C2H, C4H, C6H and C3N leads to the formation of the negative ions, C2H -, C4H -, C6H -and C3N -, respectively. The H -, CH2 -, CH3 -, OH -and CN -anions can be produced by thermal electron capture by the neutral molecules but the rate coefficients have not been determined. Radiative attachment is likely to proceed at a rate only slightly faster than that of an atomic species. According to Chandrasekaran et al. 49 the radiative electron attachment (REA) leading to the formation of C6 -is estimated to be 3 × 10 -7 cm 3 s -1 at 300 K.

In planetary ionospheres, where the pressure is low, radiative attachment can be a very efficient process. Together with dissociative attachment of ozone, radiative attachment to O and nondissociative electron attachment on O2 (three-body stabilization) ensures that anions out number free electrons in the Martian atmosphere below 30 km. 20

Radiative association of anions.

According to Senent et al. and Vuitton et al. , 50,51 anions in astrophysical environments could be formed by radiative association of anions with neutrals. An example of this mechanism is the reaction between Cn -and H, that yields to the formation of exited CnH -anion which then relaxes by emitting a photon.

Using a selected ion flow tube apparatus (see chapter 2), Eichelberger et al. 2007 52 have determined the rate coefficients for the reactions of Cn -and HCn -with H, N, and O atoms in order to characterize their interstellar chemistry. Reactions between the carbon chain anions and H and O occur readily, with rate coefficients ranging from mid to high 10 -10 cm 3 s -1 . These studies further demonstrated that radiative association is only feasible for large Cn -ions with n > 6, since associative detachment outcompetes the association process for smaller species.

Another example for the radiative association process is the reaction between H -and H2, that yields to the formation of H3 -, this reaction takes place in the interstellar medium, according to M. Ayouz et al (2011). 53

Radiative ion-pair formation

Photo-ionization can induce the formation of a positive and negative ion pair (AB + hν→ A -+ B + ). The cross-section of this process is several orders of magnitude lower than that of the formation of a positive ion and a neutral. Anion formation through this mechanism has been included in models of Titan's ionosphere (Vuitton et al 2009). 46 For example, the photoionization of HCN and of C2H2 leads to the formation of CN -/H + C2H -/H + ions pairs respectively.

Growth and destruction of ions in space 1.5.1 Cations growth and destruction

Cation-neutral reactions are the most efficient process for the recycling and growth of cations in planetary atmospheres and interstellar medium. According to Beck et al, 54 the abundance of the atomic ions in the CSE, is strongly affected by the charge-transfer reactions (CT) between neutrals and ions. The charge transfer reactions can also occur between molecular ions and neutrals. For example, in Martian atmosphere, the charge transfer reactions between N2 + , Ar + , CO + and CO2 is an important ionization pathway, leading to the formation of CO2 + ions. The latter undergo a CT reaction with O2 leading to O2 + ions. In addition, through the same process between O2 + ions and NO, the NO + ions can also be formed. 55 In addition to charge transfer reactions, cation-neutral transfer reactions in which a part of neutral molecule is transferred to the ion is an important ions destruction pathway in space.

Furthermore, three-body ion-molecule clustering reactions (ion clustering) exerts also a broad influence on the physics and chemistry of planetary atmospheres. As an example, cluster ions have been detected in Titan's ionosphere, HCO + •H2 and CH5 + •CH4•are especially prevalent at altitudes below 80 km. 56 Radiative association reactions contribute in the formation of more complex species in the interstellar medium. For example, radiative association of C3H + and H2 leads to the formation of C3H3 + , which can exist in two forms, a cyclic (c-C3H3+) and a linear one H2CCCH + . These two isomers can undergo dissociative recombination forming neutral unsaturated hydrocarbons, according to D. Smith and N.G. Adams. [START_REF] Smyth | The Ion-Molecule Chemistry of C3H3 + and the Implications for Soot Formation[END_REF] UV radiation is not only an important production source of ions but also an efficient destroyer of ions (photodissociation reaction). For example, according to Van Dishoeck et al. , 58 UV radiation dissociates CH + and OH + ions into smaller ions in Herbig Ae stars.

Reactions between cations and electrons are considered to be an important cations destruction pathway. It exists different types of these reactions: radiative and electronic recombination, ionpair formation, dissociative excitation, dissociative ionization, and dissociative recombination (Table 1.3). For example, in Titan's atmosphere, the dissociative recombination of protonated nitriles and the larger hydrocarbon ions is often the only destruction route. 3

Anions growth and destruction

The main destruction mechanisms for anions under astrophysical conditions are: photodetachment, ion-neutral associative detachment, collisional detachment, electron-impact detachment, and mutual neutralization with cations (ion-ion recombination). A recent experimental work by Zabka et al. showed that in the multiple collision regime, negative ion chemistry could lead to the formation of heavy anions of rapidly growing size. 59,60 For example, for CN -and C3N -anions, the photodetachment by interstellar UV photons is the most important destruction mechanism for these anions in the circumstellar envelope of the carbon-rich star IRC+1021. 61 The collisional detachment mechanism is an important destruction pathway for weakly bond anions. For example, for the HCN -anion, the extra electron is weakly bound in the dipole potential of the HCN molecule by only 1.56 meV, thus making the collisional detachment an efficient destruction pathway. 62 In the early universe, the associative detachment of between H -anion and H atoms leads to the formation of neutral H2 molecule (H -+ H → H2 + e -). This molecule plays a fundamental role in the cooling of molecular clouds during the formation of first generation of stars. 63 In Earth, and in Mars ionosphere at low altitudes, ion-ion recombination is an efficient pathway for the destruction of the ions, for example, the recombination between NH4 + (H2O)n and HSO4 -(H2O)n leads to their destruction. 29 Polymerization reactions is an important route leading to anions destruction, in which anions incorporate in neutrals molecules can lead to higher mass anions. For example, referring to Descheneaux et al. , 64 the repeated insertion of acetylene molecules results in an anion sequence with anions that peaks at the C2nH -species. Therefore, C2H -anions can trigger a consecutive chain of polymerization (C2H -+ C2H2→ C4H -+ H2; C2nH -+ C2H2→ C2n+2 H -+ H2).

The precise rate coefficient of these reactions cannot be found in the literature, but base on ions growth rate in silane discharge the rate coefficient for these reactions is estimated to be around 10 -12 cm 3 s -1 . 65, 66

Conclusion:

This chapter provided an overview on the observation of ions (cations and anions) in spatial environments. The most important ions formation, growth and destruction processes in space were outlined. Among all these processes, ion-neutral reactions play a crucial role in the chemistry of the astrophysical environments, and they are a possible synthetic route for the formation of prebiotic molecules in space. 67 In contrast with cation-neutrals reactions which are more recognized, there are less than a dozen measurements on the gas-phase kinetics of anions at low temperatures below 200 K. The determination of the temperature-dependent kinetic rate coefficients and the branching between the different exit channels is of outmost importance for feeding the photochemical models of astrophysical environments. In the next chapter we will focus on the capture models for the estimation of the rate coefficient and experimental techniques used to study the ion-neutral reactions.

( Chapter 2:

Studying Ion-neutral reactions in the laboratory

Theoretical aspects

Theoretical approaches are required for a better understanding of ion-neutral reactive collisions.

Collisions between species can be classified as elastic (only kinetic energy is exchanged between the partners), inelastic (kinetic and internal energy is exchanged) and reactive (the chemical structure of the collision partners is varied). This section is dedicated to the theoretical treatment of ion-neutral reactions employing a selection of models that are based on the long-range capture theory. In this section, we will recall first some basic chemical kinetics before moving to capture theories that can reasonably well model, usually barrierless, ion-molecule reactions.

The well-known capture models which are the Langevin model, the Locked dipole model (LD), the average dipole orientation model (ADO), and the Su and Chesnavich model (SC) have been employed to predict the rate coefficient of ion-neutral reactions, and to understand experimental results that will be presented in next chapters.

Chemical reaction rate

We distinguish different types of elementary reactions, depending on the molecularity, which refers to the number of the reactant particles involved in the reaction. Molecularity can be described as unimolecular (when a molecule rearranges itself producing one or more products), bimolecular (involves the collision of two particles), or termolecular (where two reactants form an initial complex that reacts with the third reactant to form products).

Consider the standard alphabetical reaction between a moles of species A and b moles of species B going to c moles of C and d moles of D given in Equation (2.1):

aA + bB → cC + dD Eq. (2.1)

The rate of the reaction is related to the density of the species multiplied by a rate coefficient written generally as:

ν = k[A] α [B] β Eq. (2.2)
where ν is the rate of the chemical reaction, k is the rate coefficient, α and β are the partial orders for A and B respectively (α + β is equal to the global order of the reaction), [A] and [B] are the densities of the reactants. The parameters a and b can be any number (including non-integer numbers) and for simple reactions represent the stoichiometry of the reaction.

A chemical reaction could have zero, first, second or even a third order for astrophysical interest.

The first-order rate equation for the removal of a reactant may be written in terms of the rate of change of [A] with time as:

Rate= 𝑑[𝐴] 𝑑𝑡 = -k[A] Eq. (2.3)
By integration of the last equation, we obtain the density of species A after a reaction time t:

[A]t= [A]0 exp(-kt) Eq. (2.4)

Where [A]0 is the initial density of species A.

For the second and third order reactions, the rate coefficient is expressed respectively as follows:

Rate= k[A][B] Eq. (2.5) Rate= k[A][B][C] Eq. (2.6)
The pseudo-first-order conditions is obtained when one of the reactants is introduced in excess with respect to the other one (for example [A]>>[B]): its density doesn't change significantly during the reaction, and the second order reaction behaves as a first order reaction.

Eq. (2.4) thus becomes:

Rate = k[A][B] = k1st [B] Eq.(2. 4)
Where k1st is defined as pseudo-first-order rate coefficient of the reaction, k1st = k [A]. Using this method, the second-order/bimolecular ion-neutral reactions that we are interested in studying, are simplified to the pseudo-first-order reaction.

In the pseudo-first-order conditions, the density of reactant B as a function of time is simply given by:

[B]t= [B]0 exp(-k1stt) Eq.(2.7)

Where [B]0 is the initial density of species B.

The unit of the rate coefficient k is cm 3 s -1 (or cm 3 molecule -1 s -1 ), and cm 6 s -1 (or cm 6 molecule -2 s -1 , due to the contribution of the density of the third neutral partner) for bimolecular and termolecular reactions respectively.

Temperature dependence

Laboratory measurements of gas-phase reactions show that many follow an empirical temperature dependence of the rate coefficient described by the Arrhenius equation:

k(T)= A exp [ -𝐸 𝑎 (𝑇) 𝑅𝑇 ]
Eq. (

Where:

• A (cm 3 s -1 ) is the pre-exponential factor which represents the frequency of reactants collision

• R is the perfect gas constant (8.314 J mol -1 K -1 ),

• Ea is the activation energy for the reaction (J mol -1 ) which is defined as the minimum energy that must be provided into compounds to initiate the reaction.

According to this equation, the rate coefficient increases with the temperature, and the reaction proceed faster. This is due to the increase of the internal energy of molecules, the kinetic energy, and the number of collisions between species. However not all the chemical reactions follow this law (e.g. some gas phase reactions involving ions and radicals). In other cases, the rate coefficient and the temperature are inversely proportional due to quantum effects (e.g. quantum tunneling effect) that is more important at low temperatures. This is known as non-Arrhenius temperature dependence. The Arrhenius equation was updated to Arrhenius-Kooij expression 1 (or the modified Arrhenius equation) and expressed as:

k(T)= α ( 𝑇 𝑇 0
) β . e -γ/T Eq.(2. 9) with β a temperature exponent, which is zero in the original equation. In Eq. (2.9), α replaces A, γ replaces Ea/R, and T0 is the temperature reference (usually 300 K).

Collision theory states that for a chemical reaction to occur, the reacting particles must collide with one another with sufficient energy (activation energy), and with the proper orientation. The rate of the reaction depends on the frequency of collisions. As stated earlier, collision between two reactants can result in the two molecules bouncing off one another and if no energy is exchanged this is called an elastic collision but if some kinetic, rotational or vibrational energy is exchanged it is called non-elastic. The third option is called reactive scatter and this leads to breaking of bonds and possibly the formation of new bonds. From kinetic theory, the collision rate ZAB between two molecules A and B, assuming that they are hard spheres, is given by: ZAB = NANBσAB [

8𝑘 𝐵 𝑇 𝜋𝜇 ] 1/2
Eq. (2.10)

where Ni are the number density of molecules, σAB (m 2 ) is the collision cross-section πr 2 (where r is the radius of the molecule, considering the spherical approximation), 𝑘 𝐵 is the Boltzmann constant (J K -1 ), T is the temperature and μ is the reduced mass. The SI unit of Z is cm 3 s -1 .

By adding the steric factor, P, which is the ratio between the frequency factor (the pre-exponential factor) and the collision frequency, the rate coefficient k (cm 3 s -1 ) is expressed by: k(T)= P NANB σAB [

8𝑘 𝐵 𝑇 𝜋𝜇 ] 1/2 exp [- 𝐸 𝑎 𝑅𝑇 ] Eq.(2.11)
The collision theory gives relatively accurate values for pre-exponential factors of reactions between monatomic species, but often fails for reactions between molecules with more complicated structures by several orders of magnitude. The reason for this is that the reacting molecules are able to react in all directions, because they have been considered to be spherical.

Capture models

For ion-neutral reactions, collision theory has evolved into capture models by considering the polarizability, the dipole moment, and the orientation of dipole moment of neutral reactant.

Capture models assume that the reaction rate is governed by long-range interactions leading to the capture of the collision partners, while the short-range reaction probability is unity.

Langevin model

The Langevin model is the simplest theoretical approach to estimate the rate coefficient between an ion and polarizable neutral colliders. 2 This method has been used extensively to calculate the collision rate coefficient in many environments, including the interstellar medium, where atomic hydrogen, molecular hydrogen, and atomic oxygen and nitrogen are the most abundant species.

We consider the collision of a point ion with a neutral (sphere) interacting via the charge-induced dipole potential. The total energy of the system in the center of mass frame of reference Erel can be expressed as:

Erel = 𝑃 𝑟 2 2𝜇
+ Veff(r,L) Eq. (2.12)

where

𝑃 𝑟 2 2𝜇
is the translational energy, Pr is the radial momentum of the collision partners at separation r, μ is their reduced mass, and the effective potential, Veff, is given by the following expression:

Veff (r,L)= 𝐿 2 2𝜇𝑟 2 - 𝛼𝑞 2 2𝑟 4 = 𝜇𝑣 2 2 × 𝑏 2 𝑟 2 - 𝛼𝑞 2 2𝑟 4
Eq. (2.13) where:

• L is the orbital angular momentum of the system (L= μ.v.b, MKS unit kg m 2 s -1 )

• 𝐿 2
2𝜇𝑟 2 is the orbital rotational energy 𝐸rot

• α is the polarizability of neutral molecule (Å 3 )

• b is the impact parameter

• q is the ion charge.

• 𝜇𝑣 2 2
is the relative impact (translational) energy 𝐸𝑟

• (- 𝛼𝑞 2
2𝑟 4 ) is the classical electrostatic attractive potential. The Langevin potential has a maximum along the reaction coordinate, it is equal to:

Vmax= 𝐸 𝑟 2 𝑏 4 2𝛼𝑞 2
Eq. (2.14)

The critical impact parameter corresponding to this potential is: bmax= (

2𝛼𝑞 2 𝐸 𝑟 ) 1/4
Eq. (2.15)

When b = 0, the ion is attracted directly and reacts. When b < bmax the capture collision probability is 1, and when When b > bmax the capture collision probability is 0.

In this case, the cross-section is derived as:

𝜎 L = 𝜋b max 2 = 𝜋 ( 2𝛼𝑞 2 𝐸 𝑟 ) 1/2
Eq. (2.16)

By taking into consideration that molecules follow a Maxwell-Boltzmann velocity distribution, the Langevin rate coefficient which is independent of temperature is expressed by: k L = 𝜎 L v= 2πq (α⁄μ) 1/2 Eq. (2.17)

This model therefore gives a temperature independent rate coefficient, which is a lower limit in the case of dipole molecules. One of studied reactions in our work is the reaction between N2 + ion and allene CH2CCH2, which is a nonpolar molecule, the rate coefficient dependence with temperature of this reaction follows the Langevin model, and it will be described in detail in chapter 4.

Locked dipole model

The Locked dipole model was determined early by Moran and Hamil 1963, 4 it states that the contribution of ion dipole potential to the collision cross section cannot be ignored for target molecules with significant permanent dipole moment. It describes the interaction between an ion (charged point) and a neutral molecule (sphere) with a permanent dipole μD. In this model, the rotation is considered as frozen and the dipole orientation is locked toward the ion direction.

The effective potential Veff between an ion and polarizable molecule having a dipole moment μD which is oriented in the position of minimum energy is:

V eff = 𝐿 2 2𝜇𝑟 2 - 𝛼𝑞 2 2𝑟 4 - 𝑞𝜇 𝐷 𝑟 2
Eq. (2.18)

If no alignment of the dipole with the incoming charge, the interaction averages to zero and Langevin theory is applied, If the molecule dipole "locks-in" as the ion approaches, then the iondipole interaction is taken into consideration.

The rate coefficient for thermal ions is expressed by: kLD= kL + 2πq

µ 𝐷 √2 √µ𝜋𝑘 𝐵 𝑇 Eq. (2.19)
Where kB is Boltzmann's constant = 1.380610 -16 erg.K -1 (cgs), and T is the temperature (K). In this model, the ion field doesn't completely quench the rotational angular momentum of the polar molecule, then the dipole effect (therefore the rate coefficient value) is overestimated in this model.

Average Dipole Orientation model ADO

Developed by Su and Bowers in 1973. 5 This theory is formulated in terms of an r-dependent average orientation angle 𝜃 (r) between the dipole and the line of centers of collision. The ADO model predicts the dipole dependence in proton transfer, charge transfer, and momentum transfer collisions. The ADO model considers that no net angular momentum between the rotating molecule and the ion-molecule orbital motion (not conserved, oscillations) when the dipole moment of neutral μD is increasing, the locking of the dipole increases. The potential energy is expressed by: V(𝑟)=𝑉𝐿 -qμD cos(𝜃 (𝑟))/𝑟 2 Eq. (2.20)

The average dipole orientation capture rate coefficient is given by:

𝑘ADO =𝑘𝐿 + 2𝜋𝑞𝜇 𝐷 𝐶√2 √𝜇𝜋𝑘 𝐵 𝑇 Eq. (2.21)
Where C is the effectiveness of locking the dipole or the locking dipole constant, it is given a value between 0 and 1, qualitatively considered as cos<θ> where <θ> is the average orientation angle of the dipole. In Eq. ( 2.21), one can say that when C=1, the expression is reduced to locked dipole limit, and when 𝜇=0, the expression returns to Langevin model.

The locking dipole constant is in function of the polarizability and the dipole moment: 𝐶=(μ/√𝛼).

In order to calculate the Average dipole orientation rate coefficient, it is necessary to know the value of (μ/√𝛼) for the polar molecule of interest, which then allows the determination of the value of the constant C, as shown in figure 2.2. By comparing the experimental measurements of the rate coefficient with Langevin, ADO and LD models, we can see that, in some cases, the Langevin model underestimates the rate coefficient, the LD model overestimates it, and the ADO approximation predicts the rate coefficient with a reasonable accuracy. 7 This result is reasonable because the conservation of angular momentum results in smaller values of the effective potential and hence enhances the possibility for capture.

Su and Chesnavich model

Previous models (Langevin, LD, and ADO) are informative, they are unable to give information about the classical dynamics of the ion-dipole capture, and on the motion in non-central field. 8 For this reason, Su and Chesnavich developed a theory which is the combined variational state theory (also known as the classical trajectory study). 9 In this framework, the angular momentum is conserved, the ion in considered as a point charge, and the neutral molecule is considered as two-dimensional rigid rotor. It couples rotational angular momentum of the dipole (J) to orbital angular momentum of the system (L): 𝐿 ⃗ and 𝐽 are collinear.

The rate coefficient 𝑘 𝑆𝐶 = 𝐾 𝑐𝑎𝑝 × 𝑘 𝐿 is temperature dependent, where:

• 𝑘 𝑐𝑎𝑝 = 0.4767𝑥 + 0.6200 when 𝑥 = µ 𝐷 √2𝛼𝑇𝑘 𝐵 ≥ 2,
• kcap= (𝑥+ 0.5090) 2 /10.526 + 0.9754 when 𝑥 < 2.

One of studied reactions in our work is the reaction between N2 + ion and propyne (also known as methylacetylene CH3CCH), which is a polar molecule (µ = 0.78 𝐷𝑒𝑏𝑦𝑒), 10 we have observed that the rate coefficient increases when the temperature decreases, following the Su and Chesnavich model, that will be presented in details in chapter 4.

