Chapter II: Experimental Methodology

Chapter I: Introduction 1.1 General introduction

Radiation chemistry is a branch of chemistry that studies chemical transformations in materials exposed to high-energy radiations. It uses radiation as the initiator of chemical reactions, as a source of energy that disrupts the sensitive energy balance in stable systems. In that way it is a younger sister of photochemistry, which does the same, but uses another type of electromagnetic energylightas the initiator. Radiation chemistry does not deal with radioactive elements (as radiochemistry does), except to use them as a source of radiation, always physically separated from the irradiated system.

Figure 1.1 The key role of fundamental studies in radiation chemistry

Understanding radiation-induced processes is of vital significance in such diverse fields as waste remediation in environmental cleanup, [1][2][3] radiation processing of polymers 4,5 and food [6][7][8] , medical diagnosis and therapy, 9,10 catalysis of chemical reactions, [11][12][13][14] environmentally benign synthesis, 1,15 and nuclear energy production. [16][17][18] Radiation chemistry provides for these fields fundamental quantitative data, such as reaction rate coefficients, diffusion coefficients, radiation chemical yields, etc., as well as providing useful quantitative information of technological and medical importance, radiation chemistry is also a valuable tool for solving fundamental problems in chemistry and in material sciences. Exploiting the many facets of radiation chemistry requires a thorough and comprehensive understanding of the underlying chemical and physical processes. An understanding of the structure and dynamics of "tracks" produced by ionizing radiation is a central issue in the field. There is a continuing need to study the ultrafast processes that link the chemistry and physics of radiation-induced phenomena. This is especially true for practically important, but less well understood, nonstandard environments such as interfacial systems, supercritical media, and heterogeneous systems. These various goals necessitate the development and application of complementary programs of experiment and theory, and will involve the use of nonconventional radiation sources and the study of novel homogeneous and heterogeneous chemical systems. There is also a need to upgrade other types of instrumentation used in radiation chemistry in the national laboratories, including high field electron paramagnetic resonance, and modern analytical tools.

The development and enhancement of these various tools will allow for a much wider use of the national radiation chemistry facilities.

Water is a major constituent of all living matter, representing 70-85% of the weight of cells.

For this reason, water plays a major role in radiotherapy, 19 radiosterilization, 20 sewage treatment 21 and food irradiation 22 , etc. because its radiolysis produces reactive species which are responsible for indirect and/or direct effects. Furthermore, its absorption properties being very similar to those of the human body, water is also widely used for clinical dosimetry. For these reasons, radiolysis of water and aqueous solution has been extensively studied for more than a century. In fact, very shortly after the discovery of X-rays by Wilhelm Conrad Roentgen in 1895, the natural radioactivity of uranium compounds by Henri Becquerel in 1896, and the radioactive element radium by Pierre Curie, Marie Curie and Gustave Bémont in 1898, dissolved radium salt has been shown to decompose aqueous solutions by liberating hydrogen gas continuously, due principally to the release of α-particles from the decomposition of radium. 23 H2O2 was also found in the irradiated solution. [24][25][26] The mechanism accounting for the radiolysis of water has been presented after a century's survey. Nevertheless, it is unclear in the earliest term of the radiolysis of water because of the limitation of the technology.

Quantification of radiation

Energy emitted from a source is generally referred to as radiation. Ionizing radiation is the type of radiation that the emission occurs with enough energy so that during an interaction with an atom, it can remove tightly bound electrons from the orbit of an atom, causing the atom to become charged or ionized. Quantified parameters are essential to understand the radiation process that occur in the system.

Definition of linear energy transfer (LET)

Radiation effects depend on the type of irradiation and its mechanisms of deposition of energy into the irradiated medium. From the quantitative point of view, the impact of radiation with matter can be characterized as the rate of energy loss per unit length of track of the particle. The rate of energy loss per unit length of track of the particle is known as the stopping power or linear energy transfer (LET). LET could be expressed as:

 xr dE LET dx       EQ. 1-1
The units used for LET can be keV mm -1 , eV m -1 or eV Ǻ -1 .

The rate of energy loss of an energetic particle passing through a liquid is non-uniform; it increases as the kinetic energy of the particle decreases. When the ionizing particle is a high energy electron, the ionization events occur in small clusters called spurs that are widely separated along the track and the radiation is classed as low LET (Figure 1.2). For energetic nuclear ions, e.g. H + , He 2+ , etc., the rate of energy loss is much higher so that the spurs are much closer together and can overlap to form cylindrical columns of ionized molecules in the liquid. In this case the radiation is classed as high LET. Actually, low LET radiation has a high LET component due to low-energy secondary electrons, and high LET radiation has a low LET component due to high-energy secondary electrons, so it is common practice to classify types of radiation by their average LET over the length of the particle track. evolution with time by diffusion and reaction up to homogeneous distribution at ≈ 10 -7 s. 27 In most cases, the LET is equal to the "stopping power". The Bethe theory of stopping power describes the mean energy loss by a charged particle due to the electrostatic interactions between the particle and the electrons of atoms in the medium. 28 For non-relativistic ions, the Bethe formula can be written as:  m 0 is the mass of electron.

 N is target electron density.

 I is the average of ionization potentials of the target.

Several important aspects are illustrated in this formula. Since 2 2 dE Z dx V  , the LET of two ions of the same charge is higher for the ion with the lowest energy. This can be understood by noting that a slower ion has more time to interact with electrons of the medium than a faster ion; thus, the LET of the ion with the lowest energy is higher. EQ. 1-3

Today, the most common unit of G value is mol J -1 . It can be converted as:

1 molecules / 100 eV=1.036 × 10 -7 mol J -1 EQ.1-4

Generally, G value could be determined by:

() ()    Ct Gt Dose EQ.1-5  C(t)
is the concentration of the species at a given time t (in mol L -1 ).

 Dose is the absorbed energy per unit of mass by the system (in J kg -1 or Gy).

 ρ is the density of the sample (g mL -1 ).

Knowing the G value is particularly important for both the sample and the dosimetry. Usually, the G value of the given species itself is not measured directly. It is mostly based on a yield of primary radicals that has been estimated from measured yield at infinite dilution of all scavengers.

These yields represent the number of the primary radicals that diffuse out of the tracks formed by the ionizing radiation and their secondary electrons. Immediately after the arrival of the high energy radiation, the tracks contain a high density of radicals and other fragments, and many of the radicals combine with each other. Thus only a fraction of the original primary radicals escapes into the bulk solution and is measured. diluted solute S acting as a radical scavenger. 9 The radiolysis of water is usually divided in three more or less overlapping stages: physical (<10 -15 s) (Figure 1.3), physicochemical stage (∼10 -15 -10 -12 s) and non-homogeneous chemical stage (∼10 -12 -10 -6 s). This time division is controversial, it gives a convenient way to describe the mechanisms involved in water radiolysis.

Radiolysis in water

1.3.1 Physical stage (<10 -15 s) Direct ionization of water produces a radical ion and a free subexcitation electron (E < 7.4 eV)

Energy transfer can produce a water molecule in an excited state.

*

Excitation H O H O  

EQ.1-6

Direct ionization of water produces a radical ion and a free subexcitation electron (E < 7.4 eV).

22

Ionization

H O H O e

     EQ.1-7

The time scale for the creation of these species is on the order of 10 -16 seconds.

1.3.2 Physicochemical stage (~10 -15 ~10 -12 s)

The ionized and excited molecules created during the physical stage are highly unstable.

During this stage, lasting ∼10 -12 s, the ionized and excited water molecules dissipate their energy by energy transfer to neighboring molecules and bond rupture. The sequence of events of the physicochemical stage is, in fact, not well characterized experimentally. However, some important physicochemical stage events worth mentioning: proton transfer to a neighboring molecule, dissociation of excited water molecule and electron thermalization and solvation.

The proton transfer to a neighboring water molecule is very important, because it leads to the production of an OH • radical:

2 2 3 H O H O OH H O       EQ.1-8
For an excited water molecule, there are several channels for the dissociation The excited water molecule can dissipate excess energy by bond breakage to produce hydroxyl and hydrogen radicals. It takes ~5eV to break the O-H bond. Where O( 1 D) and O( 3 p) are the singlet and triplet state of the atomic oxygen, respectively.

* H O H O 

EQ.1-12

The excited water molecule can also return to its fundamental state without dissociation by heat loss.