Experimental techniques

Over the last decades, a notable development of laboratory instruments has been realized in the studies of ion-neutral reactions at room and low temperatures. Laboratory experiments on elementary reaction processes seek to provide two types of information: the first is the measurement of the rate coefficient k, and the second is the determination of the nature of reaction products. If more than one set of products is energetically accessible, the experimental determination of the branching ratios is necessary. For cold gas-phase reactions, reactants are typically produced either through collisions with cryogenically cooled walls of a reaction cell, or by the expansion of gas from high to low pressure. In this section, we present a range of experimental techniques for application in reaction studies between ionic and neutral atoms/molecules.

Flowing Tubes

Flowing Afterglow

A major advance in the afterglow method came with the development of the Flowing Afterglow Langmuir Probe (FALP) apparatus by Smith and Adams. 11 The FALP apparatus is equipped with two main measurement tools, the Langmuir probe (LP) and the Quadrupole Mass Spectrometer (QMS), both movable along the axis of the main chamber.

The FALP apparatus present in Rennes is composed by three main zones: the ion source, the main chamber, and the detection chamber. It uses a helium buffer gas flow (15 slm) through a glass tube (4 cm in diameter), which is weakly ionized in the ion source by a microwave discharge operating at 2.45 GHz (power can be varied between 20 and 200 W). The flow is maintained by a roots blower (4000 m 3 h -1 ), with a pressure ≈ 0.5 mbar in the reaction zone. A turbomolecular pump is employed to maintain a vacuum of ≈ 10 -7 mbar in the detection chamber.

Generated helium afterglow plasma is formed by ground state helium, electrons, He + , He2 + and metastable helium atoms He M . Latter species can be a source of ionization. In order to solve this problem, the stainless-steel tube (8 cm in diameter and 100 cm long) has an entry port used for the injection of argon gas Ar that play a role of a quencher to destroy the metastable helium atoms.

The movable mass spectrometer is used to identify ions present in the measurement region of the afterglow and to measure the ratios of the ion signals as a function of distance along the flow and of the electron number density, and so to determine the rate coefficient. A Langmuir probe is attached to the mass spectrometer entrance cone in order to obtain the electron density, simultaneously with the measurement of the relative ion populations. An example of reaction studied recently by FALP apparatus, is the reaction between cyanide anion CN -with cyanoacetylene (HC3N) at room temperature performed by S.Carles et al. . 12 In that work, the CN -anions were generated from dissociative electron attachment on a cyanogen bromide (BrCN) precursor. The rate coefficient for the CN -+ HC3N reaction is measured k= 4.8 (0.3) ×10 -9 cm 3 /s, this experimental value is in good agreement with the Su and Chesnavich coefficient (kSC= 4.32×10 -9 cm 3 /s). The m= 26,50, 79, 81 u masses have been observed and respectively attributed to the CN -, C3N - anions and the two natural bromine isotopes anions. The C3N -anion is generated from the proton transfer reaction. These observations confirm the possible pathway of C3N -formation from the CN -/HC3N reaction which was suggested and taken into account in the Titan's atmosphere model.

By cooling the flow tube and the carrier gas with liquid nitrogen, the FALP technique was updated into cryo-FALP, enabling one to study the ion chemistry at low temperatures down to 80 K. 13,14

SIFT: Selected Ion Flow Tube

In order to study ion-neutral reactions with selection of a single ionic species, and therefore to reduce the side reactions which complicate the ion spectrum, FA technique was extended by N.

G Admas and D. Smith in 1975 to the Selected Ion Flow Tube technique (SIFT). 15 Under this scheme, ions are created in the microwave discharge ion source (low-and high-pressure electron impact sources were also previously commonly used). A quadrupole mass filter can be used to pass ions of a given mass-to-charge ratio. These mass-selected ions are injected at energy about 20 eV (in the laboratory frame Elab) via a small aperture into a flow tube, and after they are entrained by a fast-flowing carrier gas (pressure of typically 0.5 Torr). In the FALP and SIFT experiments, for molecular ions, the centre of mass interaction energy with the carrier gas, Ecm, should be less than the dissociation energy of the ion. In this regard, because of their small masses, helium or hydrogen are the favored carrier gases, since Ecm

= Elab × [Mc/ (Mi + Mc)]
where Mc, and Mi are the masses of the carrier gas and the injected ion, respectively. The main difference between FA and SIFT, is that in the SIFT the ions are created in a remote ion source and not in the carrier gas-reactant-gas mixture. This method was invented to measure kinetic rate coefficients and product branching ratios for the gas phase reactions at room temperature. To G. Adams 16 The neutral reactant and the ions are injected in lower quantity than the buffer gas. A thermodynamical equilibrium is ensured due to the collisions with the walls of the reactor, which is cryogenically cooled with liquid nitrogen through two loops going in opposite directions along the flow tube. But this technique is limited by the condensation on the wall, and then an increase of the density's uncertainty.

For example, Illia Zymak et al. 2021 17 studied the reactivity of O -ions with CO2 molecules with different ternary gases (He and N2) over the 230-340 K temperature range. This termolecular reaction is considered to be the main production channel of CO3 -anions in martian atmosphere.

The ternary rate coefficient of the reaction was measured equal to (1.24 ±0.06) ×10 -28 (300/T) 2.86±0.28 and (2.29 ±0.05) ×10 -28 (300/T) 2.06±0.13 cm 6 s -1 for He and N2 carrier gas, respectively. The results of this VT-SIFT study are in a good agreement with previous measurements obtained using helium carrier gas in flowing afterglow instruments and those obtained using oxygen carrier gas. These experiments have some interest in the study on the effect of the nature of the carrier gas on the rate coefficient for termolecular reactions. A voltage between the ends of the flow-drift tube is applied in order to establish a uniform electric field, E, along its axis. The variation of E and the carrier gas number density, n, allows the reduced field strength E/n to be varied at will. This technique allows one to change the kinetic and the internal energies of the reagent ions and also their residence times in the flowdrift tube. In addition to study the kinetics and the products distribution of the ion-neutral reactions, this technique can be employed to investigate the ion mobility, the determination of the collisional cross section, the collisional dissociation. 18,19 …

The optimal working parameters for SIFDT-MS are summarized in Table 2.1 and compared with the corresponding parameters used in SIFT-MS technique. This reveals the enhanced and attractive features of the SIFDT analytical method. The SIFDT tube is extended by the use of different forms of collision-induced dissociation, CID.

The in-tube CID was implemented by applying electrostatic potential difference between the resistive glass flow-drift tube downstream end and the nose cone of a quadrupole mass spectrometer. This method allows, basing on the structural differences and corresponding changes in fragmentation patterns with CID energy, a real-time identification of isomeric products of ion-molecule reactions. 20

Guided ion beam GIB

The guided ion beam mass spectrometer (GIB-MS) is a versatile technique that allows one to study the ion-neutral reactions over a wide energy range with high sensitivity, and also determination of the integral cross sections with high accuracy. 21 This technique employs two octupole ion guides (O1, O2) and two quadrupoles (Q1, Q2), connected in the O1, Q1, O2, Q2 sequences. (Figure 2.8) A powerful approach for dealing with low-energy ion beams is based on the use of radio-frequency octopole guides, that confine ions in the transverse direction.

The first octupole is used for cooling the ions produced by electron impact, in which their excitation energy is eliminated by collisions with an inert gas (such as helium or argon). These ions are introduced in the first quadrupole to select the mass of the primary ion. Therefore, the mass-selected ions are transferred into the second octupole ion guide located in a chamber which the neutral reactant is introduced. The beam enters a second quadrupole where the ionic products with low kinetic energy (down to 0.04 eV) are collected, mass analyzed and deflected to reach the deflector.

Ion trapping method

Another technique to study ion-neutral reactions at low temperatures (down to 10 K) is the ion trapping method developed by Gerlich D. 23 This cryogenic approach is constituted of 22 pole electrodes on which nonhomogeneous radiofrequency are applied in order to produce an attractive potential to trap the ions.

Ions are generated by electron impact (for example), and after, they are selected by mass by a quadrupole mass filter and are then injected in the trap's interior. The collisions with the trap's cryogenic surface leads to the cooling of reaction zone (employing e.g liquid nitrogen as a cryogenic coolant). The trapping time vary between milliseconds and few minutes. The density of neutral reactant can be varied between 10 9 molecules/cm 3 and 10 14 molecules/cm 3 . By choosing an appropriate combination between these two parameters (the trapping time and the density), this method can be used to measure the rate coefficient of fast and slow bimolecular reactions. Upper part: Effective potential in the x-z-plane. Equipotential values are given in meV 24 As an example, Sunil S. Kumar rt al. 2018 25 used the cryogenic ion trapping technique to make the kinetic study for the reactions between OH + and H2O + ions with H2 at low temperatures ( 21to 155 K). These reactions contribute in the cycle of formation of water molecules in the diffuse interstellar medium (ISM). Their low-temperature measurements show that both OH + and H2O + ions react very fast with molecular hydrogen, with rate coefficients almost approaching the classical Langevin collision limit, kL~ 1.5 × 10 -9 cm 3 s -1 . rates for OH + and H2O + as a function of H2 density at two different temperatures. The data points at the lowest densities resulted from the residual H2 in the trap, which varied with trap conditions and temperature. Those points, while fully consistent with the trend, were not included in the fit. From Sunil S. Kumar et al. 2018. 25 Flowing Afterglow and ion trapping techniques use cryogenic cooling to obtain low temperatures, and therefore difficulties caused by condensation are encountered at the lowest temperatures (below∼200 K) when using reactant gases other than H2 or He 26 , that makes the absolute reactant concentration difficult to determine.

Long time trapping technique leads to high sensitivity and long interaction times, allows one to determine very small rate coefficients at low temperatures, eg. for radiative association reactions.

For example, the radiative association reaction between CH3 + and H2 has been studied at 13 K by S. E. Barlow et al. 1984 using the ion trapping technique, where the rate coefficient is equal to (1.8 ± 0.3)×10 -13 cm 3 s -1 . 27

Free jet

The free jet flow reactor is a technique used for the determination of ion/molecule reaction dynamics at unusually low temperatures. The free jet developed by Mark Smith was formed by a supersonic expansion of a gas mixture into a vacuum chamber, employing a pulsed nozzle.

Using this technique, a very cold gas is produced: down to 1 K. 28 A free jet is a nonequilibrium medium. The density and temperature both drop with increasing distance from the nozzle (strong temperature and density gradient), this means that the determination of the exact temperature associated to the measurement of the rate coefficient is difficult. 26 An example of an ion-neutral gas phase reaction using free jet technique, is the study made by L. K. Randeniya and M. A. Smith (1991). They measured the bimolecular rate coefficients for the reactions of N2 + ions with CH4, O2, and n-H2 at temperatures below 15 K. N2 + ions were generated by Resonance-enhanced multiphoton ionization (REMPI). Reaction with CH4 shows only a weak temperature dependence, the rate coefficient approaches to the Langevin capture rate at low energies. The rate coefficient for reaction O2 increases rapidly when the temperature is decreased from 300 K and approaches a limiting value at low energies. The energy dependence of reaction with n-H2 shows a minimum which resembles the reactions of NH3 + and C2H2 + with H2. 29

Crossed Molecular beams

The crossed molecular beam (CMB) technique was demonstrated in 1953 by Taylor and Datz of Oak Ridge National Laboratory. Dudley Herschbach and Yuan T. Lee developed this technique, for which they were awarded the Nobel Prize in Chemistry in 1986. They refined the apparatus and began probing gas-phase reactions in unprecedented detail. This method is particularly efficient in the case of reactions where product ions are distributed over a limited number of well-spaced energy levels. By inserting a skimmer into the supersonic zone of the free jet expansion, a molecular beam with a defined velocity is generated. The interaction between two molecular beams corresponds to the molecular crossed beams method. This technique enables one to study the gas phase reactions at different collision energies that can be accessed by varying the angle of collision. 31 For example, Friedrich et al. 1984, used the CMB technique to study the charge transfer reaction between N2 + (X 2 Ʃg, v=0) ions with N2 (X 1 Ʃg, v=0) at room temperature, at two different collision energies (0.74 and 9.94 eV). They have demonstrated the existence of two distinct mechanisms for charge transfer in N2 + /N2 collisions at low energy. One of these is a long-range direct mechanism, while the second proceeds via a complex surviving several rotational periods, with extensive energy transfer. At higher energy, the direct mechanism is dominant, with little energy exchange. 32 Despite that the crossed molecular beam technique enables experimentalists to determine the absolute integral cross-sections and the collision dynamics over appreciable interaction energies, especially at energies of astrophysical interest, it fails to find the rate coefficient in thermal equilibrium conditions. CMB technique encounters limitation at ions energies below a few eV.

CRESU technique

The CRESU technique is an efficient approach to study ion-neutral reactions at low temperatures.

The CRESU technique and our experimental setup coupling a mass selective ion source to a CRESU chamber will be presented in details in chapter 3.

Conclusion

In this chapter, we have presented some capture models used to estimate the rate coefficient of chemical reactions and introduced the experimental techniques employed in the investigation of ion-neutral gas phase reactions at room and low temperatures. Experimental measurements of the rate coefficient and the branching ratios supported by theoretical calculations (ranging from empirical approaches to quantum-chemical calculations) are crucial to understand the chemistry and physics in the astrophysical environments. Experimental kinetic measurements are in particular used in the construction of photochemical models of astrophysical environments. The purpose of the theses models is to reproduce the abundance for the various observed chemical species in a wide range of environments ranging from dense molecular clouds, 33 diffuse interstellar medium to planetary settings such as Titan's atmosphere. 34 Chapter 3:

Experimental methodology: the

CRESU-SIS machine

The CRESU technique (French acronym for "Cinétique de Réaction en Ecoulement Supersonique Uniforme" or "Reaction Kinetics in a Uniform Supersonic Flow") developed in the 80's by B. Rowe and J.-B. Marquette, 1 belongs to the family of flow reactors. The CRESU technique can generate cold (down to 6 K 2 ) uniform supersonic flows using a type of rocket engine nozzle, the Laval nozzle, giving the advantage of a wall-less flow reactor where no condensation occurs in the scrutinized medium. This method opened new horizons in the field of laboratory astrochemistry and chemical physics.

The CRESU-SIS device developed in Rennes (Figure 3.5) allows measuring the absolute rate coefficient and the branching ratios of targeted gas-phase ion-molecule reactions at various low temperatures.

The CRESU technique

The CRESU technique is based on the production of a uniform supersonic expansion of a buffer gas into a low-pressure (0.1 -2 mbar) chamber through a Laval nozzle. The nozzle is designed by solving the nonlinear Navier-Stokes equations for viscous fluids. The optimally shaped convergent-divergent nozzle produces a flow at the exit which is at a constant Mach number (∼ 5), the consequence being that the density, pressure, and the temperature at all points within the subsequent postnozzle flow are constant for tens of centimeters, which means a time scale of hundreds of microseconds. Some nozzles could generate uniform supersonic expansions with different temperature and pressure conditions, using binary mixtures or various buffer gases. The experimental characterization of the uniformity length of supersonic flows is fundamental before application to chemical kinetics studies.

In order to achieve uniform flow conditions, the buffer gas flow rate can reach 100 slm (standard liter per minute) thus requiring substantial pumping capacity (up to 60 000 m 3 h -1 ). The density of the flow is in order of 10 15 to 10 17 molecules cm -3 : the high density, i.e. the high number of collisions, ensures a local thermodynamical equilibrium (LTE) and rapid rotational relaxation of the molecular species (charged or neutral). These conditions allow us to measure the absolute rate coefficient and the branching ratios of molecular and atomic ions (cations and anions) with neutral reactants under the cold conditions found in astrophysical environments. The density of the flow is higher than the density in interstellar medium and planetary atmospheres, but this does not affect the kinetic study and branching ratios measurements in the case of bimolecular reactions. The neutral reactant is continuously injected in the supersonic flow with a controlled flow rate of a few sccm (standard cubic centimeters per minute) corresponding to a density of Where M is the Mach number, (M=u/a; a is the speed of sound in the surrounding medium),  is the ratio of the specific heat capacities (J mol -1 g -1 ) at constant pressure and volume (= Cp/Cv). The flow pattern of a supersonic flow depends on the profile of the Lava1 nozzle (i.e. convergingdiverging) which generates it. The flow temperature, T, and flow pressure, P, and the density, ρ, are expressed by the isentropic relationship:

T 0 T = 1 + M 2 (γ-1) 2
Eq. (3. 5)

P 0 P = (T0/T) (γ/ γ-1)
Eq. (3. 6)

ρ 0 ρ = (T0/T) (1/ γ-1)
Eq. (3. 7)

In equations 3.5-3.7, T0, P0, and ρ0 are the temperature, pressure, and density in the stagnation region (reservoir), respectively, and T, p, and ρ are the temperature, pressure, and density in the gas beam, respectively.

The gas expansion through the de Laval nozzle converts the internal energy of the gas species into kinetic energy. For a compressible perfect gas with a constant specific heat capacity Cp and with a well-defined temperature, the energy conversion is driven by the following very simple equation:

CpT+ u 2 /2=CpT0 Eq. (3. 8)
where T0 and T are the temperatures of the gas prior and after expansion respectively and u is the supersonic flow velocity.

By neglecting the effects related to the viscosity and by supposing that the expansion is adiabatic, the conservation of the mass across a section A is expressed by the Hugoniot relation 4 :

du u (M 2 -1) = dA A Eq. (3. 9)
The characterization of the uniform supersonic flow in terms of pressure, temperature and density is performed using a Pitot Tube moving along the gas expansion. The Pitot tube is a 10 cm of 1/8 inch stainless steel tube mounted on a support capable of moving 7 cm in the y-axis, and 60 cm in x-axis. It provokes a perturbation and creates a shock-wave detached from this obstacle.

From the pressure measured behind the shock wave, one can retrieve the Mach number of the flow and the flow pressure, density and temperature. 5 According to the laws that govern the behavior of flows in a supersonic regime, the chamber pressure (Pchamber) should be equal to that at the exit of the Laval nozzle (Pflow). If it is not the case, the flow losses its uniformity. So, after the expansion of the gas throw the Laval nozzle, there are three possible scenarios: these cases are simulated by our colleague Abdessamad Benidar as shown in Figure 3.3.

The first case is when the pressure of the flow is inferior to that in the chamber (Pflow<Pchamber), the flow converges at the exit of the Laval nozzle (Figure 3.3 a). The second case is when the pressure of the flow in superior to that in the chamber (Pflow>Pchamber), the flow diverges near the exit of the Laval nozzle (Figure 3.3 b). The last scenario is when the pressure of the flow is equal to that in the chamber (Pflow=Pchamber), in this case we talk about a well-adapted case (Figure 3.3 c). In order to limit the high gas consumption required to generate high-quality USFs (Uniform Supersonic Flows), in the mid 1990's, M.A. Smith (University of Arizona, USA) and co-workers developed a pulsed version of the CRESU technique. A similar apparatus was constructed in the group of S. Leone at Berkeley (USA), using a pulsed Laval nozzle. 6 An example of reactions studied using a pulsed Laval nozzle expansion is the reaction of OH radicals with propene and isoprene, in the temperature range of 58-300 K, by Troe's group. 7

Selective Ion Source

The CRESU-SIS method combines the CRESU method with a mass-selective ion source. In the present set-up, the mass selective ion source is developed by Fasmatech (Greece), which is coupled to a CRESU chamber. The proof of concept of the CRESU-SIS experimental setup was published in 2019 10 . In that work, Rennes' group studied the reactivity of Ar + with C2H6, N2O, and N2. The reaction between O2 + ions with ethane C2H6 was also investigated. The results are in good agreement with previous measurements (FA, SIFT, CRESU, ICR) and empirical models.

Generation of ions

The ions are produced in a cold, stable, and dense plasma generated by a hollow cathode discharge (HCD) (P∼2 mbar, Udischarge ∼395 V, Idischarge ∼950 mA) obtained with a continuous flow of helium (0.750 slm) and a small quantity of precursor gas (for example we inject 5 sccm of nitrogen N2 to create N2 + ions). The typical conditions for the operation of the plasma source are summarized in table 3.1. The cathode is connected to a high-power supply, and the anode is held at ground potential. Because the plasma is globally neutral, the sum of negative charges is equal to the sum of positive charges. As most of negative charges are carried by electrons, we observe that the number of cations is one order of magnitude higher than the number of anions.

The ions generated by the HCD are injected into an ion funnel which is driven by high-frequency low-amplitude RF waveforms (3.5 MHz).

Helium

Mass-selective ion transfer line

The mass selection of the ions is provided by a radio frequency quadrupole mass filter (QMF)

giving an output ion current of about 1 nA. Ions are introduced into the QMF with an optimal kinetic energy of 10 eV. The QMS is a device that can separate charged atoms or molecules according to their mass-to-charge ratio. Ionized molecules are accelerated (∼ few eV) towards the quadrupole analyzer region where a combination of radio-frequency RF (V0 cos(ωt)) and direct-current DC (U) fields are applied to cylindrical rods arranged along the axis of the ion beam. Ions entering a field experience a deflecting force, depending on the strength of the field and the mass-to-charge ratio of the ion. For given values of the RF and DC fields, only ions with a specific mass can pass longitudinally through the rods. Several quadrupoles (P≈10 -5 mbar) and octopoles (P≈10 -2 mbar) ion guides are used to transport ions from their production zone, through the selection mass filter and finally down to the uniform supersonic flow with a good transmission. Studies of Douglas and French (1992) 12 on the collisional focusing effects in RF quadrupoles show that, provided a low initial ion kinetic energy

(1-30 eV), the ion transmission through a small aperture at the exit of a quadrupole increases as the pressure increases to reach a maximum near 10 -2 mbar, which is very close to the pressure conditions of our octupole ion guides. In order to maximize the transmission and counteract collisional cooling, a weak DC gradient is applied across the segments of ion guides. All DC and RF potentials applied to the ion transfer line are controlled electronically through our own computer interface. We use two types of power supplies: bipolar dc power supply units (PSUs) that are employed for the transfer line, that can be operated in positive and negative ion modes, and a single RF PSU delivering a 3.5 MHz sinusoidal waveform is used to drive the funnel, the ion guides, and the hexapole integrated upstream the detection QMS, while independent DC PSUs are employed to control all static potentials applied to the lens electrodes and to establish the axial DC gradients across the ion guides. Ions selected in the QMF are focused using a set of lenses through a differential pressure aperture into the octopole ion guide.

The final ion current at the bottom of the SIS is around 600 pA. The ions arrive at right angle compared to the neutral uniform supersonic flow axis requiring for the injection a deflector with adapted applied potentials (around +1 V and -30 V for radial and vertical axes respectively) which allows the alignment of ions trajectories with the supersonic flow towards the entrance of the detection system.

The selective ion source can work in positive and negative modes. Negative ions are detected at the end of the mass filter, and we have measured a quantity of ≈ 100 pA at the lens at the end of the selective ion source. To work in negative mode, we should invert the polarity of all applied potentials. The buffer gas of the CRESU chamber flows back into the ion source, due to the relative high pressure in the chamber (∼mbar) comparing with the pressure in the quadrupole ion guide at the end of the selective ion source (∼10 -2 mbar). In order to study the effect of the nature of the buffer gas on the ion transmission efficiency, Fasmatech performed some simulations (C2H5 + ions in this case), considering two buffer gases predominantly used in our Laval nozzles: helium and the nitrogen. Simulations show that in the case of helium buffer gas, the transmission is more than 60%, but in the case of nitrogen buffer gas, the ion transmission was strongly inhibited (< 5%). The gas-dependent transmission could be explained by the higher collision cross-section of the molecular nitrogen N2 (σ= 0.43 nm 2 ) 13 relative to the helium He (σ= 0.21 nm 2 ), 13 which lead to more ion-molecule collisions and scattering, and therefore lower transmission. 14 (Figure 3.10)

Detection part

The monitoring of ions along the de Laval expansion is performed by a movable quadrupole mass spectrometer (QMS) housed in a differentially pumped chamber (P≈10 -5 mbar). The detection chamber, mounted on a position-controlled translating stage, was recently modified to incorporate a rf hexapole ion guide combined with a molecular beam skimmer (orifice diameter D usually 0.8 mm) to enhance sampling efficiency of reactant and product ions. The skimmer was polarized by a few volts (∼ -2 V for cations). In our case, the resolution of the QMS is typically 30 and the m/q range is 0-100 u. At the end of the QMS, a channel electron multiplier (also known as channeltron, CEM) is used to detect ionic species in the flow. In our experimental setup, we are using the Magnum 5901 CEM delivered by Photonis (gain ≈ 5× 10 7 ) well adapted to operate at reltively high pressures (10 -4 mbar). Ion trajectories, and therefore their detection in the USF are very sensitive to the voltages applied to the deflector. Applying adapted voltages on the deflector allowed for more than 95% of the selected ions to be drifted toward the skimmer. The measurements showed that, in the absence of neutral reactant, the ion signal must not vary greater than 5% along the flow, for a distance up to 20 cm (see for example the figure 4.3 in chapter 4) to provide adequate condition for kinetic measurements. We found that a change of ±5 V in the deflector potential leads to its complete disappearance of ion signal.

As an example, figure 3.13 shows a mass spectrum recorded in the flow by our movable QMS at 24 K. The mass filter strongly reduces the transmission of other ions than the desired one produced in the source such as H3O + by a factor of ⁓ 100.

Gas introduction

The buffer gas, the precursor, and the neutral reactant are introduced into the source and the reservoir of the CRESU chamber using mass flow controllers (Bronkhorst or MKS). These controllers are calibrated regularly in order to reduce the errors in the density of introduced gas.

The neutral density should be known precisely to determine an absolute rate coefficient. The vibrational relaxation of molecular ions by neutrals is generally efficient because of the added electrostatic potential between the ions and neutrals. 16 As presented in the work of Baptiste Joalland et al., 10 due to the high number of collisions, the Ar + ions are de-excited from ( 2 P1/2) to

( 2 P3/2) in the sub-microsecond time scale, directly in the dense plasma of the source, while for O2 + ions, the vibrational deactivation of O2 + (v = 1, 2) with different neutral partners is also achieved during their travel in the ionization source. In some cases, to ensure the quenching of exited ions, and to avoid their presence in the reaction zone, we inject a small quantity of a quencher gas in the source. The nature of the quencher gas depends on the studied ion.

The determination of the rate coefficient

Because of the constant flow velocity, the moveable detector allows to follow the time evolution of the ion signals for a hydrodynamic time h of typically 150-200 µs. The slope of the evolution of ion signal with time at fixed neutral density corresponds to the pseudo-first order rate coefficient k1st (the loss rate, s -1 ), and the slope of the loss rate as a function of the neutral reactant density determines the rate coefficient of the collisional chemical reaction.

The temperature and the velocity of injected ions

One of the key aspects of the experimental scheme is to ensure the good thermalization of the injected ions, that itself allows the derivation of meaningful rate coefficients at a given flow temperature k(T).

In order to study the velocity and temperature of injected ions, our colleague Abdessamad Benidar did some simulations using COMSOL software of the injection of Ar + ions (in which all parameters needed to the simulations, such as collision cross sections, exists in the program database) of initial kinetic energy of 5 eV (similar ions energy used in the simulations of Fasmatech, section 3.2.2) are guided and confined by the octopole to be injected into the CRESU using the deflector. In these simulations, the uniform supersonic helium flow is defined with a temperature of 50 K, a speed of 1630 m/s with an isentropic core of 16 mm. The deflector is brought to a ddp of 28 V (Utop= -1; Ubottom = -29V). Figure 3.15 shows a simulation of the injection of Ar + ions in the CRESU's supersonic flow. The ions velocity can be expressed by the Maxwell-Boltzmann distribution: This expression was used in order to deduce the temperature of the injected ions in the flow.

f(v) = ( 2 π ) 1/2 ( m k B T ) 3/2 v 2 exp (-
Results show that at a distance 2 < x < 3 cm from the injection zone, the ions have a temperature ⁓ 58 K, and after few centimeters, their temperature decreases to reach the temperature of the cold flow ⁓50 K at a distance 7 < x < 8 cm. Usually, in our experiments, the first measurement of the ion signal is around 7 cm, so we can assume that the ions are "safely" injected in the flow, and they have temperature close to the temperature of the isentropic core, due to the high density of the flow (therefore high number of collisions between the ions and the buffer gas of the CRESU expansion).

Conclusion

This chapter gave a brief overview of the CRESU technique, and described the CRESU-SIS experimental setup developed in Rennes 1 University.

The design of the ion transfer line, which takes advantage of the relatively high pressure in the reaction chamber and of the combination of quadrupolar and octopolar radio frequency field orders, makes possible to concurrently:

(i) Produce a great variety of positive and negative ions through its modular ion source.

(ii) Select the primary ion of interest for a given m/z ratio, that makes the measurements of the rate coefficient and the branching ratios more precise.

(iii) Maximize the ion transmission through large pressure gradients between the ionization source (∼mbar), the quadrupole mass filter (∼10 -5 mbar), and the de Laval expansion (∼mbar).

(iv) Ensure the prompt rovibrational thermalization of the mass-selected ions.

(v) Redirect the ion cloud along the propagation axis of the cold de Laval expansion readily. In the next chapter, the CRESU-SIS method will be applied to investigate the reactivity (kinetics and branching ratios) of the N2 + ions with propyne CH3CCH and allene CH2CCH2 at low temperatures (down to 24 K).

Chapter 4:

Kinetics and Branching for the Reactions of N 2 + with C 3 H 4 Isomers at Low Temperatures and Implications for Titan's Atmosphere Cyclopropenylidene C3H2 has recently been detected in Titan's atmosphere. One of the precursors of this carbene has been identified as the cyclopropenyl cation (c-C3H3 + ) that can form C3H2 through dissociative recombination. This combined experimental/theoretical work explored the reactivity of N2 + ions with C3H4 isomers, namely propyne (CH3CCH) and allene (CH2CCH2), as possible pathways to the formation of c-C3H3 + , the smallest aromatic species.

For the first time, both reactions were investigated at low temperatures between 24 and 71 K.

Rate coefficients, product identity, and branching ratios were measured within uniform supersonic flows combined with a newly developed mass-selective ion source and an improved detection scheme. The experimental results were complemented with coupled cluster ab initio calculations to enable the firm identification of the possible reaction paths and product isomers.

The results show that the rate coefficients are large and their temperature dependence in remarkable agreement with capture models. The derived reaction rates and branching ratios have been included in a photochemical model of Titan's atmosphere. The results point to a secondary role of the N2 + + C3H4 reactive pathways in the formation of c-C3H3 + .

Introduction

The Cassini-Huygens mission to Titan, which explored Saturn's largest moon from 2004 to 2017, has revealed a fascinating and complex world that could provide a good analogue of Earth during its early history. Under irradiation from UV solar photons, bombardment from galactic cosmic rays and high energetic electrons from Saturn's magnetosphere, the dense atmosphere of Titan, dominated by nitrogen (N2) and methane (CH4), leads to the production of a wealth of organic species. Our knowledge of the atomic and molecular constituents of Titan's atmosphere, their spatial distribution and their temporal variations has also seen some significant progress through combined Earth-based and space-based observations and in situ measurements. The composition of the dense troposphere has been determined by the Cassini Gas Chromatography/Mass Spectrometer (GCMS) measurements. 1 For the stratosphere, advances have relied on ground-based mm-wave instruments (IRAM, ALMA,…) 2,3 , infrared (IR) space observatories ISO 4 and Herschel 5 , and the Composite Infrared Spectrometer (CIRS)

embarked onboard Cassini 6 . The mesosphere has been characterized by the Cassini Visible Infrared Mapping Spectrometer (VIMS) 7 and Ultraviolet Imaging Spectrograph (UVIS) 8 . The much dilute high layer of the atmosphere, the thermosphere, has been solely investigated by the Ion Neutral Mass Spectrometer (INMS), which conducted mass spectrometric measurements of both ions and neutrals onboard Cassini. 9 In this particular region, a plethora of hydrocarbons [two-carbon acetylene (C2H2), ethylene (C2H4), and ethane (C2H6), the three-carbon propyne/allene (C3H4), propene (C3H6) and propane (C3H8), the four-carbon diacetylene (C4H2)

and the six-carbon benzene (C6H6)] and nitriles [hydrogen cyanide (HCN) and hydrogen isocyanide (HNC), cyanoacetylene (HC3N), acetonitrile (CH3CN), acrylonitrile (CH3CCN), propanenitrile (C2H5CN), cyanogen (C2N2)] have been detected.

The INMS mass spectra spanning the 0-100 u range show peaks arranged in clusters separated by 10 to 15 u that coincide with the sequential addition of hydrogenated carbon or nitrogen atoms. Noteworthy, a good correspondence is found between the ion and neutral spectra.

However, due to the low resolution of the embarked mass spectrometer (m/m~100), and the absence of isomer identification inherent to the method, the assignment of the peaks cannot be self-sufficient and relies heavily on a good knowledge of the chemistry. The latest modeling study based on INMS measurements 10 puts forward key hydrocarbon ions such as CH3 + , CH5 + , C2H5 + , C3H3 + , C3H5 + , C4H3 + , C4H5 + , C6H7 + , C7H7 + , and nitriles such as HCNH + . 11 Many other ions stay under the radar because of mass overlap; they include N2 + since HCNH + is the dominant species at m/z=28 u, and also N2H + concealed by C2H5 + (m/z=29 u). To assess their presence, well-known reactions dominating their loss processes can be employed. The best tracer of N2 + is considered to be CH3 + as it can only be produced massively through the reaction of N2 + with the second most abundant species in the atmosphere, methane (CH4). 12,13 The N2 + cation is of particular importance as it is considered as a primary ion along with N + and CH4 + .

These three cations are directly produced upon interaction of N2 and CH4 with solar photons and suprathermal photo-and secondary electrons.

Propyne CH3CCH has been first uncovered in the Voyager IR spectrum of Titan in the 80's. A stratospheric abundance of C3H4 of 3×10 -8 was determined for the mid-latitude region. 14 Further observations by ISO in 2003 led by Coustenis et al. 4 provided a disk average value of 1.2 10 8 .

During targeted flybys 15,16 , Cassini/INMS measured the abundance of neutral C3H4 in the ionosphere and derived a mole fraction for propyne in the 2 7 10 6 range. Propyne was also detected by the CIRS instrument through its 633 cm -1 emission feature 17 . Propadiene, also referred to as allene, CH2CCH2, an isomer of propyne, was unambiguously identified by the Texas Echelle Cross Echelle Spectrograph (TEXES) on the NASA Infrared Telescope Facility through its emission near 12 m with a measured volume mixing ratio of 6.9 10 10 at the altitude of 175 km 18 . CIRS measurements of propyne made at the same time indicate that the abundance ratio of propyne to propadiene is 8.2 at the same altitude. Propyne and propadiene constitute the first hydrocarbon isomer pair detected on Titan. Both isomers have been included

in several photochemical models. 11,19,20 According to Dobrijevic et al., 21 the main pathway for the formation of both isomers in the stratosphere is the associative reaction between hydrogen and propargyl radicals (C3H3):

C3H3 + H (+ M) → C3H4 (+M) Eq. (4.1)

Below 200 km, the CH2CCH2 molecule can be produced additionally by the photodissociation of propene (C3H6).

In the ionosphere, C3H4 can be generated through the reaction of the CH radical with C2H4 and through the recombination of C3H5 + with electrons:

C3H5 + + e -→ C3H4 + H Eq. (4.2)
The primary loss channel of C3H4 is photodissociation back to propargyl C3H3:

C3H4 + hѵ → C3H3 + H Eq. (4.3)
Conversion from propadiene to propyne can take place through the reaction:

H + CH2CCH2 → CH3CCH + H Eq. (4.4)

The ion and neutral chemistry proposed above is however incomplete as major sources of uncertainty remain, besides the fact mentioned above that the INMS measurements cannot alone rule out isomers and lead to a firm identification of the alleged species. In particular, major sources of uncertainty come from the lack of information on product identity and branching as well as their temperature dependence (available data are often restricted to room temperature). While this is especially problematic for radical-neutral reactions, with only one study below 50 K reported so far 22 , several techniques have been developed to measure the low-temperature kinetic rate coefficients as well as the products of ion-molecule reactions:

crossed and merged molecular beams, ion traps and flow reactors. Cryogenic ion traps are for instance tailored to examine the reaction kinetics of ions over a wide dynamic range. 23 In practice however, condensation of the neutral co-reactants onto the walls of the chamber limits their use to light neutrals such as H2 or He. 24 Uniform supersonic flows, which belong to the family of flow reactors, are well adapted to explore quantitatively the reactivity of ions with large neutral molecules [25][26][27] it is one of the rare techniques to provide rate coefficients obtained under well-controlled thermalized conditions and over a wide range of low temperatures 28 . Here, we use a new approach, which combines a CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme) reactor with a mass-selective ion source, 29 to measure for the first time the low-temperature rate coefficients and product branching of two key Titan's atmospheric reactions involving isomeric hydrocarbon targets: N2 + with propyne CH3CCH and propadiene CH2CCH2 at 24, 36, 49 and 71.6 K. The experimental results are complemented with quantum chemical calculations to help identify the relevant products and chemical pathways.

Experiments

The low-temperature reaction of N2 + with C3H4 (propyne and allene) was investigated using a novel mass selective ion source (SIS) combined with a CRESU flow reactor. The CRESU-SIS device allows measuring absolute rate coefficients and the branching ratios of targeted gasphase reactions at various low temperatures (down to 15 K). Details on this new approach can be found in chapter 3. The neutral cold and dense uniform supersonic flow is generated by the isentropic expansion of a continuous buffer gas (here helium or a mixture of helium and nitrogen) through a Laval nozzle. The density, the velocity and the temperature in the core of the supersonic flow are uniform over several tens of centimetres (See Table 4.1 for the characteristics of the Laval nozzles used in this work). The supersonic flow, which can be viewed as a wall-less chemical reactor, can be inseminated with a small quantity of the studied ionic and neutral reactants allowing reactive collisions. The neutral reactants propyne and allene are commercially available from Air Liquide with purity higher than 96% and 98%

respectively. These species are continuously injected in the supersonic flow with a controlled flow rate of a few sccm (standard cubic centimeter per minute) corresponding to a density of about 10 12 molecule cm -3 .

In the present set-up, ions are produced in a plasma generated by a hollow cathode discharge (P∼2 mbar, Udischarge ⁓ 395 V, Idischarge ⁓ 950 mA) obtained with a continuous flow of helium (0.750 slm) and a small quantity of nitrogen N2 (5 sccm). The mass selection of the N2 + ions is provided by a radio frequency quadrupole mass filter giving an output ion current of about 1 nA. The final ion current at the bottom of the SIS is about 600 pA. According to Kato et al. 30 and Ferguson 31 , the collisional vibrational relaxation coefficient of N2 + (v =1) by N2 is about 5-8×10 -10 cm 3 molecule -1 s -1 , meaning that the relaxation to the ground state is achieved in a few µs, i.e. directly in the plasma of the SIS start station. As a result, the N2 + ions are in their ground vibrational state before being injected into the uniform supersonic flow.

Results

Reaction Products

Figure 1 shows two mass spectra recorded at 49 K after a reaction time of t = 50 µs and with a reactant density [C3H4] = 1.4×10 12 molecule cm -3 for propyne and allene. For both reactions, the main ion product detected is C3H3 + , followed by C3H2 + and C3H4 + ions (respectively m/z=39, 38 and 40 u). In addition, the mass spectra reveal traces of C3H5 + (m/z=41 u) and C3H + (m/z=37 u) as can be seen on the inset of Figure 1. Without any neutral reactant, ions other than N2 + , namely N2H + at m/z = 29 u, NO + at m/z = 30 u, O2 + at m/z = 32 u, H2O + at m/z= 18 u, and H3O + at m/z= 19 u, are also detected. experiments at room temperature and the only observed exit channel for both reactions is C3H5 + + H2O with reaction enthalpies ΔH = -0.83 and -0.87 eV and rate coefficients k = 1.7-1.8 and 1.4×10 -9 cm 3 molecule -1 s -1 for propyne and allene, respectively. 30,31 ii. The H2O + + C3H4 reaction is considered as a source of C3H5 + and potentially C3H4 + in line with the proton affinities of propyne (748 kJ/mol) and allene (775.3 kJ/mol) which are larger than the ones of OH ( 593.2 kJ/mol) and H2O (691 kJ/mol). 32 The contribution of the H2O + + C3H4 reaction to the BR of C3H4 + is weak. As an illustration, the H2O + ion signal drops by less than 5 % in the presence of neutral reactant with a density [C3H4] of 610 12 molecule cm -3 , meaning that H2O + is not highly reactive with C3H4. In spite of its small contribution, this effect is taken into account in the determination of the branching ratios of the N2 + + C3H4 reactions.

iii. According to Milligan et al., N2H + reacts with C3H4 to form the C3H5 + ion exclusively as it has been observed in FA-SIFDT experiments at 300 K 31 . The experimental rate coefficients are k = 1.5 and 1.4×10 -9 cm 3 molecule -1 s -1 and the corresponding reaction enthalpies are ΔH = -2.87 and -2.91 eV for propyne and allene, respectively.

iv. The NO + + C3H4 (propyne and allene) reactions have also been studied in SIFT experiments 30 at 298 K. Propyne only generates the [C3H4 NO] + adduct with a rate coefficient of k = 6.3×10 -11 cm 3 molecule -1 s -1 , while the reaction with allene exhibits two exit channels corresponding to the [C3H4 NO] + adduct (90%) and the C3H3 + ion (10%).