Figure 1. 4 Cartoon showing the formation of the solvated electron upon solvent ionization. 9 Most secondary electrons have low energy, 30 but some of them may have energy in the keV or even in the MeV range. These electrons lose their energy into medium by doing further ionization, excitations until they eventually reach thermal equilibrium with the liquid. The electron, which is negatively charged, interacts with the dipoles of the surrounding water molecules and becomes trapped in a state called "trapped electron". Finally, this electron becomes a "hydrated electron", which behaves like chemical species (Figure 1.4). This rapid sequence of events can be written as follows: Besides, the electron could react with H + to form a H atom in acidic condition: The concentrations of the radicals depend on the energy deposited in water. Thus, the three initial species H2O*, H2O Most of these reactions remove chemically reactive species from the system. With time (by ~ 10 -6 s) all of the reactive species have diffused sufficiently far that further reactions are unlikely.

The chemical development of the track is over by 10 -6 s.

At the end of the physicochemical stage, the species H • , OH • , H2, H2O2 are created in very high concentration in the spurs. Other species such as O( 3 P) may also be present, in small quantity. They diffuse in the medium and eventually encounter chemical species created in other spurs, which gives them the opportunity to react. In general, the radical species (such as H • , OH • and aq e  ) react in ∼1 μs to form the molecular species H2 and H2O2.

Concluding remark

Deuterated water (D2O), generally known as heavy water, is a form of water that contains a larger than normal amount of the hydrogen isotope deuterium ( 2 H or D). D2O is one of the distinct feathers of pressurized heavy water reactor (PHWR), where it is used as coolant and moderator because it exhibits a thousand times enhanced neutron-moderating ability compared to H2O. In PHWR, the nuclear core is cooled by liquid heavy water that undergoes radiolytic decomposition induced by an intense ionizing radiation field. The incident energy comprises low LET X-rays, fast neutrons whose energy is transferred to protons and oxygen nuclei resulting in the emission of charged particle recoils of high LET, and also much higher LET radiation (recoils of 7 Li ions and α-particles). The aqueous radiolysis products must be assessed because they can induce unwanted deleterious corrosion, hydriding, and cracking of sensitive materials both in the core and in the various piping components of the reactors. [31][START_REF] Muroya | A Re-Evaluation of the Initial Yield of the Hydrated Electron in the Picosecond Time Range. Chapter III: Time Dependent Yield of the Solvated Electron and the OD • Radical in Deuterated Water at Low LET Studied by Picosecond Pulse Radiolysis 99[END_REF][33] The basic interest of nuclear power engineers is to know the real concentrations of the oxidizing products and to select conditions at which their formation could be suppressed. Consequently, knowledge of the radiolytic yield of the product in D2O formed by ionizing radiation is essential. Additionally, it is captivating to explore the isotopic effect of H-D on the radiolytic behavior of the species produced in the water after irradiation.

In this thesis, the subject of interest is on the time dependent radiolytic yield of the solvated electron and the hydroxyl radical (Chapter III) and the reactivity of the precursor of hydroxyl radical-the water cation radical (Chapter IV) in deuterated water on picosecond range. Chapter II: Experimental methodology

State of art

Radiation chemistry has been strongly influenced by the techniques to make measurements.

The radiation chemists have developed and utilized powerful facilities to study both time resolved and steady-state chemical phenomena resulting from the interaction of ionizing radiation with matters. Nowadays the instruments in the field of radiation chemistry are able to explore the chemical process in various time scale, temperature, pressure and phase stage. It is notable that the special pattern of the reactive species produces by the radiolytic energy deposited within the sample depends on the type of incident radiation and its energy. When the energy deposited in the sample is by phonic radiation (Xand gamma ray), it is well separated interactions within the sample. In the case of highly charged nucleons, it would produce much denser deposition patterns with higher probability of overlapping region of ionized species. The effect is quantified as liner energy transfer (shorted as LET). Photons and electrons are considered to be low LET radiation whereas protons and heavy nucleons (heavy ions, alpha particles and neutrons) are high LET. The technical methods in radiation chemistry can be quickly summarized as:

High LET i. Heavy ion

Heavy ion beams occupy an important role in radiolysis experiments as a result that heavy ions have higher LET values, which deposits energy to material at a high density per thickness.

Generally, heavy ion radiolysis experiments are performed at large, multipurpose accelerator facilities with cyclotrons or tandem Van de Graaff accelerators. The yield of radiolytic products as a function of LET and the effects on the concentration of scavengers on these yield have provided critical favor for the understanding of the radiolytic process. For an example, when applied to modification of polymers, ion beam caused less deterioration compared with electron beam. 1 By far, ion beams have been used to fragment polymers to form membranes, and conversely, to polymerize precursor substrates to form forests of polymer rods on surfaces. 2 Additionally, the localized damage due to heavy ion radiation plays unreplaceable role in radiation biology and radiation medicine. Proton therapy is a precise way to deliver large radiation doses to kill tumors.

Besides, hydroperoxyl radical HO2 • , only produced by the heavy ion radiolysis in water not by low LET radiolysis in the absence of molecular oxygen. It is a significant method to give rise to oxidative damage to hypoxic tumors.

ii. Alpha particles and neutrons Alpha particles are the largest particles emitted from the nucleus of an atom. Alpha particles will quickly interact with an atom and lose their energy soon after they are emitting because of their relatively low velocity and large charge. Alpha particles travel only a few centimeters in the air. A piece of paper, normal work clothes, or the dead layer of skin on a human body can stop an alpha particle. Because of their high energy, alpha particles can deposit a significant internal dose if inhaled or ingested. The early radiation chemistry studies were predominately performed with αparticles because they were available and the chemical effects were large enough that they could be observed using the techniques of the time. Nowadays, there are many ways in which science successfully uses alpha radiation in a beneficial way. In cancer treatment, alpha radiation is used to treat various forms of cancer what is called unsealed source radiotherapy. This process involves inserting tiny amounts of radium-226 into cancerous masses. The alpha particles destroy cancer cells but lack the penetrating ability to damage the surrounding healthy cells. Moreover, alpha radiation is also applied as static eliminator, spacecraft power and so on. In the past, neutron radiation normally was found only in the immediate vicinity of a nuclear reactor or nuclear weapons burst. Today there are more and more industrial use of neutron-emitting radionuclides. Neutron-emitting radionuclides are being applied in some specialized radiographic procedures, in well logging and soil moisture analysis. If accelerator energy is above 8 MeV, neutrons may be an important component of the radiation generated.

Neutron radiation is extremely difficult to detect. Because neutron radiation is extremely penetrating, protecting workers is equally difficult. Effective shielding for neutron radiation usually involves a material that has a high hydrogen atom concentration. Such shielding might employ several feet of soil, paraffin (materials saturated with hydrogen atoms), or concrete.

Low LET i. Gamma and X-rays

Gamma radiation is by far the most penetrating of the three common types of radiation (alpha, beta, gamma). Gamma rays have no mass or charge and are considered to be pure energy. They X-rays differ from gamma radiation only in their origin and the fact that they tend to be less energetic. Whereas gamma radiation originates in the nucleus of an atom, x-rays are generated by the shell transition of orbital electrons (Figure 2.2). They are also formed from the deceleration of electrons interacting with matter (known as bremsstrahlung). Nowadays particles accelerators can be used as continuous or pulse X-ray source by stopping the particle beam (typically electrons) in a high atomic number material (gold or tungsten).

ii. Charged particles by accelerators In radiation chemistry there are many methods of accelerating particles beam for pulse radiolysis. Generally, acceleration could be sorted by the type of the force applied by electric field.

The field may be direct current as produced by electrostatic potential or oscillating in time and space as produced by radio frequency, microwave or optical laser radiation (Figure 2.3) The large doses are well suited for the study of radical-radical reactions and gas phase pulse a)

radiolysis. The electrical noise generated by the spark gaps and the discharge of the Febetron is a major problem for detection systems, requiring much signal averaging and background correction.

Accelerators that use electromagnetic radiation to produce the accelerating field could generate particles with much higher energies than that produced by electrostatic accelerators. Higher beam energies provide more penetrating power for elaborate experimental setups such as pressure vessels and cryostats, and the ability to better focus more charge into a smaller target to produce a higher radiolytic dose. However, the beam energies above 40 MeV for electrons are less desirable for pulse radiolysis as a practical consideration. The reason is that when the beam energy increase, the fraction of the beam energy that converted into bremsstrahlung instead of being deposited in the sample in radiolytic events will increase along with increasing the beam energy. The most common type of oscillating-field accelerator for pulse radiolysis is Linear accelerator (LINAC). Figure 2.6 shows a diagram of LINAC. There is a series of resonant cavities for radiofrequency in the accelerating sections. The frequency could range from 100 MHz to 10 GHz, and 1-3 GHz is the most common one. Keeping in phase with the radio frequency supply, the charged particles are accelerated in the gaps between the electrodes. In other words, initially, the tube in front of the particles has an opposite charge to that of the particles, and hence attracts the particles.