However, the rate coefficient k = 2.5×10 -11 cm 3 molecule -1 s -1 for the NO + + CH2CCH2 reaction is too low to be competitive here. The lack of a significant drop in the NO + signal at low temperatures (a few percent) and the absence of an ion signal at m/z=70 u allow to discard any interference from the NO + + C3H4 reactions. propyne and allene respectively 30 . Dicker et al., also studied the O2 + + CH2CCH2 reaction and derived a rate coefficient k = 1.3×10 -9 cm 3 molecule -1 s -1 , but they observed three exit pathways: the charge transfer (94%), the C3H3 + + HO2 exit channel (4%) and the CH2CO + + CH2O one (2%) with calculated corresponding enthalpies ΔH = -2.39, -2.87 and -2.91 eV 34 .

In the work of Wilson et al. 30 , the presence of vibrationally excited O2 + ions was assumed.

In the latter experiments of Decker et al. 34 , this issue was considered and O2 was added to quench the excited O2 + ions. This can explain the different results from the two SIFT experiments. Then, the C3H4 + and C3H3 + ions can also be formed by the O2 + + C3H4 reaction, this is considered in the branching ratio determination (see thereafter).

vi. For the C3H + product, all the reaction enthalpies calculated in this work for the different exit channels of the N2 + + C3H4 reactions are positive (see Figure 2), suggesting that the observed C3H + traces come from a secondary reaction between C3H4 and some impurities.

These reactions do not affect the evolution of the N2 + density and therefore the derived rate coefficient of the studied reactions.

In order to qualitatively analyze the stoichiometry of the species observed in the mass spectra we performed a reduced theoretical analysis by calculating the enthalpies of formation and the corresponding relative reaction enthalpies by means of high level ab initio theories. The ab initio calculations were performed using the spin unrestricted coupled-cluster theory with single and double excitations (UCCSD) along with the augmented correlation-consistent valence double zeta (aug-cc-pVDZ) basis set. All calculations were carried out with the MOLPRO (version 2015.1) quantum chemistry software package 35,36 . The structure and energy properties of all molecular species were determined in their minimum configuration, i.e. following full geometry optimization. Various starting geometries were examined in order to find the structures and properties of the most stable [N2-C3H4] + complexes.

Harmonic vibrational frequency and normal mode calculations were performed to compute the zero-point vibrational energies (ZPVE) and the total enthalpies of formation. More details about the geometries and energies of the species are given in the Supplementary Material.

Figure 2 shows the schematic diagram of the relative reaction enthalpies for the observed hydrocarbon cations and a few other related ones. Since the experiments were carried out at various temperatures, the reaction enthalpies were calculated at 0 K only, i.e. taking into account the total electronic and zero point vibrational energies exclusively. Note that many more cyclic complexes exist but our analysis was limited to the two most stable [N2C3H4] + ones. Given the low temperatures of the experiments, the N2 + + C3H4 reactions can only form exoergic products i.e. CH2CCH2 + and CH3CCH + cations (pure charge transfer) as well as the different C3H3 + and C3H2 + isomers. According to our calculations the endoergic exit channels towards the C3H + cations remain out of reach. As one can also see in Figure 2, the most exoergic N2C3H4 + species have a cyclic structure. The first of them (1H-imidazole cation) has a very large relative exoergicity of about -7.22 eV, while the second isomer (pyrazole cation) has a slightly higher relative enthalpy, with about -6.20 eV. Both of these cations are highly exoergic with respect to the reactants and the C3H4 + , C3H3 + and C3H2 + product formation channels. However, since the formation of these cyclic species implies strong re-orientations and multiple internuclear rearrangements which possibly involve many intermediate transition states, we expect that such N2C3H4 + complexes do not form effectively from the linear molecular reactants.

Charge transfer reactions (both dissociative and nondissociative), which lead to the formation of the observed C3H4 + , C3H3 + and C3H2 + products, most probably proceed via the formation of non-stable, quasi-bound complexes (i.e. complex with large intramolecular distance, in agreement with the capture theory), producing excited C3H4 + cations preliminarily. These complexes are represented by close superposition of the colliding molecular species, where their bond lengths and angles are not distorted, but where the electron exchange from C3H4 towards N2 + can effectively proceed, involving electronic transitions between multiple potential energy surfaces. This mechanism is supported by the Franck-Condon principle, according to which the electronic transitions proceed much faster than nuclear movements and rearrangements. If the primary cations are formed by conserving a large amount of internal energy, their dissociation proceeds according to a unimolecular decomposition scheme.

Rate coefficients.

Kinetic data were obtained under pseudo-first order conditions, i.e. with a [C3H4] neutral density in excess compared to the [N2 + ] density. The loss rate of N2 + was determined by the evolution of the N2 + signal with time for a given neutral reactant density. The variation of the N2 + loss rates with the neutral reactant density gives the absolute rate coefficient k(T) of the reaction at a fixed temperature, as illustrated in Figure 3 for N2 + + CH2CCH2 at 24 K. The results are summarized in Table 2. Several arguments concur to rule out termolecular processes for the N2 + + C3H4 reactions.

First, no peak at m/z= 68 u, corresponding to the association product, is detected in the mass spectra. Second, the rate coefficient of the N2 + with CH3CCH reaction is not affected by changing the buffer gas, i.e. pure helium at 49.1 K and helium-nitrogen mixture (1:1) at 57 K. Thirdly, an increase of the total density or a decrease of the temperature both lead to a rise of the rate coefficient in a termolecular process; yet the measured rate coefficients for propyne and allene follow closely the Su and Chesnavich and the Langevin models, respectively (see Figure 4), including between 24.1 K and 36.1 K where the total density is changed by more than 3-fold (18.3 10 16 and 5.28 10 16 molecule cm -3 , respectively). This set of observations strongly supports a bimolecular process only.

Branching ratios

The C3H2 + , C3H3 + and C3H4 + ions are clearly identified as the primary products of the N2 + + CH3CCH / CH2CCH2 reactions. The enthalpies of the N2 + + C3H4 reactions producing C3H + are positive and the formation of C3H5 + is well explained by the reactions between N2 + with impurities. As mentioned above as well, the O2 + + C3H4 reaction represents however a small but non-negligible source (up to a few percent) of C3H3 + and C3H4 + ions, even if the initial abundance of N2 + ions is much higher-by a factor of 16than the abundance of O2 + . In order to determine accurate branching ratios for the N2 + + C3H4 reactions, we have also explored the low-temperature reactivity of O2 + with propyne and allene with our CRESU-SIS experimental setup-the results will be presented in detail in an upcoming publication. In this case, the detected products are C3H3 + , C3H4 + , and C2H2O + (with branching ratios (T), (T), and (T), respectively), in agreement with Decker et al.'s observations for allene at 300 K. The small contributions of the O2 + + C3H4 reactions measured while studying the N2 + + CH3CCH / CH2CCH2 reactions were eliminated by subtracting (T)O2 + and (T)O2 + to each of the C3H3 + and C3H4 + integrated peaks, respectively, with O2 + representing the decrease of the O2 + ion signal.

The apparent BRs for the three exit channels C3H2 + , C3H3 + and C3H4 + , corrected to eliminate the contributions of the O2 + + C3H4 reactions, are shown in Figure 5 The BRs of the studied reactions and their temperature dependence, obtained after correction for the O2 + + C3H4 reaction contributions and extrapolation to a null [C3H4] density, are shown in Figure 6 and summarized in Table 4. For both reactions, the BR of the main exit channel leading to C3H3 + decreases as the temperature decreases, while no significant variation is observed for the other dissociative C3H2 + exit channel. Hence, a small yet significant increase of the BR associated with the nondissociative charge transfer reaction leading to C3H2 + is observed.

Discussion

Reaction dynamics

To better understand the outcomes of the title reactions, we first invoke the reaction of Ar + with ethylene C2H4 studied in a flowing-afterglow apparatus by Tsuji et al. 41 , as Ar + is only slightly more energetic than N2 + (recombination energy of 15.76 eV for Ar + versus 15.58 eV for N2 + ). The reported branching was 0.76:0.20:0.04 for C2H2 + :C2H3 + :C2H4 + . The C2H2 + :C2H3 + ratio was found to increase further while increasing the internal energy of C2H4 + . From these measurements it was inferred that 95% of the CT excess energy is released into internal energy of the C2H4 + ion before its decomposition. The authors concluded that the reaction proceeds via two competing mechanisms:

i. A near-resonant, dissociative CT occurring at long range without significant momentum transfer. This mechanism is dominant and leads to the formation of C2H2 + and C2H3 + . It involves nonadiabatic transitions between charge states of N2 + + C2H4 and N2 + C2H4 + , which populate some of the C2H4 + excited electronic states.

ii. A nonresonant, nondissociative CT occurring via an "intimate collision" that enables significant momentum transfer. This mechanism is responsible for the C2H4 + product, which In the case of reactions of N2 + with hydrocarbons, Gichuhi and Suits have measured the product branching for the reactions of N2 + with methane CH4, acetylene C2H2, and ethylene C2H4 at 40 K using a modified velocity map imaging apparatus. 42 The reaction with acetylene was found to form only C2H2 + via nondissociative CT, while the reaction products for the reaction with ethylene led to the formation of C2H2 + and C2H3 + whereas no C2H4 + was observed, suggesting that this reaction proceeds only via the near-resonant dissociative mechanism. Interestingly, the branching of 0.76:0.24 is remarkably close to the one measured for the Ar + + C2H4 reaction.

In light of these previous results, the two CT mechanisms (dissociative and nondissociative) are likely responsible for the outcomes of the reactions studied here, although a careful mapping of the potential energy surfaces of the N2 + + C3H4 system would be necessary to fully validate this assumption. The measured rate coefficients, which are all collisional, indeed suggest that these fast reactions take place at long range via CT. The CT can deposit more than 5 eV of internal energy in the C3H4 + precursor (based on the NIST Chemistry WebBook, the CT between N2 + and propyne releases 5.22 eV; the CT between N2 + and allene releases 5.89 eV). C3H4 + can then undergo rapid internal conversion to the ground state and possibly rearrange before the cleavage of C-H bonds occurs. The potential energy diagram of C3H4 + , adapted from Mebel and Bandrauk 43 , is shown in Figure 7. The lowest-energy dissociation channel leads to cyclopropenyl c-C3H3 + + H, with an energy of 1.68 eV and a transition state (TS) at 1.87 eV above CH2CCH2 + , i.e. about one third of the total excess energy. This pathway implies the rearrangement of CH2CCH2 + and CH3CCH + into c-C3H4 + prior dissociation. The pathway leading to l-C3H3 + + H is barrierless and the exit channel lies 2.84 eV above CH2CCH2 + -about one half of the available energy. At similar energies lie the two lowest-energy H2 loss channels, namely CH3CCH + HCCCH + + H2 (2.79 eV, TS at 2.93 eV), and c-C3H4 + c-C3H2 + + H2 (2.56 eV, no TS found). The only other exit channel energetically accessible after CT with N2 + is the loss of H2 from CH2CCH2 + , (4.39 eV, no TS found) which requires more than two thirds of the excess energy to be funneled into. shown that the main isomer formed is cyclic C3H3 + , [44][45][46] corresponding to the lowest-energy dissociation channel on the C3H4 + potential energy surface. Liu et al. 47 investigated theoretically the isomerization barrier to form c-C3H3 + from linear propargylium H2CCCH + at the QCISD(T)/6-311+G(d,p)//B3LYP/6-31G(d) level of theory. They reported a direct pathway with a 3.8 eV barrier and a pathway through TSintermediate -TS with a 2.1 eV barrier, both well above the relative enthalpy of 1.1 eV. In case of an internally highly excited C3H4 + cation, both cyclic and linear C3H3 + isomers may form via unimolecular decomposition processes. The allene and propyne cations formed in electronic/vibronic excited states via CT might not necessarily dissociate following the lowest energy pathway, as e.g. pointed out in a work on the photoabsorption and photoionization of propyne 46 . The examination of unimolecular decomposition process which has guided part of our analysis so far may provide some further hints. In the absence of quantitative determination of branching ratios between isomers from photoionization, one can turn to electron-impact ionization studies. Infrared spectroscopic measurements conducted by Brünken et al. 48 showed that electron-impact ionization of allene CH2CCH2 leads to the dominant production of the cyclic C3H3 + isomer (81%) over the linear one (19%) and that the isomeric ratio is independent of the electron energy over the 14-70 eV range. Only a fraction of the energy of the colliding electron will effectively contribute to ionize and excite the molecule as most of it will be carried away with the electron itself after the encounter, since elastic electron scattering cross sections are typically orders of magnitude higher compared to the inelastic ones at such energies (see, e.g. Ref. 49 ). The ionization process will take away the amount of energy corresponding to the adiabatic ionization potential of the target, allene, which is 9.7

eV and the remaining excess energy will be deposited as internal energy for the cations.

Considering the electron energy range covered, one could expect that typically only a few of eV (see subsection 2.4 in Ref. 50 ) ends up in internal energy of the molecular target. Then, although direct photoionization and electron-impact ionization differ, one can expect a similar trend, supporting the hypothesis of a cyclic product to dominate. According to the energy diagram, this should also hold in the case of propyne.

The formation of less prominent C3H2 + is thought to occur directly from the CH3CCH + structure-note that we observed slightly more C3H2 + with propyne than with allene. The cyclic isomer of C3H2 + is also accessible, possibly without a barrier, but both these channels are not expected to contribute significantly.

The temperature dependence of the branching ratios gives further insights into these two competing CT mechanisms. While it has been shown previously that the near-resonant dissociative CT is largely independent of collision energy, 51 the nondissociative CT mechanism is not. Here, the slight but consistent increase of the yield toward C3H4 + with decreasing temperature, associated with nonresonant, nondissociative CT for both target hydrocarbons, simply reflects the greater probability to form a bound complex at low collision energy, even after the long-range CT deposited such internal energy into C3H4 + .

Since the electron transfer processes are mainly driven by nonadiabatic couplings at long range, a quantitative explanation for this mechanism cannot be provided as the treatment of such complex processes is out of the scope of the present work. However, we can expect that, at lower collision energies, the charge transfer mechanisms lead to C3H4 + cations with less internal energy and higher kinetic energy, which can be observed in the mass spectra as well. At higher temperatures, the primary C3H4 + cation is formed with more internal energy which may effectively initiate a unimolecular decomposition into smaller species.

Respectively, higher branching ratios are expected for the C3H3 + and C3H2 + products with less amount of C3H4 + in this case.

C3H3

+ in Titan's atmosphere Latest models coupling ion and neutral chemical networks 10,19 , supported by Cassini observations, have simulated the vertical density profile of many ionized and neutral organics in Titan's atmosphere. In terms of mole fraction, ionized species concentrate in the ionosphere spanning the 700-1400 km altitude range, where ionic processes play a key part in the chemistry 19 and in the formation of heavy molecules acting as aerosol embryos 52 . The generative or destructive role of ions is however not restricted to charged species and can notably include the production of neutrals, in particular through dissociative recombination. At an altitude of 1000 km, the most abundant hydrocarbons ions are C2H5 + , C3H3 + , C3H5 + followed by CH5 + , C5H5 + and C5H7 + 53 . Among them, the C3H3 + ion possesses two isomers, which cannot be observationally distinguished:

the most energetically stable aromatic cyclopropenyl cation c-C3H3 + and the linear propargyl cation l-C3H3 + . The major loss process of ionic aromatic species in Titan's atmosphere is commonly considered to be dissociative recombination. Recent photochemical models 11,48 confirm that (neutral) benzene is mainly generated in the thermosphere by ion chemistry and Loison et al. note that the large abundance of C6H6 in the upper atmosphere is not the result of efficient formation pathways, but instead is due to the fact that once generated, the aromatic ring is preserved from photodissociation in the upper atmosphere. 19 The major loss process for c-C3H3 + in Titan's atmosphere was also formally identified as dissociative recombination, leading to the formation of c-C3H2 or c-C3H 10 . In the work of McEwan et al. 55 , the authors pointed to the probable presence c-C3H2 and encouraged its search. Two years ago, the smallest cyclic hydrocarbon cyclopropenylidene c-C3H2 was effectively discovered in Titan by Nixon et al. with the help of the Atacama Large Millimeter/sub-millimeter Array (ALMA) radiotelescope, hence confirming this prediction. 56

Chemical reactivity of C3H3 +

The reactivity of the C3H3 + cation was investigated in the mid-90's in the laboratory at room temperature by employing a low pressure ICR trap ( 10 -6 mbar) 55,[START_REF] Smyth | The Ion-Molecule Chemistry of C3H3 + and the Implications for Soot Formation[END_REF] and high-pressure SIFT methods ( mbar) 55,58,59 . The linear isomer of C3H3 + appears to be significantly more reactive with hydrocarbons than the cyclic isomer of C3H3 + , due to a greater efficiency in forming a long-lived intermediate and also because the latter is more energetically stable. At high pressure or when the trapping time is long enough, this intermediate complex can be stabilized faster by collisions leading to larger rate coefficients such for the C3H3 + + CD4, C2H2 and C2H4 reactions where the ionic adduct have been observed. For instance, the reactions between l-C3H3 + and several small hydrocarbons of interest for Titan as C2H2, C2H4, C3H4 (allene and propyne) and C3H6 were measured with rate coefficients around 10 9 cm 3 molecule -1 s -1 in SIFT or ion trap experiments at room temperature, which is close to the Langevin rate constant. In summary, laboratory works evidenced the low chemical reactivity of the c-C3H3 + ion, making it a terminal species of the chemical network.

Formation pathways of C3H3 + in photochemical models

The formation route of this carbocation is not completely secured. Recent photochemical models favor the ionmolecule reactions to form C3H3 + because the radiative association of C3H + with H2 to form C3H3 + , a process which takes place in the interstellar cloud, presents a very low rate (below or around 10 -12 cm 3 molecule -1 s -1 ). These ionmolecule reactions mainly lead to the formation of the most stable cyclic c-C3H3 + . In the work of Ali et al., a combination of calculations and experiments leads to the conclusion that under low temperature and pressure conditions of Titan's atmosphere, the hydrocarbon chemistry could play a fundamental role in the synthesis of organic compounds. The CH3 + + C2H2 or C2H4 reactions generate the C3H5 + ion which is followed by monomolecular decomposition yielding the cyclopropenyl cation c-C3H3 + 60 . In others recent models, key ionmolecule reactions involve the three most abundant ions, C2H5 + , C2H4 + and CH3 + with a density of above 10 cm -3 (420, 120 and 31 cm -3 respectively at 1125 km) and neutral acetylene C2H2, one of the most abundant hydrocarbons with a peak molar fraction around 3.1×10 -4 at 1077 km. These bimolecular reactions are:

C2H5 + + C2H2 → c-C3H3 + + CH4 Eq. (4.5) C2H4 + + C2H2 → c-C3H3 + + CH3 Eq. (4.6) CH3 + + C2H2 → c-C3H3 + + H2 Eq. (4.7)
with rate coefficients (in cm 3 molecule -1 s -1 units) of 6.84×10 -11 , 6.47×10 -10 for the reactions ( 5) and ( 6) in the Vuitton et al. 10 and Dobrijevic et al. 19 Indeed, at the exception of an ICR study of the C2H4 + + C2H4 reaction for which the C3H3 + product has been observed 61 , the numerous theoretical and experimental studies all conclude that the C2H4 + + C2H4 and C2H5 + + C2H4 reactions do not form the C3H3 + product 62 . In contrast, C2H4 can react with CH3 + to preferentially produce the c-C3H3 + isomer through the monomolecular decomposition of C3H5 + but this reaction is not a preferred pathway due to its low rate coefficient (the experimental value obtained by ICR measurements is 9.2×10 -10 cm 3 molecule -1 s -1 ) and its branching ratio leading to the C3H3 + formation (5%) 62 . At last, the fourth most abundant ion C2H3 + with a density around 14 cm -3 at 1125 km 10 , reacting with C2H2 or C2H4 doesn't form C3H3 + 62 .

N2 + + C3H4: a new pathway to form c-C3H3

+ in Titan's ionosphere? Among all the detected hydrocarbon species, the two isomers of C3H4 have been included in several photochemical models 10,18,19,63 . Their total abundance is comparable to C2H2 and surpasses the one of the other considered C3 hydrocarbons. In the coupled ion-neutral photochemical model of Vuitton et al. 10 , the abundances of propyne CH3-C2H and allene CH2CCH2 peak in the upper atmosphere with very similar calculated maximum mole fractions, e.g. of 1.5×10 -5 and 1.8×10 -5 cm -3 at 1075 km. Their combined contribution (3.3×10 -5 molecule cm -3 ) falls in between the uncorrected (6.310.24×10 -6 molecule cm -3 ) and corrected (1.40.9×10 -4 molecule cm -3 ) mole fractions from INMS 15,53 . Around the same altitude, from both Dobrijevic et al. 19 and Vuitton et al. 10 photochemical models, the N2 + abundance peaks at around 50 cm -3 (around 1100 km) for the first model and at 37 cm -3 (1125 km) for the second one which is the same order of magnitude as the CH3 + density. The concentration of this primitive photolysis product cannot however be directly established by mass spectrometry because of the overlap with HCNH + considered to be 40 times more abundant and in a lesser extent with C2H4 + .