Once in the tube, the polarity of the tube changes. The particles are repelled from the tube and attracted to the next tube, and so on. Since the frequency of the electric field is constant and the particles' speed increases, the tubes get progressively longer to ensure that the particles spend the same amount of time in each tube and therefore keep in phase with the electric field. In older type of LINAC the cylindrical electrodes ware called drift tubes which are aligned coaxially with the chamber. No drift tubes are used in the modern high-energy. The radio frequency is boosted at regular intervals along the chamber by means of klystrons (electron tubes used for the amplification or generation of high frequency waves). Only a small magnetic field, supplied by magnetic lenses between the radio frequency cavities is required to focus the particles and keep them in a straight line. Typical rates of energy gain in a LINAC are 7 MeV per metre for electrons.

Detection technique for ultrafast radiolysis

To start the chemical processes under investigation, irradiation produces chemically highly reactive species at intervals generally much shorter than the observation period. The detection apparatus consists of appropriate devices depending on the physical properties being studied. For the long alive species produced by γ-source, an analyzing light source and a photodetector for optical detection is applicable. A voltage generator and an electrical detection circuit for conduct metric studies is essential meanwhile a device for recording the kinetic voltage vs. time curves is also necessary. A computer may be used to store the transient curves for later analysis or may even be used to control the entire experiment for picosecond pulse radiolysis. With the development of technology, more and more elaborated detection methods have been applied to investigating the radiation process. In pulse-probe transient absorption spectroscopy, a short pulse of light is used to measure the absorbance of a sample as a function of time delay between the electron pulse and the probe beam. Time resolution using this method is typically on the order of the electron pulse width. Originally, Cerenkov light generated from the electron beam itself was used as a variable-delay probe beam. 6,7 Later, a free-running Ti-Sapphire oscillator was used with a time-amplitude converter to measure spur decay of the solvated electron. With the advent of laser photocathode electron gun systems that provide picosecond-synchronized laser pulses, high-resolution pulse-probe kinetics measurements have nearly become routine. Timing improvements 8 and streak-camera jitter detection have made such measurements possible on thermionic, pre-bunched picosecond linacs as well. For transient spectroscopy, two methods of probe pulse generation are used to measure kinetics at wavelengths other than the fundamental of the gun-driving laser system (~ 800 nm). At some facilities, a white light continuum is generated by focusing intense laser pulses into a substrate such as a sapphire plate, fused silica or D2O, [9][10][11] while at others a specified probe wavelength is produced by an optical parametric amplifier pumped by the gun-driving laser system. 8,12 Broadband detection of the white-light probe is accomplished with a spectrograph/CCD or diode array combination,10 while single-wavelength detection is done with a pair of diodes. [10][11][12][13] Broadband detection works within the sensitivity range of silicon detectors (up to 950 nm), while NIR-sensitive photodiodes (Ge and InGaAs) extend the range of single-wavelength detection up to 1700 nm. Pulse-probe transient spectroscopy has been used to follow the formation of Xe2* excimers in the radiolysis of supercritical xenon, 14 measure dissociation rates of aryl halide radical anions, to re-evaluate the yield of hydrated electron at picosecond times, 15 and to observe the solvation of excess electrons in ionic liquids.

ELYSE -A picosecond pump-probe radiolysis platform

ELYSE is named after Lysis (Greek for degradation) by Electrons. This Fast Kinetics Centre at Orsay is based around a femtosecond laser facility used for triggering the photo-cathode and a picosecond electron accelerator used for pulse radiolysis experiments. The combination of laser and electron beams will allow the physical chemistry community to perform pumpprobe experiments with high time resolution. The electron pulse is extracted from a Cs2Te photocathode by a femtosecond laser pulse. The synchronization of laser and electron beams allows the physical chemistry community to perform pump-probe experiments with high time-resolution. Irradiation of chemical samples will be possible with both electron beams (pulse radiolysis) or laser beams (also used independently for ultra-fast photochemistry). The photocathode is triggered by a 263 nm laser pulse obtained from a 780 nm, frequencytripled, amplified Ti-sapphire laser (120 fs, 2 mJ, 10-50 Hz). Using an 80 MHz quartz oscillator controlling both the pump laser beam and the klystron RF of the laser-triggered electron beam provides a precise synchronization of both laser and electron beams. Therefore, a small fraction of the 780 nm laser beam (10%) can be deviated by a beam splitter (BS) to be used as a synchronized probe pulse, of this wavelength or after generating a pulsed white light continuum through a sapphire plate, to detect the kinetics of the transients. The ELYSE facility is shown schematically in Figure 2.9 along with the areas used for ultrafast pulse radiolysis, the whole being surrounded by radiation shielding. Three experimental areas are available for irradiation 16 EA-1 at the exit of the direct electron beam, EA-2 and EA-3 at the exit of the deviated lines. The laser beam at 1-50 Hz is transferred through a hole in the concrete/lead shielding between the laser and the accelerator rooms down to the photocathode after being frequency tripled to 263 nm. The energy per pulse at 263 nm is up to 120 µJ. In order to obtain normal incidence of the laser beam onto the photocathode, and thus to produce photoelectrons with a time structure as close as possible to that of the laser, the beam, which is injected into the vacuum gun chamber through a transparent window, is reflected for almost normal incidence onto the cathode by a laser entrance mirror (LME) (Figure 2.9). The mirror is in fact slightly shifted relative to the electron beam axis so as to prevent irradiation. The laser pulse energy delivered to the photocathode is measured, by using a switchable laser exit mirror to deflect it onto a virtual cathode (VC) located on a side line perpendicular to the main gun axis and equipped in situ with a Joulemeter.

The intensity and position of the electron beam is observed upstream of each experimental area by a wall current monitor and a moveable Faraday cup. Ceramic screens are used to indicate the transverse beam profile. The bunch length is measured from the pulse length of Cerenkov light which is emitted by sapphire screens and transferred through a series of mirrors to the detection room. The need to extract high charges with µJ laser pulses requires the use of Cs2Te photocathodes, which are presently considered to be the best choice for high quantum efficiency 17,18 and acceptable lifetime. Since the Cs2Te coating is sensitive to oxidation by air, 19,20 it should be prepared in situ under extremely low-pressure conditions. A vacuum chamber based on the chamber used at CERN/CTF 17 and dedicated to Cs2Te preparation was built for this purpose, 16 representing the first such installation in a pulse radiolysis facility. The photocathode preparation chamber is connected to the accelerator cavity by a vacuum valve that allows the chamber to be isolated from the gun. The diameter of the laser spot on the cathode is about 5mm. The charge extracted from the Cs2Te photo-cathode is 1-5nC (Figure 2.13). Therefore, the charge due to the dark current becomes a negligible part of the total charge. 16 When the cathode is freshly prepared, the current is somewhat larger. The photocurrent decreases slowly as a function of photo-cathode ageing. The current emitted by the photo-cathode over 2 yr of running is remarkably stable. In the absence of any vacuum leak in the chamber (Figure 2.14), the Cs2Te deposit is particularly robust and does not need to be prepared very frequent. At a fixed laser pulse energy (about 15 mJ), the charge extracted from the photocathode increases with the size of the laser beam spot on the cathode due to lower charge recombination.

Charges as high as 7.5 nC have been measured at the beam exit EA-1 for a spot diameter of 15 mm. However, the pulse width also increases up to about 15 ps. The charge depends linearly on the laser pulse energy up to 4 mJ, with a slope of   0.25% electron per 263 nm photon absorbed.

Between 4 and 18 µJ, the current still increases but the quantum efficiency decreases. 16 After turning on microwave power to the machine, about 2 h are required for reaching a stable electron beam and reproducible pulses in width and intensity (or less than 45 min after short stops for manipulations in the machine room). The duration of the pulse is measured using the Cerenkov light pulse emitted by sapphire screens moved across the electron beam. The screen can be located inside the vacuum tube or outside the electron beam exit window. The light beam is transferred by the mirrors of an optical line and through a hole in the concrete/lead wall to the entrance window of the fast streak camera Hamamatsu located in the detection room. Figure 2.15 shows a streak camera profile at 500 nm obtained from Cerenkov radiation of the electron pulses obtained on the deviated line EA-3. As an example, the pulse width (FWHM) for 9MeV electrons is 6.3 ps.

To conclude, the ELYSE accelerator has produced, at a repetition frequency of 25 Hz, a few nC, 6 ps pulses of 9MeV electrons for the direct line EA-1 and pulses of a few nC, 3.6 ps pulses, 9MeV, for the deviated line EA-3, using a Cs2Te photo-cathode prepared in situ in a vacuum chamber. The response of the cathode is particularly stable when ageing. The ratio of dark current to photocurrent is less than 2% at 5 and 9MeV for the direct line EA-1 and the deviated line EA-3, respectively.

EA-1 -Picosecond pulse-probe setup

In the experiments, the electron pulse constitutes the pump of the pulse-probe methods. It generates the transient species in the solution by radiolysis of the sample. As shown in Figure 2.9, a 10% part of the 780 nm pulsed laser beam is reflected by a beam splitter, before frequency tripling (THG), to be used for probing the time-resolved absorbance in the cell. The delay between the electron pulse and the probe laser pulse, which are synchronized, is adjusted using a variable optical length with a delay line (1meter for the total length) controlled by a stepper motor (Owis). The maximum delay is 11 ns. The probe laser pulse at 780 nm is then divided by a beam splitter into two parts, 50% being used as the reference signal needed to take into account the shot-to-shot laser fluctuations of the probe light and the other 50 % being used as the absorbed signal after passing through the sample almost collinearly with the electron beam. For that purpose, a thin mirror (200 µm) is interposed in the narrow space between the electron exit window and the cell.

A mechanical chopper (Thorlabs MC1000) is located on the 260 nm beam exciting the photocathode in order to alternatively measure the intensity of the probe beam with and without excitation by the electron pulse, I, and I0, respectively. In order to avoid RF noise, both probe signals are collected and transferred by long Si/Si low OH optical fibers (400 ptm core diameter, Ocean Optics) to be recorded by the reference photodiode and the signal detectors (Thorlabs, PDA55, Si 400 -1100nm), respectively, both located in the shielded detection room.

For the measurement in the near infrared down to the visible a custom made ultrafast broadband spectrometer was used at EA-1 (Figure 2.9). 21 An adapted version with a custom-made prism spectrometer and an InGaAs array with extended spectral sensitivity was used to cover the near infrared from 700 -1500 nm. 22 A particular configuration was applied in order to measure the transient absorbance changes simultaneously in the middle UV around 260 nm and in the near UV to visible. After passing the optical delay line, two probe beams were generated in a rectangular arrangement before they were unified by a 50% beam splitter. In one arm of the rectangle, a supercontinuum was generated by focusing ~1 μJ of the laser source into a 6 mm thick CaF2 disk.

It was used as the optical probe covering a wavelength range from the visible down to the ultraviolet.

In the other arm a second probe beam at 260 nm was generated by third harmonic generation. The optical path length of these two probe beams and their beam divergence were kept similar in the rectangle. Both probe beams were coupled to each other in a collinear manner with the 50% beam splitter that was also used to split off collinear reference beams from both probe sources before the fused silica optical flow cell (FSOFC). These dual probe and reference beams were both coupled into an optical fiber, transmitted to a spectrometer, and dispersed onto a CCD. The combination of the broadband probe and the multichannel detector allowed the entire transient spectra to be recorded, independently of the shot-to-shot fluctuations and possible long-term drifts of the electron source. All measurements were made in a custom made FSOFC with a 5 mm optical path collinear to the electron pulse propagation and 200 µm thick optical windows. The electron pulses were of ~ 4 nC, with an electron energy of 6-8 MeV, delivered at repetition frequency of 10 Hz. To determine the dose deposited in the sample, the time dependent absorbance of the solvated electron in static cell is measured for each experiment. For example, in Figure 2.18 the timedependent decay of the solvated electron is recorded at 600 nm, according the formula 7 600 7 ()

t ps nm t ps A Dose Gy Gl         EQ. 2-2
 At=7 ps is the absorbance of the solvated electron measured at 7 ps.

 600nm   
is the extinction coefficient value of the solvated electron at 600 nm, i.e. 13482 M - 1 cm -1 .

 Gt=7 ps is the radiolytic yield of the solvated electron at 7 ps, i.e. 4.4 × 10 -7 mol J -1 . 15  l is the length of the optical path in the fused silica optical cell, i.e. 0.5 cm.

 ρ is the density of water, i.e.

EA-3 -Streak camera

Another approach for picosecond optical absorption detection is to use a light source with an intensity stable for a time period much longer and an optical detector with a response time much shorter than the absorbance variation to be studied. For this purpose, an original setup was developed at ELYSE based on a high-repetition Xe flash lamp 23 coupled with a high dynamic range streak camera used in absorbance mode, both being synchronized with the electron pulses. This is easily seen on streak images of the Cerenkov light (measurements for subtraction of the Cerenkov light from the absorbance signal or for the estimation of the duration of the electron pulse). At least for the highest time resolution, the time origin is therefore taken from the ultrashort Cerenkov emission signal at the corresponding wavelength. From a single shot of the accelerator, the transient spectrum at any time (sweep time/full screen 500 ps to 1 μs) and the time evolution of the short-lived species at each wavelength (from 250 to 850 nm in 360 nm sections) can be simultaneously obtained. However, when the 360 nm spectral range is recorded at 500 ps full sweep, a fogging effect appears due to the high light intensity required to get an image. This is why the highest resolution is only obtained by recording a restricted wavelength domain (± 5 nm) so that the fogging effect is negligible. Work is in progress to evaluate this effect using known reference systems, and thus to obtain the highest time resolution over the entire spectral range. The light source consists of a home-made pulse generator driving a commercial Xenon lamp having a high and constant maximum intensity for a period of about 500 ns, with a rise time of 2 μs and a repetition rate up to 25 Hz. 23 The cross section of the analyzing light beam is smaller (0.2 cm) than that of the electron beam and both are collinear. Alignment of the electron beam relative to the probe light beam is optimized by adjusting the magnets to maximize the reference absorbance signal (generally of the hydrated electron) recorded by a control photomultiplier equipped with filters selecting the wavelength range. The photomultiplier is located in the detection room at the arrival of the probe beam and systematically used to control the optimization of the absorbance before reflecting the beam to the spectrograph-streak camera set-up (Figure 2.21). The major advantage of using such a device is to obtain in a single pulse the information both on kinetics and on the transient absorption spectrum. The method is particularly well adapted to the spectral identification of the transients and to an overview of the chemical mechanism. This set-up can be used to observe transients from the end of the pulse up to rather long time, in the range of 500 ns, or with an adequate light source, up to 1 ms full scale for longer-lived species. sample to be studied: firstly, the background noise of the camera is recorded in the absence of any light signal (BRUIT) (Figure 2.22 c), then the analysis light is recorded alone (BASE) (Figure

b).

The third measurement is made on presence of the electron beam sent into the cell and the analysis lamp. Thus, the light of analysis is absorbed not only by the formed species, but also by the Cerenkov light itself (ABS) (Figure 2.22 a). In order to eliminate it, the Cerenkov light is recorded alone by sending the electron beam into the sample without any analysis light (CER).

Thus, the Optical Density can be calculated from the following Beer-Lambert law:

log BASE BRUIT OD ABS CER       EQ.2-3
The time-resolved absorbance of the species in 2D image is determined (Figure 2.22 d).

Generally, the data collected is acquired by averaging 400 pulses (or images) to improve the signal to noise ratio. Images are first received and averaged by Hammamtsu software in the streakcamera. In order to work simultaneously, the computer that drives the streak camera does not store the data but sends it directly to a second computer that has 3 fast SCSI hard disks. Thus it is possible to average the 400 images of an acquisition on a hard disk instead of acquiring another series of images on another hard disk.

t ns nm t ns A Dose Gy Gl         EQ. 2-4
 Gt=3 ns is the radiolytic yield of solvated electron at 3 ns, i.e., 3.4 × 10 -7 mol J -1 .

 660nm   
is the molar extinction coefficient value of solvated electron at 660 nm, i.e., 18 000

M -1 cm -1 .
 L is the length of the optical path in the optical cell, i.e., 1 cm.

 ρ is the density of water, i.e., 1 g mL -1 . So, the dose could be determined as:

Dose Gy Gy

      a)
b)

It should be noted that according to the purpose of the experiment, the dose can be reduced either by placing neutral densities in front of the laser in order to lower its intensity and thus generate fewer electrons; or by changing the position of a lens on the laser path to focus the beam more or less at the photocathode. All these manipulations are done without access to the accelerator room during the experiments from the control room.