Our work combining laboratory experiments and theoretical considerations (see the energetic landscape of the Figure 7) shows that the reaction of N2 + with both C3H4 isomers generates dominantly c-C3H3 + ions over the low temperature range explored (T= 24 -71.6 K) and to a lesser extent the C3H2 + and C3H4 + cations. The nature of the C3H2 + isomer remains elusive considering our incomplete understanding of the energetic landscape.

However, given the low efficiency of the C3H2 + and C3H4 + exit channels, their inclusion in the chemical network will affect only marginally the overall chemical setting. The story is not the same for the cyclopropenyl c-C3H3 + cation. Its reaction enthalpy is lower than that of l-C3H3 + by 1.19 eV, and these isomers are protected from isomerization with a significant barrier of at least 2.17 eV. 48 In a warmer environment, such as the one characterizing Titan's atmosphere above 800 km in which the gas temperature reaches 150 K, we can reasonably assume that the branching ratio into the c-C3H3 + channel will be similar. Since the exoergicity is large, the additional energy corresponding to a temperature rise from 70 to 150 K should not allow populating much more levels. The branching ratio into C3H3 + measured at 70 K of 0.85 can be then considered as a lower limit. Based on these premises and on the reported channel-specific reaction rate (see above) the reaction of N2 + with C3H4 isomers may participate in the production of c-C3H3 + .

To evaluate the possible contribution of this pathway to the formation of c-C3H3 + we included the evaluated reaction rates within a photochemical model of Titan's atmosphere 64 that has been updated with the latest chemical network coupling ion and neutral chemistry from Vuitton et al. 10 . Our calculations suggest that the N2 + + CH3C2H/CH2CCH2 contribution has a secondary role compared to the dominant production terms from the reactions of C2H4 + and C2H5 + with acetylene (Figure 8). At the production peak near 1125 km, the cumulative contribution of the N2 + + C3H4 channels (dashed black line) is comparable to other secondary production pathways such as from reaction of CH2 + with C2H2 and CH3 + with C2H2, C2H4 and HC3N. However, at its maximum contribution the N2 + pathway is ~40x lower than the total production rate (solid black line), while its column integrated contribution is about 2% of the total column production above 600 km.

Conclusion

The rate coefficients and the branching ratios for the reactions of N2 + with propyne (CH3CCH) and allene (CH2CCH2) have been experimentally determined at 24, 36, 49 and 71.6 K using the CRESU-SIS apparatus. All measured rate coefficients are large, agreeing with the classical capture models of Langevin (for allene) and Su and Chesnavich (for propyne). Three primary products C3H2 + (m/z=38 u), C3H3 + (m/z=39 u), C3H4 + (m/z=40 u), have been observed even if their isomers are indistinguishable in our present experimental setup. High-level ab initio calculations have been performed, which allowed to qualitatively analyze in many details the possible reaction pathways. Three exothermic exit pathways have been identified for the two studied reactions. The pure charge transfer is the most exothermic exit channel, while the two others correspond to dissociative charge transfer pathways leading to the C3H3 + and C3H2 + products. These three exit channels have been observed, the main one corresponding to the C3H3 + formation. The theoretical exploration of the pure charge transfer mechanism provides an explanation of the temperature dependence of the C3H4 + ion formation. The theoretical analysis also helped in the interpretation of the differences in relative reaction probabilities.

Our results indicate that the reaction of N2 + with both isomers of C3H4 generates primarily the cyclic form of C3H3 + . The low-reactivity of this isomer makes its presence likely in Titan's atmosphere. Our evaluation on Titan's atmospheric photochemistry suggests that the contribution of this pathway to the formation of cyclic C3H3 + is however of secondary importance relative to other pathways.

This work confirms the ability of the experimental approach (mass-selective ion source to inseminate uniform supersonic flows) in providing temperature-dependent branching ratios of bimolecular ion-molecule reactions. Future experiments will focus on the reactivity of key molecular ions for astrophysical environments such as CH3 + and whose neutral counterparts are not available off the shelf. Although the reaction rates and branching ratios are available for a large selection of ion-molecule reactions, mostly at room temperature, large discrepancies between reported values persist and temperature dependence studies remain rare.

Chapter 5:

Reactivity of Ar + ions with C 2 H 4 and CH 4 at low temperatures This chapter presents the results obtained on the investigation of the reactivity of Ar + ions with methane and ethene at low temperatures using the CRESU-SIS experimental setup. In the first section we discuss the reaction of Ar + ions with ethene at 18.7 and 71.6 K. We present the kinetic experimental results, the branching ratios between the product exit channels and the abinitio calculations that were performed to explain the experimental measurements. The rate coefficients and the products distribution are also measured for the reaction of Ar + ions with methane at 18.7 and 71.6 K, this will be presented in details in the second section.

Introduction

Alongside the astrophysical importance of ion-neutral reactions, such studies give also some insight into the reaction mechanisms, particularly, the charge transfer process which has been studied in various contexts for a long time. 1 The reactivity of Ar + ions with hydrocarbons has been the object of numerous investigations including saturated (CH4, C2H6, C3H8, C4H10…)

and unsaturated species (C2H2,C2H4, c-C6H6…) mainly driven by plasma physics applications. 2 Most of the studies have been performed at room temperature. Noteworthy, some discrepancies remain between studies regarding the branching into the various exit channels.

In Titan's atmosphere, the neutral argon 40 Ar has been detected by the Ion Neutral Mass Spectrometer on the Cassini orbiter. 3,4 According to WH Ip 5 and G. J. Molina-Cuberos et al., 6 the neutral argon can be ionized in the lower ionosphere (from 1 and 400 km) by the absorption of galactic cosmic rays to form Ar + . 7,8 Farther from us, Ar + ions may play some role in the chemistry of the diffuse interstellar medium.

The Ar + ions can be produced in the ISM via the reaction:

Ar + H2 + → H2 + Ar + Eq. (

or by cosmic ray ionization Ar + CR → Ar + + e -+ CR Eq. (5.2)

and then react with H2 to form ArH + , Ar + + H2 → ArH + + H Eq. (5.3) which is detected in the diffuse interstellar medium. 12 According to Schilke et al., ArH + could be a good tracer of atomic hydrogen. To that end, it is important to understand well the chemical network of Ar + , a species which plays a key role in the production of ArH + .

Besides the astrophysical importance of this work, the main object of this chapter is to understand the chemical mechanisms that occur in ion-neutral reactions, specifically charge transfer that can be, because of its quantum nature, a temperature-dependent process.

Reactivity of Ar + ions with C 2 H 4

Previous measurements

The rate coefficient and the branching ratios of the Ar + + C2H4 were measured at room temperature using the Flowing Afterglow technique (FA) by M.

Experimental results

The CRESU-SIS experimental setup is used to measure the rate coefficient and the branching ratios for the Ar + + C2H4 reaction at 18. The Ar + ions (m/z= 40 u) are generated by the hollow cathode discharge source with a He (0.750 slm) /Ar (3 sccm) mixture. After the injection of the neutral reactant in the uniform supersonic flow, we observe four primary products for the Ar + + C2H4 reaction: C2H2 + (m/z= 26 u), C2H3 + (m/z= 27 u), C2H4 + (m/z= 28 u) and ArH + (m/z= 41 u), with C2H3 + being the major product (see figure 5.4). In the spectrum recorded in the absence of C2H4 neutral reactant (figure 5.3), we observe a very small quantity of ArH + at m/z= 41 u, which is injected in the flow from the ion source. In order to verify that the ArH + ion is also a primary product for the Ar + + C2H4 reaction, we fixed the QMS at a distance = 7 cm from the deflector, and by comparing the ion signal at m/z= 41 u before and after the injection of C2H4, we remarked an increase in the ArH + ion counts. Primary products are identified by their time evolution at low initial neutral reactant densities. The branching ratio of each primary product is determined by the intersect with the y-axis. All these pathways are exothermic, the details of the energetic study are discussed in section 5.2.3. The branching ratios of these products changes between 18.7 and 71.6 K, in the goal of understanding this variation, an ab-initio calculation was performed (section 5.2.3).

Branching ratios

The branching ratios of ionic products measured by our CRESU-SIS experimental setup at low temperatures differ from those measured at room temperature. The charge transfer product, C2H4 In order to gain more insight into the exit channels of the Ar + + C2H4 reaction, and to understand the variation of branching ratios at low temperatures, specifically the increase in the branching ratio of the CT product, our colleague Sandor Demes made some ab-initio calculations. These results are presented in the next section.

Ab-initio calculations

In order to study the collision of Ar + cation with ethylene, high-level ab initio calculations were performed. The spin unrestricted (open-shell) coupled cluster theory was used with single and double excitations and triple corrections (UCCSD(T)) along with the augmented correlationconsistent valence triple zeta (aug-cc-pVTZ) basis set. All calculations were done with the MOLPRO (version 2015.1) quantum chemistry software package. 16,17 The goal of the study is to find the enthalpies of formations for the reactants, the possible reaction products, including several intermediate complexes (ICs). From the calculated quantities it is possible to compute the corresponding reaction enthalpies. The structure and characteristics of all neutral and ionic species were found following full geometry optimization calculations. Harmonic vibrational frequency and normal mode calculations were performed to compute the zero-point vibrational energy (ZPVE) corrections.

In order to examine the quality of our ab initio calculations, in Table 5.2 we compared our theoretical ionization potentials with the experimental values listed in the NIST database. 18 As one can see there is a very good overall agreement (1-3%) between the compared data, except for the C2H• radical. It is worth to mention however that C2H + was not observed in the experiments, and the theoretical calculations show, that all the reactions which lead to the formation of this cation are endothermic. Based on the results of the performed ab initio calculations we have provided a qualitative analysis of the possible reaction pathways. The analysis of reaction enthalpies shows that the C2H4 + , C2H3 + , C2H2 + and ArH + cations should be the primary products of the Ar + + C2H4 reaction due their negative (exothermic) enthalpy of formation. The results of reaction enthalpy calculations for the exothermic products are shown in Table 5.5. First, we can see that the most exothermic channel is the pure charge-exchange process: C2H4 + Ar + → C2H4 + + Ar. As we have also shown in our previous work (chapter 4), such a reaction can only proceed via crossings between multiple electronic states. This interstate crossing has a strong quantummechanical character, which is mainly determined by the non-adiabatic couplings between the particular states. These properties make the pure charge-exchange reactions more favorable at lower temperatures. Since in this type of reactions strong intermolecular motions and rearrangements are not likely to occur, the C2H4 + Ar + → C2H4 + + Ar reaction proceeds without the formation of intermediate complexes. All other reaction products resulting from the studied reaction are characterized by hydrogen exchange process, and effectively proceeds via formation of intermediate complexes (according to capture theory) 19 . We found a highly exothermic van der Waals-type [Ar + C2H4] + intermediate complex (see Figure 5.8) with a relative enthalpy of -6.6 eV (with respect to the reactants). This complex thus could be attributed as an intermediate complex towards the formation of C2H3 + , C2H2 + and ArH + cations from C2H4 + Ar + reaction. Since the structure of this complex is not very distorted (the bond lengths and angles for C2H4 within the complex is very close to those of in the stable ethylene monomer), its formation can be very efficient without crossing reaction barriers (i.e. transition states). 5.5: Product formation channels with the corresponding enthalpies calculated for the Ar + + C2H4 reaction at 0 K. "Iso" describes the C2H2 + ion isomer where the two H-atoms are in the same side, and "lin" in the case of its repartition on the two molecule sides. From the intermediate complex the formation of the final reaction products can be effective.

Molecule

Reactants

The reaction which leads to formation of C2H2 + (lin) along with H2 can be the most favorable exit channel for the first glance, since it is the most exothermic (about -2,833 eV). However, if

we analyze its geometry we can see that the formation of this cation is probably not feasible Langevin model over a large temperature range, due to the absence of dipole moment of the neutral reactant (CH4).

Branching ratios

In our measurements for the Ar + + CH4 reaction, we detected two primary products: CH2 + (m/z= 14 u) and CH3 + (m/z= 15 u). Despite an exothermic CT pathway for this reaction (ΔH= -3.1 eV), the CH4 + ion is not detected as product. An hypothesis that could explain this observation is the presence of an energetic barrier. The branching ratios measured using the CRESU-SIS technique and comparison with those obtained at 10-20 K using FJFR are summarized in table 5.4. We don't observe an important variation in the branching ratios of the Ar + + CH4 reaction at low temperatures, since the charge transfer product is not observed here.

Ar + /CH4

Branching ratios

Conclusion

In this chapter we have presented the results of the Ar + + CH4 and Ar + + C2H4 reactions obtained at low temperatures by the CRESU-SIS experimental setup. The kinetic measurements are in good agreement with the values estimated by the Langevin model. For the branching ratios, in the case of the Ar + + C2H4 reaction, the results show that the branching into the charge transfer channel (C2H4 + ) increases at low temperature (the branching ratio reaches 22 % at 18.7 K). It is explained by quantum nature of the process, which is getting more importance at low temperatures. For the reaction with CH4, the charge transfer product is not detected. In this case the branching ratios of the products (CH2 + and CH3 + ) remain constant over the temperature explored. These results provide a strong incentive to investigate the CT mechanisms. The direct implications of these results for astrophysical environments are not presented, and need more efforts to be completed.

Chapter 6:

Kinetic study and branching ratios for the reactions of CH 2 CN + with C 2 H 4 and C 2 H 6 at 49 K

Introduction

According to NASA, Titan is one of the most earth-like worlds found by far. 1 An exo-moon baring such features attracts scientists to explore its atmosphere. The chemistry in Titan's atmosphere is one of the most complex known in the Solar System, and leads to the formation of solid organic aerosols responsible for Titan's brownish color. Nitrile ions are very abundant in Titan's ionosphere, and contribute to this chemistry. For instance, the HCNH + ion is the major species between 900 and 1200 km. In the stratosphere, a proton transfer reaction can occur between HCNH + and less abundant species such as C4H2, HC3N and CH3CN. At low altitudes, where three-body ion-molecule association reactions are favored by the lower temperatures (up to ⁓ 70 K) and higher densities (⁓ 1.5 bar), 2 the nitriles ions contribute to the formation of more complex organic-nitrogen molecules. The saturated (e.g hydrogen cyanide H-CN) and unsaturated nitriles (e.g propiolonitrile HC3N) are both detected by Voyager-1 infrared observations and by the Ion Neutral Mass Spectrometer (INMS) on the Cassini spacecraft. 3,4 These species can be formed by neutral-neutral reactions of CN radicals with unsaturated hydrocarbons. Formation of cluster ions can also take place there. For instance, HCNH + ions can associate with C2H2 to sequentially generate HCNH + •C2H2 and HCNH + •(C2H2)2 ion clusters. The latter species can be destroyed by electronic recombination, producing complex organic-nitrogen species, such as CH3CH2CN, CH2CHCN and HCCCN. 5 Furthermore, the peak at m/z=40 u is detected by the INMS. For this peak, model calculations predict a density of 2.3 cm -3 for the C2H2N + ion (including all isomers) and 0.93 cm -3 for the C3H4 + ion in the ionosphere. 4 The main formation pathway for the CH2CN + (m/z = 40 u) ion in Titan's atmosphere is the reaction between CH2 + and HCN: 6,7 CH2 + + HCN → CH2CN + + H Eq. (6.1)

In addition, CH2CN + can be formed by ionization of acetonitrile (CH3CN) through ultraviolet (UV) radiation or highly energy electron impact.

CH3CN + h𝛎 → CH2CN + + H Eq. (

CH3CN + e-→ CH2CN + + H + 2e - Eq. (

Since acetonitrile is one of the most abundant nitrogen-containing species with a fractional abundance of about 1.51×10 -6 at 1025 km above Titan's surface 8 , such a production route of CH2CN + could be efficient in this environment. In addition, CH2CN + could emerge from the ionization of the CH2CN radical that has been proposed to form in the reaction of N ( 2 D) atoms with ethene (C2H4), or N ( 4 S) with ethynyl C2H3. 9 N ( 2 D) + C2H4 → CH2CN Eq. (6.4) N ( 2 S) + C2H3 → CH2CN + H Eq. (6.5)

The CH2CN + ions could also directly be produced in Titan's atmosphere by the reaction between N + ( 1 D) or N + ( 3 P) and C2H4. 3 N + ( 1 D) + C2H4 → CH2CN + Eq. (6.6)

N + ( 1 P) + C2H4 → CH2CN + Eq. (6.7)

In addition, the reaction of acetonitrile with N2 + (one of the primary ionization products in Titan's ionosphere) preferably forms CH2CN + . This reaction was studied by Wilson K. Gichuhi and Arthur G. Suits (2012), and the results show the production of two primary ionic products: It exists four isomers for CH2CN + (figure 6.1): one cyclic c-CHCHN + (the most stable isomer), and three linear ones (CH2CN + , CH2NC + , HCCNH + ). 11 In this work, we investigated the reactivity of the CH2CN + cation with ethylene (H2C=CH2) and ethane (H3C-CH3) in the gas phase at 49 K. Regarding to the kinetic studies, the rate coefficient of CH2CN + with neutral hydrocarbons reactions has not been studied before, even at room temperature ; an additional incentive to investigate its reactivity.

Experimental results

The reaction of C2H2N + with C2H6 and C2H4 was investigated using the CRESU-SIS technique.

The detailed description of our experimental setup is presented in chapter 3. Experimentally, the ionization of acetonitrile by electronic impact (EI) can lead to the production of all C2H2N + isomers, since the EI ionization by 70 eV electrons leads to the formation of C2H2N + ions with an internal energy above the threshold of interconversion between linear and cyclic isomers. 12- 14 The cyclic isomer can be produced by using methylisonitrile and 1-H-1,2,3-triazole as precursors. By contrast, the use of iodoacetonitrile (ICH2CN) or chloroacetonitrile (ClCH2CN) as precursors preferably produces the linear isomer CH2CN + . 15 According to Gui-xia Liu et al., 16 the linear-cyclic isomerization of C3H3 + ion is endothermic with ⁓ 3.8 eV. By analogy with isoelectronic C3H3 + isomers, we can presume that the linear-cyclic isomerization of CH2CN + is highly endoergic.

In our study, the CH2CN + ions are generated by the hollow cathode discharge source in a Helium/Chloroacetonitrile mixture. In such a source, the targeted ions can be generated by CT with He + , Penning ionization with He* or through impact with high energy electrons (high energy portion of the electron energy distribution function). HCD is considered as soft ionization technique, which means that the ionization products are less energetic comparing to those produced by electron impact.

The reactivity of the CH2CN + ion with different hydrocarbons was investigated by Pantea Fathi in the group of Wolf Geppert at Stockholm University. In their study, the CH2CN + ion was produced through an electron impact of Chloroacetonitrile at 70-100 eV. 15 By using the same precursor (ClCH2CN), we can assume that the same linear isomer (CH2CN + ) is generated in our experimental work. A mass spectrum measured at the exit of the mass filter of the selective ion source is depicted in figure 6.2, it shows the presence of H2O + , H3O + , N2 + , O2 + , CHCN + , CH2CN + ions.

The uniform supersonic flow (USF) is generated by expansion of helium gas through the Ar7K de Laval nozzle. The characteristics of the flow are shown in chapter 4. The CH2CN + ions are thermalized to their ground vibrational state by numerous collisions with the helium buffer gas in the selective ion source before being injected in the supersonic uniform flow. 17 A mass spectrum in absence of neutral reactant at a distance of 12 cm (distance from the deflector to the movable mass spectrometer, that corresponds to a reaction time t of 75 μs) is shown in figure 6.3.

Reaction of CH2CN + with ethane C2H6 6.2.1.1 Reaction products and branching ratios

As an example, the figure 6.4 shows a mass spectrum recorded at the distance of 11 cm (t=68 μs) for a neutral reagent density of 9×10 Secondary reactions are highlighted in red. 14,18,19,20,21 In the case of presence of different isomers, the enthalpy of formation of the most stable isomer is considered in the calculations for the reaction enthalpies.

In order to deduce the branching ratios, it is preferable to work with low neutral reagent density, and at small reaction time. Under these conditions, the presence of secondary products is strongly reduced. That can be seen in the figure 6.5 which displays a mass spectrum recorded at t =56 μs and at a low reactant density [C2H6] = 2×10 12 molecules cm -3 . In figure 6.6, we present the fraction of primary and secondary products at different neutral densities. By comparing the height of the peaks of the secondary products in figure 6.3 and 6.4, we can clearly see that the production of secondary species is more important at high neutral reactant density. In the literature we identify that the C2H2 + + C2H6 and C2H5 + + C2H6 secondary reactions are the origin of the production of C4H7 + and C4H9 + ions 20,22 that appear with large peaks at m/z= 55 and 57 u respectively (see figure 6.3). We examined the variation of the normalized ion signals of C4H7 + , C2H2 + , C4H9 + and C2H5 + as a function of time (figure 6.7): the ion signal of i. A lower production of C2H5 + at low temperature (T=49 K). This can be interpreted by the absence of five pathways leading to the production of C2H5 + in the GIB-MS studies at ECM ⁓ 0.19 eV (T⁓ 2204 K). 14 Two of these channels are endothermic (CH2CN + + C2H6 → C2H5 + + CH2NCH, and CH2CN + + C2H6 → C2H5 + + c-NHCHCH) with ΔH= + 0.40 eV and 1.52 eV respectively. In our study at low temperature, the endothermic pathways are not allowed. This can be the reason behind the production of a major quantity of C2H5 + in the GIB-MS experiments. Another hypothesis is that in the GIB-MS experiments, the density of the neutral reactant C2H6 is sufficiently high so that to promote the production of C2H5 + ions by the following secondary reaction: C2H3 + + C2H6, that leads to the formation of C2H5 + as a major product. 23 ii. The peak at m/z= 27 u that corresponds to the C2H3 + ion increases from 2 % in the GIB-MS study to 32 % in our low temperature measurements. The same behavior is observed for the C3H5 + ion, where the branching ratio increases from 9 % at a 0.19 eV collision energy (T⁓ 2204 K) to 26 % at 49 K. The prominent decrease in the BR of C2H5 + ion leads to the increase of the branching ratios of C2H3 + and C3H5 + ionic products at 49 K.

iii. The peak at m/z= 28 u that corresponds to the HCNH + ion is not detected in the mass spectrum recorded with the GIB-MS technique at T⁓ 2204 K, in contrast, in our experiments at 49 K this peak is registered with a branching ratio equal to 28 %. The observation of an important quantity of this ion in our measurements at 49 24,25 and photochemical models predict a very high density of 4.6×10 2 molecule cm -3 . 4 The HCNH + ion is considered to be highly reactive. It can react with C2H2 and C2H4 to form the C3H4N + and C3H6N + adducts respectively. 26

Kinetic study

The measured rate coefficient of CH2CN + with C2H6 reaction at 49 K is 0.92×10 -9 molecule -1 cm 3 s -1 , that is in good agreement with the Langevin model 27 that has been used for the estimation of the rate coefficient of the ion-molecule reactions. In this case, 𝛼 = 4.25 Å 3 (referring to Kenneth J.Miller, Additivity Methods in Molecular Polarizability) 28 , 𝜇= 17.14 u leading to a Langevin value of kL=1.16×10 -9 molecule -1 cm 3 s -1 .

The measurement of the loss rate (pseudo first order k1st) corresponds to the slope of the variation of the quantity of CH2CN + ions as a function of time. The rate coefficient corresponds to the slope of the variation of k1st as a function of the density of the neutral reactant.

Reaction of CH2CN + ions with ethene C2H4

Reaction products and branching ratios

As an example, the figure 6.10 shows a mass spectrum recorded at the distance of 11 cm (t=68 μs) for a neutral reagent density of 3.14×10 13 Later species cannot be ascribed as primary products for the CH2CN + + C2H4 reaction at 49 K due to an energetic reason (endoergic pathways) and then are produced by secondary or side reactions (see figure 6.10 and table 6.3) Table 6.3: Possible products and enthalpy for each exit channels for the reaction of CH2CN + with C2H4.

Secondary reactions are highlighted in red. 18,19,20,21,15 By comparing the mass spectra presented in figures 6.10 and 6.11 we observe that the secondary products are more and more important at high neutral reagent density, so in order to deduce the branching ratios of primary products we inject a small quantity of neutral reagent. Table 6.4: Comparison between branching ratios of primary products detected by CRESU-SIS at 49 K (left) and GIB-MS at collision energy ECM ∼ 0.64 eV (T⁓7427 K) (right). In our measurements, the error bars were estimated by adding systematic (±0.02) and statistical (±5%) contributions.

By comparing the branching ratios registered by our CRESU-SIS experimental setup and those by GIB-MS, we observe:

i. The peak at m/z = 28 u is ascribed mainly to the C2H4 + ion (CT product) in the GIB-MS experiments, with an additional contribution of HCNH + ion. But the charge transfer channel is endothermic with 0.28 eV, that is not allowed in our study at low temperature.

By calculating the heat of reaction CH2CN + + C2H4 → HCNH + + C3H4 we found that this reaction is exothermic with ΔH= -0.42 eV. So, we can assume that the m/z = 28 u corresponds only to the HCNH + product in our experiments at low temperature. This can explain the decrease in the branching ratio of the m/z= 28 u product seen between the GIB-MS results at a collision energy ECM ∼ 0.64 eV (T⁓7427 K) and our CRESU measurements at 49 K.

ii. For the vinyl product (C2H3 + , m/z= 27 u), we observe an important increase in the branching ratio of this ion in our measurements at low temperature (from 9% to 39 %).

The peak at m/z= 27 u cannot be ascribed to the HCN + ion due to energetic considerations: the CH2CN ion in the GIB-MS measurements is that in this study the secondary reactions are important due to the relative high density of the neutral co-reactant (⁓ 10 13 cm -3 ): C2H3 + ions react with the neutral reactant to form C2H5 + , where the latter reacts again with C2H4 to form C4H7 + ions at m/z= 55 u. This can be confirmed by the large peaks at 29 and 55 u observed by the mass spectrum registered in the GIB-MS technique at ECM ∼ 0.64 eV (T⁓7427 K). The appearance of the C2H3 + ions in a remarkable quantity at low temperature draws attention since this ion is detected in the Titan's atmosphere by the INMS with a calculated density = 5 molecule/cm 3 . 4 These results show an additional pathway for the production of C2H3 + ion in Titan's atmosphere.

6.2.2.1: Kinetic study

The measured rate coefficient of the reaction of CH2CN + with C2H4 at 49 K is 0.85×10 -9 molecule -1 cm 3 s -1 , that is in good agreement with Langevin empirical model that equal to 1.19×10 -9 molecule -1 cm 3 s -1 . The figure 6.14 shows the variation of the CH2CN + ion signal with time at different neutral reactant densities and the corresponding loss rate coefficient for the CH2CN + + C2H4 reaction at 49 K.

Conclusion

The reactivity of CH2CN + ion with ethane C2H6 and ethylene C2H4 was investigated using CRESU-SIS method at 49 K. Experimental measurements of the rate coefficients are in good agreement with the empirical Langevin model, and this is not surprising due to the absence of a dipole moment for the neutral reactant molecules (C2H4 and C2H6). For the reaction with C2H6:

CH3 + , C2H3 + , CNH2 + , C2H5 + , C3H5 + , C2H4N + , C3H7 + ions are identified as primary products at m/z= 15, 27, 28, 29, 41, 42 and 43 u respectively. Similarly, for the reaction with C2H4, CH3 + , C2H3 + , CH2N + , C3H3 + , C3H5 + , C2H4N + , C3H4N + , C4H4N + and C4H4NH + are the primary products at m/z= 15, 27, 28, 39, 41, 42, 52, 66 and 67 u respectively. The products isomers are indistinguishable in our experimental setup. These processes are of relevance for the generation of long chain species and ionic nitriles that may be of interest for the chemistry of Titan's ionosphere. The measurements with our CRESU-SIS experimental setup show a remarkable variation of the branching ratios of primary products comparing to those registered by GIB-MS technique at a collision energies ECM ∼ 0.19 eV and 0.64 eV for the reactions with C2H4 and C2H6 respectively.

cushion" at ⁓ 1 mbar with helium gas, which avoids the nitrogen gas of the flow to back into the source, and at the same time maintains the good ion transmission.

En français:

Les ions sont omniprésents dans l'espace et contribuent à la chimie des environnements Abstract: Understanding the mechanisms governing the formation of complex organic molecules in the interstellar medium and cold planetary atmospheres relies in particular on the exploration of ionic processes under extreme conditions. The CRESU-SIS experimental setup, developed to study ion-neutral reactions in the gas phase at low temperatures, is able to meet this challenge. This unique instrument, which combines a selective ion source (SIS) with the CRESU reactor which generates a uniform supersonic flow at very low temperature, makes it possible to measure the rate coefficients of the reactions between ions and neutral molecules and to determine the branching ratios between the different exit channels. The reactivity of N2 + , Ar + , and CH2CN + ions with hydrocarbons (e.g CH4, C2H4 ...) has been studied experimentally and analyzed using quantum calculations. The reaction between N2 + and the linear isomers of C3H4 (allene and propyne) has been included into a photochemical model for the atmosphere of Titan.

Figure 1 . 1 :

 11 Figure 1.1 : cycle of matter in the universe. (Credit: Bill Saxton, NRAO/AUI/NSF/)

Figure 1 . 2 :

 12 Figure 1.2: Cross section of a circumstellar envelope, plotted in terms of the logarithm of the distance from the central star log(r), Regions of LTE (Local Thermodynamical Equilibrium) chemistry, dust formation, and UVinduced photochemistry are shown. Adapted from Lucy M. Ziurys 2006. 2

Figure 1 . 3 :

 13 Figure 1.3: Observed spectrum towards B1-bS (α2000=03 h 33 m 21.34 s , δ2000=31 o 07' 26.7") at the expected frequency of NH3D + JK=10-00 transition. The upper panel contains the raw data resulting from a total integration time of 51 hours with a spectral resolution of 48.8KHz (0.054 km s -1 ). The bottom panel shows the same data smoothed to 98 KHz. For a LSR velocity of the source of 6.5 km s -1 the derived line frequency for the observed featureis 262816.73±0.1 MHz. Adapted from J. Cernicharo et al. 2016 7

Figure 1 . 4 :

 14 Figure 1.4 : Left: The H3 + ortho-para doublet seen towards W 33A IR. Right: Two H3 + lines observed towards the Galactic Center" Quintuplet" source GCS 3-2. Adapted from T. R. Geballe 2005. 8

Figure 1 . 5 :

 15 Figure 1.5: Cassini-Huygens by the numbers, credits: NASA

Figure 1 . 6 :

 16 Figure 1.6: Correspondence of ion (upper) and neutral (lower) mass spectra measured by INMS during T19 flyby between 950 and 1000 km. Benzene molecules and their ion counterpart can be identified at 78 and 79 u in the spectra. High-pressure background in the ion source has been subtracted from the neutral spectrum. (Adapted from Waite et al., 2007) 11

  Photochemical processes

Figure 1 .

 1 7 illustrates a model of the production ion rate by photoionization in the atmosphere of Mars from 80 to 300 km.

Figure 1 . 7 :

 17 Figure 1.7 : Computed photoionization rate profiles from 80 to 300 km for 7 ions for a high solar activity 60° SZA (Solar Zenith Angle) model of Mars. 21

Figure 1 . 8 :

 18 Figure 1.8 : Ionization due to EUV/XUV solar flux in the atmosphere of Titan computed for a solar zenith angle of 40 • with a solar flux given by F10.7=80. From G. Gronoff et al. 2009 19

Figure 1 . 9 :

 19 Figure 1.9 :The scheme of the ionization chemistry for water vapor the H2O-dominated atmosphere of icy moons: The primary photolysis and electron impact processes are shown by the solid lines; the ionmolecular reactions and the reactions of the dissociative recombination are shown by dashed and dashand-dot lines, respectively. From Shematovich 2008.36 In Titan's atmosphere, where the UV and X-rays are the dominant ionization sources, it exists some contribution of the magnetospheric charged particles in the 500 km to 1000 km altitude range (according to the model calculations of Cravens et al 2007).37

[

  C4H -]/[C6H -] = 1/6, is more than two orders of magnitude smaller than that between their neutral counterparts, [C4H]/[C6H] = 45. After the detection of C6H -and C4H -, C8H -, 40 C3N -, 41 C5N -42

Figure 1 . 10 :

 110 Figure 1.10 : Two rotational transitions of C6H -anion in the laboratory and in TMC-1. Frequencies are relative to the laboratory rest frequencies, assuming the standard mean radial velocity of 5.80 km s -1 for TMC-1. From M. C. McCarthy et al. 2006.38

Figure 1 . 11 : 4 1. 4 . 1

 111441 Figure 1.11 : Negative ions measured in each CAPS-ELS energy bin at an altitude of 1015 km during the T40 encounter. The attribution of the peaks is based on the ionospheric chemistry model. From Vuitton et al. 2009 46

Figure 2 . 1 :

 21 Figure 2.1: Variation of Veff with the relative distance between the ion and neutral (r) 3

Figure 2 . 2 :

 22 Figure 2.2: Locking dipole constant as a function of (μ/√𝛼). Adapted from Su et al. 1978. 6

Figure 2 . 3 :

 23 Figure 2.3: flowing afterglow apparatus at the University of Rennes.

Figure 2 . 4 :

 24 Figure 2.4: Left: examples of ln([CN] -/[CN - 0] according to the mass spectrometer position z and for different [HC3N] densities up to 2×10 12 cm -3 . Right: Graph of -vkfirst as a function of [HC3N]. The slope is k= 4.8×10 -9 cm 3 /s, considering the measured constant flow velocity of 1.7×10 4 cm/s. The data points are the results of many repetitive runs performed with different flow meters.12

  work at different temperatures (down to 80 K), the SIFT technique has been extended into a new version: The Variable Temperature Selected Ion Flow Tube (VT-SIFT).16

Figure 2 . 5 :

 25 Figure 2.5: A schematic diagram of a typical SIFT apparatus illustrating its major features. From D. Smith and N.

Figure 2 . 6 :

 26 Figure 2.6: Left: Schematic view of the VT-SIFT instrument. Right: The count rate of O -ions (black squares, counts-per-second, c/s) measured at different concentrations of the CO2 reactant gas and for a constant flow of the helium carrier gas in the flow tube cooled down to 230 K. The exponential decay fit (the mean square roots method) is shown by the red line. Each point represents a value averaged over 20 measurements of c/s, standard measurement deviations is given with error bars. From Zymak et al. 2021.17

  By addition of a static but variable E-field along the axis of the flow tube reactor in which the ion-molecule reaction takes place, Spesyvyi et al. (2015) 18 updated the SIFT technique into SIFDT (Selected Ion Flow Drift Tube). Its general structure is similar to the familiar SIFT-MS instruments.

Figure 2 . 7 :

 27 Figure 2.7: Schematic drawing of the selected ion flow-drift tube, SIFDT, apparatus. From A. Spesyvyi et al.18

Figure 2 . 8 :

 28 Figure 2.8: schematic representation of guided ion beam mass spectrometer.22

Figure 2 . 9 :

 29 Figure 2.9: Lower part: Cross section of a CAD-model of a 22-pole trap including cylindrical end electrodes.

Figure 2 . 10 :

 210 Figure 2.10: Exemplary ion counts of OH + and H2O + as a function of storage time and loss rates versus H2 density. (Left) Decay curves of OH + and H2O + as observed in the 22-pole trap at 21 K with a H2 number density of 3.2 × 10 10 cm -3 . The inset shows a spectrum of a typical initial mass distribution produced by the ion source. (Right) Loss

Figure 2 . 11 :

 211 Figure 2.11 : Schematic views of free jet and molecular beam supersonic expansions.30

Figure 2 . 12 :

 212 Figure 2.12: Schematic views of the crossed molecular beams technique. Adapted from chemistry LibreTexts site.31

  about 10 12 molecule cm -3 . Two dimensionless numbers are important for the characterization of the uniform supersonic flow: the Reynolds number, Re, and the Knudsen number Kn. The Reynolds number Re characterizes the nature of the flow regime (laminar, transient, turbulent). It represents the relation between inertial forces and viscous forces. Viscous forces here are the actions of nozzle wall on the flow, and inertial forces are acquired by pressure differences of the fluid, converted into kinetic energy. Viscous forces are the origin of the appearance of the zone that surrounds the isentropic core of the flow, known as the boundary layers δ (see figure 3.1). The Re number is defined by: Re= 𝜌𝑢𝐷 𝜂 Eq. (3. 1) Where u is the velocity of the flow, D the characteristic length corresponding to the diameter of the exit of the nozzle, ρ is the density of the flow and 𝜂 the dynamic viscosity. The thickness of the boundary layer δ is directly related to Re by the following equation: Reynolds number Re is a good indicator of the flow quality. If the Reynolds number is sufficiently high, the dissipating phenomena are negligible, and the boundary layer δ is smaller. The Kundsen number is used to determine the flow regime in terms of flow continuity (continuous, rarefied). It is defined as: Kn = λ L Eq. (3. 3)Where λ represents the mean free path of particles in the flow (μm). Otherwise, the Knudsen number can be expressed as a function of the Mach number, M, the Reynolds number Re, and of the adiabatic index of the γ gas by the von Kármán relation:

Figure 3 . 1 :

 31 Figure 3.1: Main characteristics of the CRESU expansion.3

Figure 3 .

 3 2 shows 2D profile and 1D characteristics of the He36 K Laval nozzle obtained using the Pitot tube. In terms of supersonic flow quality, this nozzle is one of the best available in the laboratory thanks to the size of the isentropic core (≈ 25 cm length and almost ≈ 2 cm diameter) and more than 195 μs of hydrodynamic time.

Figure 3 . 2 :

 32 Figure 3.2: 2D profile and 1D characteristics of the He36 K Laval nozzle obtained using the Pitot tube.

Figure 3 . 3 :

 33 Figure 3.3: CFD simulations of the helium flow generated by a He36 K nozzle. Mach number map a) Pflow<Pchamber, b) Pflow>Pchamber, c) Pflow=Pchamber (Preservoir = 50.31 mbar, Pchamber=0.292 mbar, Treservoir= 293 K, Tchamber= 36 K, Qhelium= 88.8 slm.

  Another version of the pulsed CRESU was also developed in 2009 by Sébastien Morales and Bertrand Rowe 8 in the Astrophysics Laboratory group of Rennes 1 University, and later transferred to the department of Physical Chemistry of the University of Castilla-La Mancha (UCLM) in Ciudad Real in the group of E. Jiménez et al 9 . The apparatus was based on an aerodynamic chopper for gas phase reaction kinetic studies at ultra-low temperatures. This technique saves a factor of ∼ 20 on the pumping capacity necessary to generate continuous uniform supersonic flows and thus to achieve lower temperatures and pressures, and reduces the requirement of gas consumption.

Figure 3 . 4 :

 34 Figure 3.4: Spatial profiles of the gas temperature (A) and gas density (B) in the uniform supersonic flow in the pulsed and continuous modes. The averaged pressure in the jet was also obtained from Pitot measurements and was 0.54±0.08) millibars for the pulsed mode. From E. Jiménez et al. 2015 9

Figure 3 . 5 :

 35 Figure 3.5: The CRESU reaction chamber incorporating the new mass-selective ion transfer line. A movable quadrupole mass spectrometer equipped with a skimmer/ hexapole ion guide combination is used for monitoring ions in the de Laval expansion. High-capacity root pumps (around 60000 m 3 /h in helium) maintain the desired pressure (in order to 10 -1 mbar) in the reaction chamber. The numbers in brackets are the pressure orders of magnitude in mbar unit.

Table 3 . 1 :

 31 Typical conditions for the operation of the plasma source. For each precursor, the target ion is indicated in brackets.

Figure 3 . 6 :

 36 Figure 3.6: 3D CAD model of the ion optical line. The main components of the system are indicated. Pressureregions are also highlighted.10

Figure 3 . 7 :

 37 Figure 3.7: Ion source showing the plasma generated by the hollow cathode discharge in a He/O2 (left) and Ar/O2 (right) mixtures.

Figure 3 . 8 :

 38 Figure 3.8: Schematic diagram of a quadrupole mass spectrometer. 11

Figure 3 .

 3 Figure 3.9 shows two mass spectra (recorded without m/q selection) registered at the level of the mass filter, in the case of creation of cations (positive mode) and creation of O -and O2 -anions (negative mode).

Figure 3 . 9 :

 39 Figure 3.9: Mass spectra registered at the level of the mass filter of the Selective ion source working in positive mode (left) and negative mode (right). In positive mode, the plasma is generated by the HCD in a He/N2 mixture, with QHe= 0.750 slm, and QN2= 5 sccm. For negative mode, the plasma is generated by the HCD in a He/O2 mixture, with QHe= 0.750 slm, and QO2= 3 sccm.

Figure 3 . 10 :

 310 Figure 3.10: Simulated trajectories of C2H5 + ions in the injection region (last three segments of ion guide 2/exit lens) superimposed on velocity contours of 50 m/s for He (red) and N2 (orange) buffer gases with maximum axial and radial speeds of 620 m/s and 360 m/s, respectively.

Figure 3 . 11 :

 311 Figure 3.11: A schematic diagram of the different parts used to assemble the skimmer, the hexapole ion guide, and the quadrupole mass spectrometer 14

Figure 3 . 12 :

 312 Figure 3.12: dimensions of Magnum 5901 CEM used to detect ions.

Figure 3 . 13 :

 313 Figure 3.13: Mass spectrum registered in the flow (T= 24 K) after the selection of O2 + ions (m/z= 32 u) by the selective ion source.

Figure 3 . 14 :

 314 Figure 3.14: Map of controllers used to inject gas in the source and the reservoir of the CRESU chamber.15

Figure 3 . 15 : 4 ×

 3154 Figure 3.15: Velocity simulation showing the injection of Ar + ions in the CRESU helium flow (T= 50 K, n= 10.4 × 10 16 molecule.cm -3 , isentropic core = 16 mm).