γ-source

Cobalt-60 ( 60 Co) is the most common gamma radiation source. It is synthetic radioactive isotope of cobalt with a half-life of 5.2714 years, 25 produced artificially in nuclear reactors. [START_REF] Stuart | Pulse Radiolysis Studies of Liquid Heavy Water At Temperature up to 250[END_REF] Co decays by beta ray to stable isotope nickel-60 ( 60 Ni). The activated nickel nucleus emits two gamma rays with energies of 1.17 and 1.33 MeV, hence the overall nuclear equation of the reaction is: Co source installed in Laboratoire de Chimie Physique, Université Paris-Sud is a panoramic irradiation source of 7, 000 Ci (Curie). The container is shaped as a cylinder and the [START_REF] Stuart | Pulse Radiolysis Studies of Liquid Heavy Water At Temperature up to 250[END_REF] Co source is placed in a lead jar (Figure 2.24). The system could be controlled by an operator far distance. The source is located in a chamber with thick concrete wall, where the door is made of lead. A zigzag passage is designed among those walls on the purpose of protection. The sample would be fixed on the plate when irradiated by [START_REF] Stuart | Pulse Radiolysis Studies of Liquid Heavy Water At Temperature up to 250[END_REF] Co source. The dose rate depends on the distance to the source and determined by Fricke's dosimetry. 26 For the dose deposited in the sample, it can be estimated as in water as following: Dose (Gy) = Dose rate (Gy / min or Gy / h) × Time (min or h) EQ. 2-7

SK-Ana -MCR-ALS analysis method

In transient absorbance spectroscopy, measurements are usually performed for all the wavelengths for a given pump-probe time delay. Consequently, time-resolved absorption spectroscopy data values consist of a series of such time-gated spectra. The spectra are usually arranged in a two-way structure (a table or a matrix), where each row corresponds to a full spectrum recorded at a certain time delay and each column to the variation of the absorbance at a particular wavelength as a function of the time delay. In an ideal situation, the underlying model of the spectroscopic measurements in D would be written as a sum of the contributions of the spectra of (second-order global analysis). 28,29 Indeed, the bilinear model in EQ. 2-8 can be extended to describe the structure of the measurements of the different experiments forming a multiset. The only structural requirement when performing multiset analysis is that the information on one of the modes of the data sets, e.g., the spectral information for a column-wise data set, be the same for all the multiset structure (Figure . 2.26). In practice, multiset analysis may help to determine the correct rank of the data matrix 30 and, thus, the components of the model for data description. Müller et al. 31 In my thesis, the data treatment was involved with a multivariate curve resolution alternating least squares (MCR-ALS) approach -SpectroKinetics Analysis (SK-Ana). [START_REF] Muroya | A Re-Evaluation of the Initial Yield of the Hydrated Electron in the Picosecond Time Range. Chapter III: Time Dependent Yield of the Solvated Electron and the OD • Radical in Deuterated Water at Low LET Studied by Picosecond Pulse Radiolysis 99[END_REF] As in this type of system, the spectra of the absorbing species strongly overlap that causes the difficulty to unmix and sort out individual spectra, a global data analysis approach was used. Global matrices were built by delay-wise binding matrices with the data matrices for different time resolutions and concentrations. In this configuration, the components spectra are common to all the experiments in the global dataset, while each experiment has its own set of kinetic traces. The number of absorbing species in a matrix was assessed by Singular Value Decomposition (SVD) of the matrix, and a MCR-ALS analysis with the corresponding number of species was performed. Positivity constraints were imposed for both spectra and kinetics, and the shape of some spectra can be imposed. Based on quantum mechanical information, unimodality constraints can also be imposed to the spectra. All the calculations have been made with codes developed in-house in the R environment. 33 Our MCR-ALS code was adapted from the ALS package by K.M. Mullen. 34

k different components (

Chemicals

All the chemicals were purchased from Sigma Aldrich. Water for dilution or dosimetry was purified until the value of conductivity reaches 18 MΩ.cm by passage through a Millipore purification system. Table 2.1 gathers the properties of the chemical products used in the thesis.

Concluding remark

The primary events as a result of the interaction of ionizing radiation with matter mostly occur in ultra-short time. Thanks to the development of technology, pulsed radiolysis gives us access to study the radiation chemistry within ultra-short time scale in terms of the behavior of different solvents vis-à-vis their interactions with ionizing radiation, the physical and chemical characteristics of the formed radical and molecular species, the formation mechanisms of radical and molecular species, etc. The picosecond pump-probe radiolysis facility ELYSE is a powerful tool to study the ultra-fast chemical reaction induced by ionizing radiation. In the course of my thesis, the work was mainly performed in ELYSE. The setup-picosecond pulse-probe and streak camera, were employed to collect the time resolved absorption matrix data. The UV-Vis absorption spectra of methyl viologen cation radical in water and deuterated water was obtained by irradiation with γ-source. MCR-ALS analysis method SK-Ana was involved in data treatment. , OD • , D • , and the molecules D2, and D2O2 in irradiated D2O. However, there is important controversy for the time dependent yield of these radicals in deuterated water D2O. radiolysis measurement was performed by microsecond X-ray pulse radiolysis giving the primary yield (3 × 10 -7 mol J -1 ) and the extinction coefficient (20200 L mol -1 cm -1 at 700 nm) of the hydrated electron in D2O. 11 In 1975, G. R. Freeman first reported the primary yield (homogenous step) of the hydrated electron to be 2.7 ± 0.1 in H2O and 3.1 ± 0.1 in D2O using SF6 as scavenger at 23 °C, and obtained similar results with N2O scavenging of hydrated electron. 12 Later, transient conductivity experiments were used to measure the primary yield of the solvated electron exhibiting also a higher value in D2O (3.36 ×10 -7 mol J -1 ) than in H2O (2.83×10 -7 mol J -1 ). 13 Chase and Hunt observed the spectrum of 2 DO e  slightly shifted towards the blue as compared with that in H2O, and deduced the primary yield and extinction coefficient of 2 DO e  based on previous work with salicylate anion as scavenger. 14 It was reported that Gℰmax of 2 DO e  is higher than in H2O.

Later Freeman et al. also confirmed this point. 15 By measuring with a 30 ps time resolution, the decay of solvated electron in D2O and the mixture of D2O and water, it was concluded that the slower decay of the solvated electron in neat D2O was due to a larger distance that electron travels before its solvation. 16 That means the geminate recombination is less effective in D2O. In 2001, diffusion-kinetic calculations were analyzed to understand the isotopic effect in the radiolysis of water (Figure 3.2). 17 The study showed that the initial yield (at ps range) of s e  in H2O and D2O is equal, but the spur decay of es ⁻ is slower in D2O, resulting in a higher yield of 2 DO e  at 10 -7 -10 -6 s with low LET (linear energy transfer) radiolysis. The recent picosecond pulse radiolysis measurements of hydrated electron also found a similar initial yield of es ⁻ in H2O and D2O in 2012. 5 To have a better understanding of the evolution of 2 DO e  , the related knowledge has been remarked in Table 3 * max G in D2O was reported 25% higher than that in H2O.

However, due to its low absorbance, there is not so much knowledge on the kinetics of OD • at picosecond range. The first absorption spectra of OD • from 190 to 260 nm was given by Nielsen et al. in 1976. 20 G.V. Buxton et al. performed similar measurements from 220 to 320 nm. 21 The two absorption spectra were consistent. Buxton's simulations with low LET, predicted that the initial yield of OD • and OH • are similar (Figure 3.4). 17 )

Wavelength (nm) 20 and G. V. Buxton (in pink). 21 The solid lines are the guide to eyes.

Objective

To have a better understanding of the earliest mechanisms induced by ionizing radiation in D2O, it is important to determine the time dependent yield of

The solvated electron in D2O

As known the transient absorbance of

2 DO e  equation can be expressed as:

2 2 2 ( , ) ( ) ( )
            D O D O D O e e e A t D G t l EQ. 3-1
In the formula, l is the optic path length and  represents the density of D2O. To determine the dose deposited in neat D2O, it can be estimated as:

2 22 2 2 2 () ( ) ( ) 1 ( ) ( ) ( ) e e N H O D H O M D O D D O M H O N D O       EQ. 3-2
In this equation, M the molar mass, and e N  the total number of electron per molecule. Then, with the acceptor is stoichiometric by using the invariance of the absorbance. 23 The accurate determination of an isosbestic point requires that the time evolution of the full absorption spectra can be observed, in order to be able to examine the various wavelengths and find out the wavelengths which keep stable with time. In this method, the solute which scavenge Methyl viologen dication (MV 2+ ) can scavenge the solvated electron. 24 The product of the reaction is the radical of the methyl viologen cation (marked as MV •+ ), which has a long life time and strong absorption both in UV and visible region. Camera is lower than that measured by γ source because the resolution of streak camera is not enough high to detect the narrow peak of MV •+ around 394 nm.