Figure 3 . 16 :

 316 Figure 3.16: Temperature distribution of a group of Ar+ ions injected in the uniform supersonic flow at time between 80 μs and 100 μs.

Figure 3 . 17 :

 317 Figure 3.17: velocity distribution of the Ar + ions injected in the flow at a distance 7 < x < 8.

Figure 3 . 18 :

 318 Figure 3.18: Image of the CRESU-SIS apparatus used in Rennes.

Figure 4 . 1 :

 41 Figure 4.1: Mass spectra measured in a 49 K uniform supersonic flow inseminated by massselected N2 + cations in presence of CH3CCH (A) and CH2CCH2 (B) neutral reactants at a density of 1.4 ×10 12 molecule cm -3 . The ions are detected at a distance of 8 cm from the injection lens of the SIS, corresponding to a reaction time of 50 µs. The inset shows a closeup of the products in log scale.

  v. Two SIFT experiments have been performed at room temperature to examine the O2 + + C3H4 (propyne and allene) reactions. Wilson et al. only observed the charge transfer reaction, i.e. C3H4 + + O2, with rate coefficients of k = 1.6 and 1.3×10 -9 cm 3 molecule -1 s -1 for

Figure 4 . 2 :

 42 Figure 4.2: Schematic diagram of the N2 + + CH3CCH and N2 + + CH2CCH2 reaction enthalpies with the experimentally observed (or potential) formation channels of C3H4-x +

Figure 4 . 3 :

 43 Figure 4.3: N2 + ion signal as a function of time for a range of neutral CH2CCH2 densities (left) and corresponding loss rates as a function of CH2CCH2 densities (right) at 24 K.

Figure 4 . 4 :

 44 Figure 4.4: Comparison between the measured absolute rate coefficients k as a function of temperature for the N2 + with CH3CCH (red filled circles) and CH2CCH2 (blue filled circles)

  as a function of[C3H4] density in the case of propyne at 49 K. The values vary slightly and linearly with the [C3H4] density, due to secondary reactions of the primary ion products with C3H4. To limit the contribution of those reactions, the BRs were determined at short reaction times and by extrapolating their values for a null [C3H4] density = 0 (y-intercept of the linear fit).

Figure 4 . 5 :

 45 Figure 4.5: Evolution of the branching ratios (BRs) with the [CH3CCH] density, measured at 49 K for a reaction time of 56 μs. The BRs are corrected to eliminate the contribution of the O2 + + C3H4 reaction.

Figure 4 . 6 :

 46 Figure 4.6: Temperature dependence of the branching ratios between the primary products of N2 + + CH3CCH (left) and N2 + + CH2CCH2 (right) reactions. The error bars were estimated by adding systematic (±0.02) and statistical (±5%) contributions.

Figure 4 . 7 :

 47 Figure 4.7: Potential energy diagram of the ground state of C3H4 + , adapted from Mebel andBandrauk43 . Energies were calculated at the CCSD(T)/cc-pVTZ//B3LYP/6-311G ** level of theory and are shown relative to the global minimum, the allene cation CH2CCH2 + . All the dissociation channels accessible for the ion precursor after charge transfer with N2 + , which can deposit up to 5.89 eV into C3H4 + , are reported. Note that no transition states were found for the H2 loss channels from CH2CCH2 + and c-C3H4 + -dashed lines connect these stationary points.

  models and 2.88×10-10 for the reaction(7) in the Vuitton et al. work and 1.15×10-9 for the global reaction CH3 + + C2H2 in the Dobrijevic et al. study. Despite the same molar fraction as C2H2, ethylene C2H4 (for which the INMS detection cannot be easily distinguished from that of C2H215 ) is not considered as a possible source of c-C3H3 + through the C2H5 + , C2H4 + , CH3 + + C2H4 reactions.

Figure 4 . 8 :

 48 Figure 4.8: Contribution of different chemical pathways to the production of c-C3H3 + in Titan's upper atmosphere. The indicated values represent the percentage contributions of each reaction to the total column production rate above 600 km.

Figure 5 . 2 :

 52 Figure 5.2: Synthetic spectrum of Titan in the mid-IR at CIRS maximum spectral resolution (0.5 cm) showing the vast array of emission features from trace species in the stratosphere. 11

Figure 5 . 3 :

 53 Figure 5.3: Mass spectrum recorded in the uniform supersonic flow in the absence of neutral reactant and at the temperature T= 72 K, the QMS is positioned at 7 cm from the injection zone (t⁓ 50 μs).

Figure 5 . 5 :

 55 Figure 5.5: Ar + ion signal as a function of time for a range of neutral C2H4 densities and corresponding loss rates as a function of C2H4 densities at 18.7 K (left) and 71.6 K (right).

Figure 5 . 6 :

 56 Figure 5.6: Values of the rate coefficient as a function of the temperature for the Ar + + C2H4 reaction measured by CRESU-SIS experimental setup and comparison with previous measurements and Langevin model.

Figure 5 . 7 :

 57 Figure 5.7: Branching ratios of the Ar + + C2H4 reaction measured at 18 and 72 K by CRESU-SIS technique and comparison with the measurements at room temperature by Tsuji et al. (full symbols) and J.L Franklin et al. (open symbols)

Figure 5 . 8 :

 58 Figure 5.8: The structure of the van der Waals-type [Ar + C2H4] + intermediate complex in the Ar + + C2H4 reaction.

Figure 5 .

 5 10 shows the rate coefficient of the Ar + + CH4 reaction over the 10-20 K temperature range between (obtained by FJFR technique) and at 700 K (using the DT technique by Chatham et al. 24 ). For the value at 300 K, it exists different measurements, we have chosen the most recent value measured by M. Tsuji et al. in 1993 employing the FA technique.

Figure 5 . 9 :

 59 Figure 5.9: Loss rate of the Ar + /CH4 reaction as a function of the density of CH4, T= 71.6 K.

Figure 5 . 10 :

 510 Figure 5.10: Rate coefficient of the Ar + /CH4 reaction obtained using the CRESU-SIS technique at 18.7 and 71.6 K and comparison with previous measurements.

  7 and 71.6 K by CRESU-SIS technique and comparison with the measurements of Hawley et al. (1992) at 10-20 K. 21 The error bars were estimated by adding systematic (±0.02) and statistical (±5%) contributions.

Figure 5 . 11 :

 511 Figure 5.11: Mass spectrum registered in the uniform supersonic flow in the presence of neutral reactant. T= 71.6 K, [CH4] = 5.6 × 10 12 molecule/cm 3 , reaction time ⁓ 65 μs.

Figure 6 . 1 :

 61 Figure 6.1: Isomers of CH2CN + ion 11

Figure 6 . 2 :

 62 Figure 6.2: Mass spectrum recorded at the exit of the mass filter in the selective ion source. The plasma is generated by HCD of He/ClCH2CN mixture.

Figure 6 . 3 :

 63 Figure 6.3: Mass spectrum of ionic species from the reaction of mass-selected C2H2N + recorded by CRESU-SIS at the distance of 12 cm in the absence of neutral reactant, T= 49 K.

  ion (m/z= 27 u) is endothermic with ΔH= + 3.76 eV, this eliminates the possibility of the production of HCN + via CH2CN + + C2H6 reaction in our study at low temperature. The peak at m/z= 28 u cannot be ascribed to C2H4 + ion due to the same reason. The mass spectra recorded in the absence of neutral co-reactant show traces of CHCN + ion (m/z= 39 u), the corresponding ion signal doesn't change after the injection of C2H6, so we can assume that no primary or secondary products for the CH2CN + + C2H6 reaction are observed at m/z= 39 u.

Figure 6 . 4 :

 64 Figure 6.4: Mass spectra of ionic species from the reaction of mass-selected C2H2N + with C2H6 recorded by CRESU-SIS at t= 68 μs with [C2H6] = 9×10 12 molecule.cm -3 . T= 49 K.

Figure 6 . 5 :

 65 Figure 6.5: MS spectra of ionic species from the reaction of mass-selected C2H2N + with C2H6 recorded by CRESU-SIS at t= 56 μs and [C2H6] = 2×10 12 molecule cm -3 . T= 49 K. Only the primary products are indicated on the graph.

Figure 6 . 6 :

 66 Figure 6.6: Sum of the ion signals of primary and secondary products at different neutral densities. Dashed lines correspond to linear fits.

Figure 6 . 9 :

 69 Figure 6.9: CH2CN + ion signal at 49 K as a function of time for a range of C2H6 concentration (left) and corresponding loss rates as a function of C2H6 density (right), the slope of which determines the rate coefficient of the CH2CN + + C2H6 reaction at 49 K.

  molecule.cm -3 . The reaction between CH2CN + and C2H4 produces CH3 + (m/z= 15 u), C2H3 + (m/z= 27 u), CH2N + (m/z= 28 u), C3H3 + (m/z= 39 u), C3H5 + (m/z= 41 u), C2H4N + (m/z= 42 u), C3H4N + (m/z= 52 u), C4H4N + (m/z= 66 u), and C4H4NH + (m/z= 67 u) as primary products, and C2H2 + , C2H5 + , C4H5 + , C4H7 + , C3H6N + , C4H9 + , C5H5 + , C5H9 + as secondary products at m/z= 26, 29, 53, 55, 56, 57, 65 and 69 u respectively.

Figure 6 . 10 :

 610 Figure 6.10: Mass spectrum of ionic species from the reaction of mass-selected C2H2N + with C2H4 recorded by CRESU-SIS at t =68 μs, [C2H4] = 31.4×10 12 molecule/cm 3 . T= 49 K.

Figure 6 . 11 :

 611 Figure 6.11: MS spectra of ionic species from the reaction of mass-selected C2H2N + with C2H4 recorded by CRESU-SIS at t= 68 μs (distance of 11 cm), T= 49 K, and [C2H4] = 4×10 12 molecule.cm -3 Only the primary products are labelled in the figure.

Figure 6 .

 6 Figure 6.12 presents the variation of primary and secondary products with the C2H4 density, the graph shows that the primary products react once formed with C2H4 to generate secondary species.

Figure 6 . 12 :

 612 Figure 6.12: Sum of the ion signals of primary and secondary products at different C2H4 densities. T= 49 K, reaction time t= 68 μs.

Figure 6 . 13 :

 613 Figure 6.13:The product ion mass spectrum registered by GIB-MS associated with the C2H2N + /C2H4 reaction at P(C2H4) = 1.1 ×10 -5 mbar and measured at the collision energy ECM ∼ 0 .64 eV in the center of mass frame.15

Figure 6 . 14 :

 614 Figure 6.14: CH2CN + ion signal at 49 K as a function of time for a range of ethylene densities (left) and corresponding loss rates as a function of C2H4 density (right), the slope of which determines the rate coefficient of the CH2CN + + C2H4 reaction.

  astrophysiques. Ces ions sont détectés à distance particulièrement à l'aide de radiotélescopes, ou in situ par des sondes spatiales qui visent à explorer les planètes et leurs lunes dans le système solaire (par exemple la mission Cassini-Huygens). Parmi tous les processus ioniques (tels que la photoionisation, l'attachement des électrons…), les réactions ion neutres jouent un rôle crucial dans la formation et la croissance des espèces ioniques. L'astrophysique de laboratoire pour les réactions élémentaires cherche à fournir deux types d'informations : les mesures des coefficients de vitesse k et la détermination de la distribution des produits (les rapports de branchement entre les différentes voies de sortie).La méthode standard pour l'étude des réactions ion neutres (également des réactions neutresneutres) à basse température est basée sur le refroidissement par collision cryogénique d'un gaz. Cette méthode est contrainte par la condensation des réactifs neutres sur les parois du système, qui limite la fiabilité des résultats, et la plage de température. La technique CRESU génère un flux supersonique uniforme froid (jusqu'à 6 K) offrant l'avantage d'une expérience sans paroi où aucune condensation ne se produit dans le milieu scruté. Le deuxième avantage est le LTE dû à la densité élevée qui permet de considérer une température unique déterminée pour la réaction. En utilisant notre technique CRESU-SIS, qui couple une source d'ions sélective en masse à une chambre CRESU, nous avons étudié la réactivité des ions N2 + avec les isomères C3H4 (propyne et allène) à basse température (de 71,6 à 24 K). Ces espèces sont détectées dans l'atmosphère de Titan. En collaboration avec Sandor Demes et François Lique, un calcul ab-initio a été réalisé afin de comprendre la variation des taux de branchement des produits. L'ion c-C3H3 + du cation cyclopropényle aromatique a été observé comme produit principal. Une augmentation du rapport de ramification du C3H4 + a été observée à basse température, cela peut s'expliquer par la nature quantique du mécanisme de transfert de charge pur (CT), qui procède par croisements entre plusieurs états électroniques puisqu'il n'est possible que par une dynamique non adiabatique, et il est plus important à basse température. Les coefficients de taux mesurés montrent un bon accord global avec les modèles empiriques. Pour la réaction avec le propyne, Titre : Réactions ion-molécule à basse température avec des écoulements supersoniques uniformes et investigations sur la chimie des environnements astrophysiques Mots clés : CRESU, astrophysique de laboratoire, collisions réactives, processus ioniques, chimie du milieu interstellaire, atmosphère de Titan Résumé : La compréhension des mécanismes régissant la formation des molécules organiques complexes dans le milieu interstellaire et les atmosphères planétaires froides repose notamment sur l'exploration de processus ioniques en conditions extrêmes. Le dispositif expérimental CRESU-SIS, développé pour étudier les réactions ion-neutres en phase gazeuse à basse températures, est en mesure de répondre à ce défi. Cet instrument unique, qui combine une source d'ions sélective (SIS) au réacteur CRESU qui génère un écoulement supersonique uniforme de très basse température, permet de mesurer les coefficients de vitesse de réactions entre des ions et des molécules neutres et de déterminer les rapports de branchement entre les différentes voies de sortie. La réactivité des ions N2 + , Ar + , et CH2CN + avec des hydrocarbures (e.g CH4, C2H4…) a été étudiée expérimentalement et analysée à l'aide de calculs quantiques. La réaction entre N2 + et les isomères linéaires de C3H4 (allene et propyne) a été incluse dans un modèle photochimique pour l'atmosphère de Titan. Title: Ion-molecule reactions at low temperature with uniform supersonic flows and insights into the chemistry of astrophysical environments Keywords: CRESU, laboratory astrophysics, reactive collisions, ionic processes, chemistry of the interstellar medium, Titan's atmosphere

Table 1 . 1 :

 11 Molecules in the Interstellar Medium or Circumstellar Shells (source: https://cdms.astro.uni-koeln.de/)

	2 atoms 3 atoms 4 atoms	5 atoms	6 atoms	7 atoms
	H2	C3*	c-C3H	C5*	C5H	C6H
	AlF	C2H	l-C3H	C4H	l-H2C4	CH2CHCN
	AlCl	C2O	C3N	C4Si	C2H4*	CH3C2H
	C2**	C2S	C3O	l-C3H2	CH3CN	HC5N
	CH	CH2	C3S	c-C3H2	CH3NC	CH3CHO
	CH +	HCN	C2H2*	H2CCN	CH3OH	CH3NH2
	CN	HCO	NH3	CH4*	CH3SH	c-C2H4O
	CO	HCO +	HCCN	HC3N	HC3NH +	H2CCHOH
	CO +	HCS +	HCNH +	HCCNC	HCCCHO	C6H -
	CP	HOC +	HNCO	HCOOH	NH2CHO	CH3NCO
	SiC	H2O	HNCS	H2CNH	C5N	HC5O
	HCl	H2S	HOCO +	H2C2O	l-HC4H*	HOCH2CN
	KCl	HNC	H2CO	H2NCN	l-HC4N	HCCCHNH (2020)
	NH	HNO	H2CN	HNC3	c-H2C3O	HC4NC (2020)
	NO	MgCN	H2CS	SiH4*	H2CCNH (?)	c-C3HCCH (2021)
	NS	MgNC	H3O +	H2COH +	C5N -	l-H2C5 (2021)
	NaCl	N2H +	c-SiC3	C4H -	HNCHCN	MgC5N (2021)
	OH	N2O	CH3*	HC(O)CN	SiH3CN
	PN	NaCN	C3N -	HNCNH	C5S
	SO	OCS	PH3	CH3O	MgC4H
	SO +	SO2	HCNO	NH4 +	CH3CO + (2021)
	SiN	c-SiC2	HOCN	H2NCO +	C3H3 (2021)
	SiO	CO2*	HSCN	NCCNH +	H2C3S (2021)
	SiS	NH2	H2O2	CH3Cl	HCCCHS (2021)
	CS	H3 +(*)	C3H +	MgC3N
	HF	SiCN	HMgNC	NH2OH (2020)
	HD	AlNC	HCCO	HC3O + (2020)
	FeO?	SiNC	CNCN	HC3S + (2021)

Table 1 . 2 :

 12 Molecules in the Interstellar Medium or Circumstellar Shells (source: https://cdms.astro.uni-koeln.de/)

Table 1 . 3 :

 13 Non-exhaustive list of the main chemical processes of ions interest in the astrophysical environments.

Table 2 .

 2

1: Comparison of the optimal operating parameters for the SIFDT-MS instrument and the SIFT-MS instrument. From Anatolii Spesyvyi et al.

18

Table 4 . 1 :

 41 Characteristics of the supersonic uniform flows determined from impact pressure measurements.

	Nozzle	Buffer gas	Tjet (K)	Mach	τ	n (10 16 molecule.cm -3 ) (µs)	Pchamber (mbar)	flow rate (l/min)
	He6K	He	24.1	5.84	196	18.3	0.630	93.4
	He36K	He	36.1	4.63	195	5.28	0.293	84.7
	Ar7K	He	49.1	3.87	150	10.4	0.770	67.0
	Ar7K	He/N2 (1:1) 57.0	4.08	158	6.91	0.550	20:20
	Ar50K0.3	He	71.6	3.08	163	6.01	0.640	97.7

Table 2 :

 2 Absolute rate coefficients k of the N2 + + CH3CCH and N2 + + CH2CCH2 reactions measured with the CRESU-SIS device at different temperatures in ×10 -9 cm 3 molecule -1 s -1 unit. The uncertainties take into account statistical and systematic (10%) contributions. CH3CCH = 5.55 Å 3 )40 , we determined for all the temperatures a value of the x parameter greater than 2 for the studied N2 + + CH3CCH reaction. Then the estimated values of the rate coefficients from the Su and Chesnavich model are k SC =3.49, 3.01, 2.70, and 2.38×10 -9 cm 3 molecule -1 s -1 at 24, 36, 49, and 71.6 K respectively. Figure4, which displays the experimental values obtained in this work and the estimated values obtained from the simple capture models of Langevin and Su and Chesnavich, illustrates their remarkable agreement.

	T (K)	24.1	36.1	49.1	57.0	71.6
	N2 + + CH3CCH 3.73 ± 0.41	3.4 ± 0.36 2.52 ± 0.34 2.39 ± 0.25 2.13 ± 0.21
	N2 + + CH2CCH2 1.52 ±0.16		1.40 ±0.2		1.56 ±0.22
	For N2 + + CH2CCH2, the rate coefficients show a quasi-independence with temperature. The
	measured values are in good agreement with the Langevin value of kL=1.38×10 -9 cm 3
	molecule -1 s -1 estimated from kL=2πe√α μ ⁄ , where e is the elementary charge, α the static
	dipole polarizability of the neutral molecule (α CH2CCH2 = 5.69 Å 3 from Ref. 37 ), and μ the
	reduced mass of ion-molecule system. For N2 + + CH3CCH, the Langevin model is not
	suitable since the permanent dipole moment µD of the neutral reactant is not considered. For
	a polar neutral reactant, the Su and Chesnavich model 38 gives a rate coefficient k SC =
	K µD √2αTk B	≥ 2. Considering the dipole moment of propyne (µD=0.784 D) 39 and its polarizability

cap × k L which is temperature dependent with K cap = 0.4767x + 0.6200 when x =

Table 4 . 4

 44

: Branching ratios of the reactions of N2 + with the two C3H4 isomers at 24.1, 36.1, 49.0 and 71.6 K. These values take into account the natural isotopic abundance of carbon 13.

  Tsuji et al. in 1992, 13 where C2H2 + , C2H3 + and C2H4 + were identified as primary ionic products. All these reactions pathways are exoergic. The authors of this study suggested the presence of two CT (Charge Transfer) mechanisms in the Ar + /C2H4 reaction. One is a dominant near-resonant dissociative CT leading to C2H3 + and C2H2 + , and the other is a minor non-resonant, nondissociative CT leading to the appearance of C2H4 + . Another study was performed earlier at room temperature by J. L.Franklin et al. (1961)14 using the MS technique, with the detection of C2H2 + and C2H3 + ions as products. They suggested in that work that the formation of C2H4 + by charge exchange with any of the rare gas ions is quite exothermic, and all of ethylene ions (C2H4 + ) break down into smaller fragments. These previous results are summarized in table 5.1. To our knowledge the Ar + /C2H4 reaction was not investigated at low temperatures below 300 K.