Similarly to the approach previously used for the hydrated electron in water by J-L. Marignier et al, 27 in deuterated solutions containing methyl viologen and tert-butyl alcohol, the kinetics were measured at different wavelengths. To avoid any other competitive reactions, 0.5 mol L -1 tert-butyl alcohol was added to the solution to scavenge OD • radicals:

    3 2 3 32 ˙ ˙ OD CH COH CH CH OH HOD    EQ. 3-4
Assuming that the rate of this reaction in D2O is similar to that in H2O i.e. 6 × 10 8 L mol -1 s - 1 28 in our conditions, the halftime of the OD • scavenging is around 3 ns. In these conditions, the decay of the hydrated electron is only due to the reaction with methyl viologen. In fact, the hydrated electron might exclusively react with MV 2+ that is in large excess (5.5 × 10 -4 mol L -1 ) while the concentration of the hydrated electron is only few 10 -5 mol L -1 compared to the hydrogen atom (with a concentration per pulse 4.5 times lower than that of the hydrated electron, the rate constant of this reaction is 6.0 × 10 8 L mol -1 s -1 in H2O) as well as by the radical of tertbutyl alcohol (with almost the same concentration per pulse than hydrated electron) can occur but only at longer time (The rate constant of MV 2+ and tert-butyl alcohol in H2O is 1 × 10 7 L mol -1 s -1 ). 29 However, these reduction reactions are negligible during the reduction of MV 2+ by the hydrated electron. Therefore, as recently reported by Marignier et al. 27 As an example, three kinetics are reported within 150 ns in Figure 3.10. During the first 100 ns the signal is stable at 415 nm while decreasing at 420 nm and increasing at 408 nm, respectively.

The increase of the absorbance at 410 nm and the decrease at 420 nm show that the absorption coefficient of 2 DO e  at 410 nm is lower and at 420 nm is higher than that of MV 415 nm constitutes an isosbestic wavelength, consequently the molar absorption coefficient of the hydrated electron in deuterated water is the same than that of the MV •+ at 415 nm. The molar absorption coefficient spectrum of MV •+ with a maximum of 13700 ± 300 L mol -1 cm -1 at 606 nm in H2O, has been investigated by Watanabe and Honda. 26 According to their work, we calibrated the measured spectra recorded after 100 ns, and we evaluated that the molar absorption coefficient of hydrated electron in deuterated water at 415 nm is 3000 ± 200 L mol -1 cm -1 .

                D O D O neutral e e OD OD A t D l G t G t EQ. 3-6 ( , ) ( )          acid OD OD A t D l G t EQ. 3-7
From the difference of these two absorbance the extinction coefficient shape of the

II. Visible and NIR region

For the wavelength range from 400 nm to 1500 nm, as only The maximum of the absorption band of the hydrated electron is found at 705 nm with the extinction coefficient of 22900 ± 400 L mol -1 cm -1 . The Appendix Table at the end of this chapter reports the numerical value for the absorption band of 2 DO e  from this work. Note that contrary to the present case, in the case of hydrated electron in H2O by using MV 2+ as scavenger three isosbestic points were found. The reason is that the value of the extinction coefficient of the solvated electron in D2O is higher than that of the hydrated electron in water, which was recently determined to be 19700 ± 400 L mol -1 cm -1 at 713 nm. 27 Chapter III: Time Dependent Yield of the Solvated Electron and the OD • Oscillator strength is a dimensionless quantity that expresses the probability of absorption or emission of electromagnetic radiation in transitions between energy levels of an atom or molecule.

It is of transition that could be expressed as: Where ()

 is the molar extinction coefficient in M⁻ 1 cm⁻ 1 , and  is the wave number in cm⁻ 1 . With EQ. 3-8, we found that the oscillator strength is larger in D2O than in H2O. The oscillator strength of the absorption band of the hydrated electron at room temperature in H2O was reported to be 0.75 15 and later 0.85 31 . The value of the oscillator strength in D2O estimated by Jou et al. 15 is almost the same as in H2O. The experimental data reported in Figure 3.15 give the value of 0.76 (according to the recent extinction coefficient value in H2O) 27 and 0.83 for D2O (from the present work). It is important to note that the values obtained here took into account the UV component of the absorption band only up to 4 × 10 4 cm -1 (250 nm). Therefore, the oscillator strength can be slightly larger than our value. The kinetics measured at 600 nm in neat H2O and neat D2O are reported in Figure 3.16. The measured absorbance of solvated electron at 600 nm in D2O is 1.20 ± 0.01 times larger than that in H2O. The difference can be either due to the extinction coefficient or due to the radiolytic yield.

The kinetics within the first 500 ps superimposed by normalization show that there is no important isotopic effect on the decay of solvated electron within 500 ps (Figure 3.16 inset). The decay is almost same within the first 500 ps. It is worth noting that the dose per pulse can have an effect on the time dependent yield of solvated electron. In 1975, J E. Fanning gave the most cited results about the accelerated decline of the solvated electron from 40 ns to 1 µs. 33 The decay of the hydrated electron which accelerates beyond a few tens of nanoseconds has been interpreted by Trumbore et al. as an effect of the dose per pulse described essentially in terms of density, diffusion and cluster overlap. 34,35 These authors

Time-dependent yield of OD • radical

As described in (220 ~ 320 nm), respectively. Moreover, the results of both of their works are consistent. To obtain the time dependent yield of OD • radical, first of all it is essential to find the condition to measure the time dependent decay of OD • radical. For this purpose, the measurements were performed at 260 nm by picosecond pump-probe.

The absorption measured at 260 nm in neat D2O by picosecond pulse radiolysis is due to three absorption components: the solvated electron, the transient produced in the solid statethe windows of fused silica optical flow cell (marked as FSOFC), and the OD • radical. Therefore, the measured absorption can be written as:

260 260 260 260 ( ) ( ) ( ) ( ) s FSOFC e measured OD A t A t A t A t              EQ. 3-9
                EQ. 3-10
For the purpose of deducing the transient absorption of OD • at 260 nm, the absorption due to FSOFC also needs to be confirmed and then subtracted in first approximation from EQ. 3-9. In 1950s, the coloration of the fused silica after irradiation was reported by several works. The nature of the corresponding absorption bands is studied from then on. [37][38][39][40] On 1976, the transient absorbed spectrum in fused silica from 180 to 300 nm was measured by Nielsen et al. 20 ), is higher for an empty cell than for a cell full of water. The difference between the absorbance depends on the optical path length. the probe beam at 260 nm and the supercontinuum (for example at 600 nm, the wavelength used for measuring the dose per pulse/ the solvated electron concentration) are not probing the same volume of the sample, a dose difference could occur and the value of the yield of the radical OD • could be affected. As the diameter and the position of the probe beams at 260 nm and 600 nm are not necessarily exactly the same, a pinhole is positioned with a diameter of 300 µm located just after the sample and before the detection system (Figure 3.23 Bottom). The pinhole ensured that the same irradiated volume of the sample would be probed and in consequence that there would be the identical dose per pulse for probing at 600 and 260 nm. Due to the attenuation of probe beam intensities by the pinhole, the acquisition time had to be increased in order to maintain a low noise level, which is necessary to detect the transient signals on the low mOD level.

Transient absorption induced in FSOFC

To gain better understanding of the effect of the scattering of the electron, we performed experiments in the empty optical cell and a fused silica plate with a thickness of 200 µm. For the same dose per pulse, the kinetics were recorded in empty optical cell ( 260 () In the same dose condition, the transient absorbance induced in the windows of the empty optical cell is not two times higher than that induced in the single fused silica plate of 200 µm. The absorbance induced in the empty optical cell, 260 () FSOFC At  , is 1.5 time larger than that in single fused silica plate (1.5 ×10 -3 OD). As the fused silica state was placed in the same position with the entrance of FSOFC, the dose received by the exit window (the second window) is almost two times less than that received by the entrance one (the first window) when the optical cell is empty. When In collaboration with Z. Zong at the Institut de Physique Nucléaire d'Orsay, the electron beam scattering by water in the optical cell was estimated by Monte Carlo (MC) simulation with EGSnrc code, 43 which is a general purpose program to simulate the coupled transport of electrons and photons in an arbitrary geometry. Particles with energies above several keV up to hundreds of GeV can be processed in EGSnrc. For easily modeling the geometry, the EGSnrc C++ class library 44 is used to follow the corresponding geometric layout of the experiments. In the simulations, the electron beam exiting from the accelerator is set as a circle shape, and the electron density distribution follows a Gaussian function centered at z = 0, with a sigma value of 0.1 mm for the radius of the circle. Inside the sample FSOFC, air-filled and water-filled cases were simulated, respectively.

As the dose deposited in the windows of FSOFC is sensitive to the offset from the beam axis (z-axis), the dose scoring regions are set as cylinders centering on the beam axis, with different radius in the first and the second FSOFC windows. The integral dose as a function of the radius can be obtained as D1 (r) and D2 (r) for the entrance and exit windows, respectively. Thus, the ratio As the absorbed dose is proportional to the irradiated surface, it means that the dose is around 4 time smaller at the exit window than at the entrance one. Therefore, the absorbed dose at the exit window should be between 3 ~ 4 times less according to EGSnrc calculations and beam imaging, respectively. As the signal of the transient induced in the entrance window is already low (1.5 ×10 - 3 OD), the contribution of the second one is within the precision (~0.001). Therefore, in water filled FSOFC, the value of the dose received by the windows is almost equal to that received by the first window. According to the above consideration, in our condition the absorbance due to the radicals produced in liquid can be found by using the equation: By taking into account these pointsthe consideration of the electron beam scattering in the cuvette and a precise positioning and sizing of the two probe beams with a pinhole, it becomes possible to measure precisely the time dependent yield of OD • radical. We improved investigations in the impact of the transient absorption induces in the fused silica and the solution inside in this work. As discussed, the absorbance due to the fused silica is particularly important in the UV region, even when the thickness of the windows of FSOFC is reduced to 200 µm. Moreover, consideration of the scattering effects not only by the fused silica but also by the solution in the treatment make the determination of the absorbance of the species absorbing in UV e.g. OD • radical correctly. If the scattering effects are not taken into consideration in the subtraction, important effort on the analysis can be the consequence. A flow jet without undesirable contribution and the necessary post-acquisition data treatment could be more applicative in the critical case.

Conclusions

The solvated electron is a unique species that can be produced in most polar and low polar solvents. The study of its properties is an important key factor to understand the dynamics and kinetics reactions in solutions. Moreover, the hydrated electron in neat D2O is used for the measurements at high temperatures to study the temperature effect on the radical yield and the spur kinetics. 15,[START_REF] Stanbury | Reduction Potentials Involving Inorganic Free Radicals in Aqueous Solution[END_REF] Then the knowledge of its properties and its reactions in various solvents is a key issue in physical chemistry. In radiation chemistry, the measurements of the absolute value of the radical yields at shortest time is obviously necessary to obtain two important information: the energy deposition in the spurs of ionization and the yield of secondary radical which can be produced when the solutes are present in the solutions. For this objective, it is necessary to obtain two independent parameters of the hydrated electron in neat D2O: the initial yield and the absorption spectra with molar absorption coefficient. Here, careful and systematic measurements, by taking into account the absorption induced in the windows of the optical cell and the radical hydroxyl, allowed us to determine precisely the absolute absorption spectra of the hydrated electron in D2O from 250 to 1500 nm. The shape and its amplitude are different from those reported previously in the literature. The amplitude of the absorption band as found here is different by 10% from that reported in the literature. 11 Additionally, the shape of the absorption spectra of the solvated electron reported by Freeman often used is not accurate. For example, the ratio of max () s Ge 

 between D2O and H2O in Freeman's work is 1.25, but in this work it is 1.15, and the extinction coefficient value of the hydrated electron in D2O at 300 nm is almost 20% higher than that reported by Freeman(Table 3.4). 15,22 those found in H2O, but the kinetics of the solvated electron are slower in D2O than in H2O.

Meanwhile, the kinetics of the hydroxyl radical presented no difference in D2O and H2O. The isotopic effect was explained initially by the fact that the initial spatial distribution between electron and hole in D2O is 1.18-1.41 times larger in D2O than in H2O. 13 However, laser photolysis measurements 16 showed that the main reason of slower decay of the solvated electron in D2O is due to the lower rate constant of the two main reactions responsible for the decay of the solvated electron in water at short time: But the yields are not comparable when the solvated electrons are produced by laser photolysis or by ionizing radiation. In our work, thanks to the determination of the initial yields (7 ps) of the hydrated electron and of the hydroxyl radical that are found to be same in neat water and in neat deuterated water, it can be concluded that the geminate recombination reactions are not affected significantly in D2O. Therefore, the slower decay can be explained well by the change of reaction rate of EQ. 3-13 and 3-14. The reaction EQ. 3-14 is diffusion controlled and as the diffusion coefficient is lower in D2O than H2O, the slower decay of solvated electron in D2O is to be expected. The reaction EQ. 3-13 is not fully limited by diffusion. In D2O, however, it needs higher reorganization energy and it can be considered slower in D2O than in H2O. Another point should be considered: due to the isotopic effect and the change of the vibration frequency which is responsible of hydroxyl radical formation through proton transfer, the formation of OD • through deuterated of D2O •+ should be slower than that of OH • and we had expected a lower yield of OD • and 2 DO e  at picosecond range. 10 Because the geminate recombination between electron and the radical water hole (D2O •+ ) should be favored when the proton transfer is slowed down by isotopic effect. However, the initial yields are found to be the same within the measurements precision.

Therefore, our conclusion is that the change of water hole lifetime does not affect significantly the geminate reaction. Liquid water is the key compound for our existence on this planet and it is involved in nearly all biological and chemical processes. The role of contemporary physics and chemistry of liquid water exposed to ionizing radiation (photon, fast electron, X-rays, heavy ions, etc.) is significant in such diverse fields as photon science, radiotherapy, nuclear reactors, radiation chemistry and nuclear waste management etc. [1][2][3] Since the discovery of X-rays and natural radioactive phenomena, the chemistry derived from water radiolysis has been studied intensively over the last one hundred years. The interaction of highly energetic photons or charged particles with water initially results in the ejection of a quasi-free electron from the valence shell in general, leaving behind a positively charged radical cation (H2O •+ ). The earliest time of H2O •+ formation is estimated to be on the timescale of attoseconds (10 -18 s) or subfemtosecond based on the uncertainty relationship (ΔΕΔt ≈ ħ). 4 In addition to their recombination, both of these two charged species (e⁻ and H2O •+ ) subsequently follow their own pathway of chemical reactivity. The hot electron relaxes into solvent molecules and gets trapped as a hydrated electron ( s e  ) whilst H2O •+ rapidly forms the oxidizing OH • radical via proton transfer.

H O H O OH H O

      EQ. 4-1
The chemical framework in which water radiolysis involving OH • and s e  occur is now well understood experimentally as well as theoretically. The OH • radical is thought to be mainly responsible for an important number of the radiation-induced oxidation that occurs in dilute aqueous solutions and biological system. Even if this statement is still consistent for an important number of situation, the ultrafast chemistry of its precursor, water hole (H2O •+ ) immediately following ionization of liquid water, is a puzzling and very challenging subject in radical chemistry induced by ionizing radiation. The situations where H2O •+ undergoes ultrafast one-electron oxidation in competition with proton transfer have important practical influences upon nuclear waste storage, nuclear fuel processing and radiotherapy, and remain to be clarified in order to better assess the safety concerns such as the component corrosion and hydrogen emission. Until now, real-time observation of this species H2O •+ is still lacking because of the limitation of the time resolution in currently available high energy pulse techniques to typical picosecond time scales. The proton transfer rate constant in the gas phase is about 8 × 10 12 L mol -1 s -1 . 6 The extrapolation of this constant by simply taking into account the concentration of water gives an extremely short lifetime which is estimated at approximately twenty femtoseconds, i.e. 18 × 10 -15 s. Indeed, ab initio molecular dynamic simulations suggested the lifetime of this radical cation in liquid phase is actually less than 40 fs. 7 Alternatively, the chemical reactivity of H2O •+ was arisen not long after the discovery of hydrated electron in 1962. Hamill et al. 8,9 suggested that holes in water could be trapped prior to hydration using sufficiently high concentrations of solutes such as NaX (X = Cl, Br, F, I) or sulfate salts. The high values for the yields of secondary radicals NO3 absorbance around 410 nm. The half-time of the ion-molecule reaction (EQ. 4-1) was estimated as 100 fs in light water, vs. 170 fs in heavy water (Figure 4.1). However, the charge localization step of H2O •+ towards water molecules was suggested to require less than 40 fs by theoretical simulations. 5 Nevertheless, very recently, a polarization anisotropy measurement estimated that the lifetime of this transient species in H2O is as long as 196 ± 5 fs (Figure 4.2). 11 It was suggested that after an ultrafast X-ray core ionization of water, a doubly charged species, with two positive charges located on different water units, [H2O •+ … H2O •+ ]aq can be formed. 12 This pioneering experimental and theoretical works brought insights into the electronic signature of this radical cation and prompted further investigations using more sophisticated dynamic technology with a time resolution of several femtoseconds or even better. In addition, the water cation radical produced in an inner hydration layer of DNA was considered as a direct-type damage based on the ESR (electron spin resonance) measurements at low temperature. 13 These early studies suggested that the H2O •+ radical could initiate oxidation processes both in the homogeneous solutions and in interfacial biological systems prior to the generation of OH • radical, but the results are very speculative because the time scales for these reactions are very fast (∼100 fs). Moreover, as the direct effect of radiation on the solute produces similar chemical species with those produced by the oxidation of the solute by H2O •+ , difference between these two reactions was not obvious, and the role of H2O •+ in electron transfer reaction remained controversial. In general, the direct measurement of the H2O •+ decay has not been performed in pure water. Additionally, pulse radiolysis techniques can also produce the strong oxidizing SO4 •-radicals to study the one-electron oxidation reactions when the solution contains appreciate amount of NaS2O8 salts ( s e  + S2O8 2-→ SO4 2-+ SO4 •-). 15,16 In this case, the redox potential of DNA bases (T, G, A, C) radical cation was subsequently measured, indicating the sites of hole transfer along the DNA duplex and how easily the DNA can be oxidized by free radicals. 17 In radiation chemistry, the majority of fast radicals or electrons-induced reactions occur at a diffusion or nearly-diffusion controlled process with a second-order rate constant ranging from 10 9 -10 10 L -1 mol s -1 under ambient conditions. 20,21 For this reason, over past decades, pulse radiolysis based on a nanosecond electron pulse has been shown to be sufficient to fully resolve numerous radicals' reactions on their own reaction time. However, as mentioned above, the water radical cation H2O •+ proceeds the chemical events much faster than 1 picosecond and hence a deep understanding of this radical's chemistry called for a shorter electron pulse. The femtosecond laserdriven accelerator has been established as our ELYSE facility, which achieves a high energy (7-8