	Ar + + C2H4 study	Rate coefficient (cm 3 s -1 )	Ionic products
			C2H4 + (0.04)
	Masaharu Tsuji et al. (1992)	1.10 ×10 -9 ± 20	C2H3 + (0.76)
			C2H2 + (0.20)
			C2H3 + (0.87)
	J. L. Franklin et al. (1961)	1.10 × 10 -9 ± 10
			C2H2 + (0.13)

Table 5 . 1 :

 51 Rate coefficients and branching ratios for the Ar + /C2H4 reaction obtained by the studies of FA and MS techniques at room temperature.

n (10 16 molecule.cm -3 ) Pchamber

  7 and 71.6 K. This technique is described in details in chapter 3, and only the results are presented in this chapter. The characteristics of the Ar50K0.3 Laval nozzle that generates the flow at 71.6 K are shown in chapter 4, table 4.1, and those for the He20K nozzle are presented in Table 5.2.

	Nozzle	buffer	Tjet	Mach	τ			flow rate
		gas	(K)		(µs)		(mbar)	(l/min)
	He20K	He	18.7	6.6	120	3 .7	0.11	45.7

Table 5 . 2 :

 52 Characteristics of the supersonic uniform flow generated by the He20K nozzle determined from impact pressure measurements.

Table 5 . 3 :

 53 + , increases to reach 22 % at 18.7 K, while this product doesn't appear in the measurements of J.L Franklin et al. (1962), and only in small quantity (4%) in the measurements ofTsuji et al. (1992) at room temperature. In previous measurements, the authors suggested that the dissociative CT product dissociates into smaller fragments. These mechanisms are endothermic, according to B. Joalland et al. with few eV.15 At low temperature, the dissociation of C2H4 + is not allowed. This can be the reason of the detection of an important quantity of C2H4 + ions at low temperatures in our experiments. In the other hand, we don't observe a variation in the branching ratios of C2H2 + and ArH + ions. The branching ratios of primary products detected in our study at low temperatures are summarized in table 5.3. Branching ratios for the products of the Ar + + C2H4 reaction recorded at 18.7 and 71.6 K using the

		CRESU-SIS measurements
	Primary products of the Ar + + C2H4 reaction
		C2H4 +	C2H3 +	C2H2 +	ArH +
	T= 18.7 K	0.22 ± 0.03	0.58 ± 0.05	0.16 ± 0.02	0.04 ± 0.02
	T= 71.6 K	0.10 ± 0.02	0.68 ± 0.05	0.19 ± 0.03	0.03 ± 0.02
	CRESU-SIS experimental setup. The error bars were estimated by adding systematic (±0.02) and statistical (±5%)
	contributions.

Table 5 . 4 :

 54 Ionization potentials of the possible fragment species from Ar + + C2H4 reaction.

		Calculated Ionization	Experimental ionization
		potential (eV)	potential 18 (eV)
	C2H4	10.496	10.5138 ± 0.0006
	C2H3•	8.460	8.59 ± 0.03
	C2H2	11.090	11.41 ± 0.01
	C2H•	14.098	11.61 ± 0.07
	Ar	15.614	15.759 ± 0.001

Table 5 . 6

 56

: Branching ratios of the Ar + + CH4 reaction obtained at 18.

  12 molecule.cm-3 . The reaction of CH2CN + with C2H6 at 49 K shows the production of CH3 , 44, 55, and 57 u, that are ascribed to C2H2 + , C2H6 + , C3H8 + , C4H7 + and C4H9 + ions respectively, cannot be produced directly via the CH2CN + + C2H6 reaction at 49 K for energetic reasons (endoergic pathways). Latter species can be produced via secondary reactions or by side reactions between impurities and C2H6 (Table6.1). The formation pathway of the HCN +

	Chapter 6
	26, 30
	130

+ (m/z= 15 u), C2H3 + (m/z= 27 u), CNH2 + (m/z= 28 u), C2H5 + (m/z= 29 u), C3H5 + (m/z= 41 u), C2H4N + (m/z= 42 u), C3H7 + (m/z= 43 u) as primary products, and other secondary products that are presented in Table 6.1. The products at m/z=

Table 6 .

 6

1: Possible products and enthalpy for each exit channel for the reaction of CH2CN + with C2H6.

Table 6 . 2 :

 62 Comparison between branching ratios of primary products detected by CRESU-SIS at 49 K and reaction time t = 56 μs, and GIB-MS at ECM ⁓ 0.19 eV. In our measurements, the error bars were estimated by adding systematic (±0.02) and statistical (±5%) contributions.By comparing the branching ratios registered by our CRESU-SIS technique at 49 K and those by GIB-MS at ECM ⁓ 0.19 eV (T⁓ 2204 K), we observe:

  in the reactivity of CH2CN + ion with C2H6 at low temperature, i.e. low collision energy (~ 0.045 eV). To clear up this fact, an additional experimental study at different low temperatures is desirable. The production of HCNH + from the CH2CN + + C2H6 reaction offers an additional pathway to generate this cation in Titan's atmosphere. HCNH + is detected by the INMS and CAPS instruments,

	m/z (u) a remarkable change product	CRESU-SIS, T=49 K BR	GIB, ECM ⁓ 0.19 eV BR
	15	CH3 +	0.02 ± 0.02	0.02
	27	C2H3 +	0.32 ± 0.04	0.02
	28	CH2N +	0.26 ± 0.03	-
	29	C2H5 +	0.06 ± 0.02	0.73
	41	C3H5 +	0.26 ± 0.03	0.09
	42	C2H4N +	0.05 ± 0.02	0.10
	43	C3H7 +	0.02 ± 0.02	0.02
				K points to

  + + C2H4 → HCN + + C3H4 pathway is endothermic with ΔH= + 2.42 eV. A hypothesis that can explain the decrease of the branching ratio of the C2H3 +

		CRESU-SIS	GIB-MS
		T= 49 K	ECM ∼ 0 .64 eV
	m/z (u)	product	BR	product	BR
	15	CH3 +	0.02 ± 0.02	CH3 +	0.01
	27	C2H3 +	0.39 ± 0.04	C2H3 +	0.09
	28	CH2N +	0.08 ± 0.02	C2H4 +	0.20
	39	C3H3 +	0.14 ± 0.03	C3H3 +	0.15
	41	C3H5 +	0.25 ± 0.03	C3H5 +	0.25
	42	C2H4N +	0.05 ± 0.02	C2H4N +	0.06
	52	C3H4N +	0.01 ± 0.02	C3H4N +	0.02
	66	C4H4N +	0.01 ± 0.02	C4H4N +	0.03
	67	C4H4NH + 0.05 ± 0.02	C4H4NH +	0.16

Acknowledgements

Neutral argon has a high ionization potential (15.759 eV), comparable to N2 (15.581 eV), so its production from solar photon ionization is expected to be small. At low altitudes the dense atmosphere reduces the penetration of photons. However, the ionization by hot suprathermal electrons and by galactic cosmic rays could contribute at altitudes below the photo-ionization peak, near 700 km and 60 km respectively. 9 From G. J. Molina-Cuberos et al. 7 Both hydrocarbons, C2H4 and CH4, are detected in situ at high altitude by the INMS instrument operating in neutral mode 10 and at distance by the Cassini Composite Infrared Spectrometer (CIRS) in the middle and upper stratosphere (altitudes from 45 to 250 km). 11

Kinetic studies

The Kinetic measurements were obtained under pseudo-first order conditions by measuring the loss rates of the mass-selected ions for various concentrations of neutral co-reactant set in excess. Figure 5.5 illustrates the time evolution of the Ar + ion signal and its loss rate variation with the [C2H4] density for the Ar + + C2H4 reaction at 18.7 and 71.6 K. The values of absolute rate coefficients k(T) at 18.7 and 71.6 K are (1.06 ± 0.13) ×10 -9 cm 3 s -1 and (1.13 ± 0.15) ×10 - 9 cm 3 s -1 respectively. The uncertainties take into account statistical and systematic (10%) contributions. Our values are close to the values calculated by Langevin model (described in chapter 2), kL = 1.2 ×10 -9 cm 3 s -1 . The rate coefficient remains constant over the temperature range, and this is not surprising because the ethene molecule is a non-polar molecule, and the rate coefficient follows the Langevin model. along with a molecular hydrogen product. If we analyze its formation together with two Hatoms (i.e. the C2H4 + Ar + → C2H2 + (lin) + H + H + Ar process), we found an endothermic reaction enthalpy (about +1.652 eV). As a consequence, the most favorable dissociative chargeexchange reaction is probably the Ar + + C2H4 → C2H3 + +Ar + H• reaction with -2.316 eV reaction enthalpy, and the production of C2H2(iso) + cations can be less efficient (with enthalpy of -0.851 eV). This is in a good agreement with the experimental branching ratios. As we mentioned earlier, according to our ab initio results, all the possible reactions which lead to C2H + production are endothermic.

It is worth to emphasize again that the mechanism of the pure charge exchange reaction, which leads to the formation of the parent cations (C2H4 + ), principally differs from all others, since it is only possible through non-adiabatic dynamics, which has a strict quantummechanical nature. At low temperatures, when the collision is slower, the quantum-mechanical effects are more important and the non-adiabatic processes are more effective. 20 This is also in a good accordance with the experimentally observed temperature dependence of the branching ratios in case of C2H4 + ion formation.

Reactivity of Ar + ions with methane

In addition to the reaction of Ar + ions with ethene, we have studied the reactivity of this ion with methane at 18.7 and 71.6 K using our CRESU-SIS technique. The branching ratios and the rate coefficient of the Ar + + CH4 reaction were studied previously by FJFR (Free Jet Flow Reactor) at 10-20 K by Hawley et al. 21 In addition, the rate coefficient of this reaction was measured at 23, 36 and 71 K by Thomas Speck et al. (2001) using the CRESU technique. 22 Furthermore, this reaction was explored at room and high temperatures (700 K) using different techniques (SIFT, ICR, FA, Drift Tube…). In all of these measurements, the major ionic products are CH2 + and CH3 + . Otherwise, the charge transfer product CH4 + is detected in many studies of the Ar + + CH4 reaction at room temperature (FA, SIFT, MS..). 23,2

Kinetic study

The rate coefficient of the Ar + + CH4 reaction has been measured at 18.7 and 71.6 K. We obtain values of k = (1.07 ± 0.26) ×10 -9 cm 3 s -1 and (1.10 ± 0.30) ×10 -9 cm 3 s -1 for the rate coefficients at 18.7 and 71.6 K respectively. The variation of the loss rate, or the pseudo first order k1st, is shown in figure 5.9, the rate coefficient corresponds to the slope of the variation of k1st as function of the density of neutral reactant. The measured rate coefficients are very close to that calculated using the Langevin model, kL = 1.12 ×10 -9 cm 3 s -1 . The rate coefficient follows the each of these species is divided by the total ion signal. Graphs show that the fractions of C4H9 + and C4H7 + decrease with time, and C2H5 + and C2H2 + increase. This correlation supports the role of the C2H2 + + C2H6 and C2H5 + + C2H6 reactions in the production of the C4H7 + and C4H9 + ions.

Conclusion and perspectives:

Ions are ubiquitous in space and contribute to the chemistry of astrophysical environments.

These ions are detected at distance mostly employing radiotelescopes, or in situ by space probes that aim to explore the planets and their moons in the solar system (i.e. Cassini-Huygens mission). Among all the ionic processes (such as photoionization, electron attachment…), ionneutral reactions play a crucial role in the formation and growth of ionic species.

Laboratory astrophysics for elementary reactions seeks to provide two types of information: the measurements of the rate coefficients k and the determination of the product distribution (the branching ratios between the different exit channels). The standard method for the study of ionneutral reactions (also neutral-neutral reactions) at low temperatures is based on the cryogenic collision cooling of a gas. This method is restricted by the condensation of the neutral reactants on the walls of the system, that limits the reliability of the results, and the temperature range.

The CRESU technique generates a cold (down to 6 K) uniform supersonic flow giving the advantage of a wall-less experiment where no condensation occurs in the scrutinized medium.

The second advantage is the LTE due to the high density which allows us to consider a determined unique temperature for the reaction.

Using our CRESU-SIS technique, that couples a mass selective ion source to a CRESU chamber, we have investigated the reactivity of N2 + ions with C3H4 isomers (propyne and allene) at low temperatures (from 71.6 to 24 K). These species are detected in Titan's atmosphere. In collaboration with Sandor Demes and François Lique, an ab-initio calculation was performed in order to understand the variation of the branching ratios of the products. The aromatic cyclopropenyl cation c-C3H3 + ion was observed as major product. An increase in the branching ratio of C3H4 + was observed at low temperatures, this can be explained by the quantum nature of the pure charge transfer (CT) mechanism, which proceeds via crossings between multiple electronic states since it is only possible through non-adiabatic dynamics, and it is more important at low temperatures. The measured rate coefficients show an overall good agreement with empirical models. For the reaction with propyne, the rate coefficient increases from (2.13 ± 0.21) × 10 -9 at 24 K to (3.73 ± 0.41) × 10 -9 cm 3 molecule -1 s -1 at 71.6 K, and seems to follow the Su and Chesnavich model that takes into account the dipole moment and the reaction temperature. In the case of the reaction with allene, the rate coefficient remains constant over the temperature examined, with values close to that estimated by Langevin model, and this is not surprising due to the absence of the dipole moment of the neutral collision partner. The formation of c-C3H3 + ion by the reaction of N2 + with C3H4 was included onto a photochemical model of Titan's atmosphere. Results show that this new pathway plays a secondary role relative to the dominant production terms of c-C3H3 + from the reactions of C2H4 + and C2H5 + with acetylene.

The reactivity of Ar + ions with C2H4 and CH4 was also investigated at 18.7 and 71.6 K. For the Ar + + C2H4 reaction, four primary products were detected: C2H2 + , C2H3 + , C2H4 + and ArH + . The pure CT product was observed in important quantities in our measurements with a branching ratio increasing from 10 % at 71.6 K to 22 % at 18.7 K. This reaction was also studied at room temperature using FA and MS techniques, where the pure CT product (C2H4 + ) is only observed with a BR of 4 % in the FA measurements. In these studies, the authors suggest that the C2H4 + product breaks down into smaller fragments. The fragmentation mechanism of C2H4 + leading to C2H2 + and C2H3 + is endothermic and it is not allowed at low temperatures. This can explain the detection of C2H4 + in our measurements using the CRESU-SIS technique. Similarly to the case of the N2 + + C3H4 reaction, the increase in the branching ratio of the pure CT product can be explained by the fact that this mechanism has a strict quantum-mechanical nature. For the Ar + + CH4 reaction, CH2 + and CH3 + ions were detected as primary products. The pure charge transfer product CH4 + was not observed. In this case, we don't observe a variation in the branching ratios in our measurements at 18.7 and 71.6 K and comparing to the previous study using free jet technique at 10-20 K. Concerning the rate coefficients, for the reactions of Ar + with C2H4 and CH4, our measurements give values close to those estimated by Langevin model due to the absence of dipole moment of neutral molecules. The direct implications of these results for the astrophysical environments are not presented and need more efforts to be completed.

To provide insights on the molecular growth mechanisms leading to large carbon and nitrogenbearing species in Titan's ionosphere, in the last chapter, the experimental findings (kinetic study and products distribution) corresponding to the reaction of CH2CN + ion with C2H4 and C2H6 at 49 K were presented. Our measurements of the branching ratios have been compared to those recorded by the GIB-MS technique at different collision energies. A remarkable difference was observed. For the CH2CN + + C2H6 reaction, our results show the production of CH2N + with a branching ratio equals to 26 %, while this product was not observed in the measurements using GIB technique. This result gives an additional formation pathway for the most abundant ion in Titan's atmosphere (HCNH + ). An important increase in the branching ratio of the C2H3 + ion was also noticed in our studies for the reactions with C2H6 and C2H4.

Recently, we have started the investigation of the reactivity of CH3 + ion with propyne, allene, ethene and methane at 18.7 and 71.6 K. The results show the production of ionic species of increasing size, with a remarkable variation in the branching ratios with the temperature. Many additional studies are desirable. In particular, we plan to extend the study of the reactivity of CH2CN + ion with other hydrocarbons (CH4 and C2H2) and at different low temperatures.

The successful investigation of the reactivity of cation-molecule reactions validated our approach. We are now in position to address more challenging objectives and target anionmolecule reactions. For example, we aim to study the reactivity of molecular anions such as CxH⁻and Cx⁻ which are identified in astrophysical environments (e.g. C2H -+ HC3N). One of the difficulties encountered in experiments with anions is their generation.

Using our CRESU-SIS technique, we would like also to focus our work on the first stages of nucleation induced by ions, specifically to explore how the injection of ions into the USF can accelerate the nucleation process from precursors such as HC3N or CH3CN. The high abundance of hydrocarbons and nitriles in the lower Titan's atmosphere could lead to the formation of aerosols by nucleation, especially polar species.

There is still a number of experimental challenges to carry out these studies. For example, it is necessary to update our plasma source in order to have a stable reliable source of negative ions, which are more difficult to produce than positive ions. Despite that our mass selective ion source makes possible to produce ions at sufficient quantity to measure the rate coefficient and the branching ratios with high precision, the bandpass of the quadrupole mass filter remains large, allowing some ions to pass and complicate the ion spectrum (in some cases), one of the challenges is to reduce this bandpass by replacing the rods of the quadrupole mass filter with longer rods, in order to have only the ion under study in the USF.

Another aspect to be improved is the helium consumption by the Laval nozzles in our laboratory usually used for ion-molecule reaction studies (the flow rate can reach 100 L/min). Considering the increase of the helium price, a proposition is to use the nozzles with nitrogen buffer gas.

But this reduces the ion transmission from the selective ion source into the USF (as shown in chapter 3). A solution in this case is to update the end of the SIS by adding an "aerodynamic le coefficient de vitesse passe de (2,13 ± 0,21) × 10-9 à 24 K à (3,73 ± 0,41) × 10 -9 cm 3 molécule - 1 s -1 à 71,6 K, et semble suivre les Su et Modèle de Chesnavich prenant en compte le moment dipolaire et la température de réaction. Dans le cas de la réaction avec l'allène, le coefficient de vitesse reste constant sur la température examinée, avec des valeurs proches de celle estimée par le modèle de Langevin, et cela n'est pas surprenant en raison de l'absence du moment dipolaire du partenaire de collision neutre. La formation d'ions c-C3H3 + par la réaction de N2 + avec C3H4 a été incluse dans un modèle photochimique de l'atmosphère de Titan. Les résultats montrent que cette nouvelle voie joue un rôle secondaire par rapport aux termes dominants de production de c-C3H3 + à partir des réactions de C2H4 + et C2H5 + avec l'acétylène. La réactivité des ions Ar + avec C2H4 et CH4 a également été étudiée à 18,7 et 71,6 K. Pour la réaction Ar + + C2H4, quatre produits primaires ont été détectés : C2H2 + , C2H3 + , C2H4 + et ArH + . Le produit CT pur a été observé en quantités importantes dans nos mesures avec un rapport de ramification passant de 10 % à 71,6 K à 22 % à 18,7 K. Cette réaction a également été étudiée à température ambiante en utilisant les techniques FA et MS, où le produit CT pur (C2H4 + ) n'est observé qu'avec un BR de 4 % dans les mesures FA. Dans ces études, les auteurs suggèrent que le produit C2H4 + se décompose en fragments plus petits. Le mécanisme de fragmentation de C2H4 + conduisant à C2H2 + et C2H3 + est endothermique et n'est pas autorisé à basse température.

Ceci peut expliquer la détection de C2H4 + dans nos mesures avec la technique CRESU-SIS.

Comme dans le cas de la réaction N2 + + C3H4, l'augmentation du rapport de ramification du produit CT pur peut s'expliquer par le fait que ce mécanisme est de nature mécanique quantique stricte. Pour la réaction Ar + + CH4, les ions CH2 + et CH3 + ont été détectés comme produits primaires. Le produit de transfert de charge pur CH4+ n'a pas été observé. Dans ce cas, nous n'observons pas de variation des rapports de branchement dans nos mesures à 18,7 et 71,6 K et par rapport à l'étude précédente utilisant la technique du jet libre à 10-20 K. Concernant les coefficients de vitesse, pour les réactions de Ar + avec C2H4 et CH4, nos mesures donnent des valeurs proches de celles estimées par le modèle de Langevin du fait de l'absence de moment dipolaire des molécules neutres. Les implications directes de ces résultats pour les environnements astrophysiques ne sont pas présentées et nécessitent plus d'efforts pour être complétées.

Pour fournir des informations sur les mécanismes de croissance moléculaire conduisant à de grandes espèces carbonées et azotées dans l'ionosphère de Titan, dans le dernier chapitre, les résultats expérimentaux (étude cinétique et distribution des produits) correspondant à la réaction de l'ion CH2CN+ avec C2H4 et C2H6 à 49 K ont été présentés. Nos mesures des rapports