MeV) electron beam with a pulse width of 7 ps. 22 The detecting system is based on the transient absorption spectroscopy with a probe light ranging from 380 nm to 1500 nm. 23 Similar picosecond pulse radiolysis facilities have been established over the world, such as the LEAF facility at Brookhaven National Laboratory and LINAC facility at Tokyo University etc, and many of them have reached their full capacity. 24,25,26 Therefore, this unique time resolved technique based on high energy electron pulse enables us, although not ideally, to explore the ultrafast chemical reactivity of H2O •+ through the scavenging method in a variety of highly concentrated aqueous solutions.

Accordingly, a short review of the key findings is presented to show the occurrence of ultrafast electron transfer of H2O •+ by using highly concentrated solutions. By observing the formation of secondary radicals such as NO3 • , SO4 •-, X2 •-(X = Cl, Br) or H2PO4 • at picosecond scale, the radiolytic yield of H2O •+ participating in the reaction as a function of concentration is obtained and isotopic effects have been comparatively studied. Our results imply the chemical situations where the reactivity of H2O •+ or D2O •+ plays a more important role in competition with proton transfer to generate OH • or OD • radicals. By further using the diffusion-kinetic simulations of the spur reactions induced by the incident electrons, we established the first semi-quantitative estimation of the H2O •+ radicals scavenging fractions for a wide range of solutes. In addition, it is suggested that H2O •+ is the strongest oxidative radical comparing with other oxidants in aqueous solutions based on an estimation of its redox potential value. Finally, it is rationalized that the oxidation of H2O •+ might be extended to the interface chemistry or biological system which constitutes as a similar water closely trapped environment with that in concentrated solutions.

Oxidation reactions by radicals

Oxidation reaction is a very important class of chemical reactions in solution. One example is corrosion, that is to say, the alteration of a material by an oxidant, or oxidative stress which is a type of aggression of the constituents of the cell, involved in many diseases. 27,28,29 Some oxidizing species such as O2 are stable, but there are also many other reactive oxidizing species, which are not stable in solution. The very oxidizing free radicals commonly known in aqueous solutions include OH • , O2 •-, HO2 • , CO3 •-, NO3 • , SO4 •-and HPO4 •-etc. and can be generated in a radiationchemical procedure. 30 These reactive radical species play a very important role because they act as intermediates in many mechanisms. Free radicals instinctually associated with unpaired electrons in their outer orbitals react with organic or inorganic compounds in various ways. They may abstract a hydrogen atom or add to an unsaturated bond, and they may also reduce or oxidize a substrate via one-electron transfer mechanism. Therefore, the chemistry of free radicals is very rich. In particular, their reactivity is very much studied in the metabolic system of the human being where they can trigger serious diseases. 31 But some of these radicals can also be used to treat certain diseases, such as cancer using radiation therapy. Because of their very high reactivity, the lifetime of these species is often very short, ranging from milliseconds to picoseconds. Their lifetime depends very much upon their environment. To observe the reactivity of these species, it is necessary to resort to sophisticated experimental setups that allow the monitoring of their reactions, sometimes ultrafast, as a function of time. Thus, to create these radical species, lasers or pulsed electron accelerators are often used, and coupled with a time-resolved detection method, thus exploiting the optical properties of transient species.

In past decades, numerous studies have been performed in relation with their properties using pulse radiolysis or photolysis methods. [START_REF] Muroya | A Re-Evaluation of the Initial Yield of the Hydrated Electron in the Picosecond Time Range. Chapter III: Time Dependent Yield of the Solvated Electron and the OD • Radical in Deuterated Water at Low LET Studied by Picosecond Pulse Radiolysis 99[END_REF] The term of oxidizing a molecule means capturing an electron from another molecule. The oxidation force of a species is defined by its redox potential.

The redox potential is an empirical quantity expressed in volts and denoted by E° for the standard potential under conditions of normal concentration. 33 The values of the redox potential are expressed with respect to the reference potential of the standard or normal hydrogen electrode (NHE) taken equal to zero. 2,34 The higher the value of the redox potential is, the more oxidizing the species is. For example, molecule oxygen is an oxidative species and forms O2 -by capturing an electron of a molecule and the redox potential of the O2/O2 The values of the redox potential of some transient radical species are reported in Figure 4.4.

It is determined kinetically by pulse techniques according to their ability to oxidize molecules of known potential. In this series, the OH • radical in acid medium is a very strong oxidizing radical in aqueous solutions, comparing with others, such as SO4 •-(2.43 eV) or H2PO4 • (2.40 eV). 36 It is reasonable because OH • radical is able to oxidize solutes such as SO4 2-leading to the formation of the secondary radical SO4 •-. In water radiolysis, however, the oxidizing ability of its precursor (H2O •+ ) due to its ultrafast proton transfer has not yet been considered. We use a simple thermodynamically cycle for a rough estimation of the redox potential of these radicals. The ionizing potential of a liquid water molecule is measured to be 11.16 eV by liquid-jet photoelectron spectroscopy, which is in accordance with the value (11.7 eV) from theoretical predictions by Mozumber. 37 The solvation energy of H2O •+ is unknown but it is assumed that this energy value is similar with that of H3O + . In light of this, the standard redox potential of H2O •+ / H2O couple is higher than 3 eV vs NHE, showing it is indeed the strongest oxidizing species in liquid water (Figure 4.4).

Comparing with the high mobility of electrons produced in water radiolysis, the mobility of H2O •+ is considered to be relatively low as the infrared spectroscopy study of water cluster radical cations (H2O)n •+ (n = 3-11) showed that it is localized and weakly hydrogen bound with its neighbouring water molecules. 38 The role of H2O •+ in previous radiation chemical studies of dilute solutions was completely ignored. In fact, its chemistry strongly depends on the environment and much differ from that of OH • radicals, which often reacts with a substance via H abstraction or addition instead of one-electron transfer. 39 One may wonder how a radical cation that has such a short life can oxidize another species. Indeed, before reacting, the radical and the species that will be oxidized should diffuse to get closer to each other. But even if the electron transfer reaction is ultrafast, the diffusion of the reactants through the solvent molecules would require a minimum time (a diffusion limit of the rate constant). For example, the radical OH • , which has a very large diffusion coefficient (D = 2 × 10 -9 m 2 s -1 ), 21 must nevertheless diffuse during about 1 μs to reach a molecule in solution at a concentration of the order of 10 -4 M. If the solute is in 1 molar concentration, this diffusion time is reduced to 60 ps. Under these conditions, the reaction is not likely to take place for the radical cation H2O •+ because of its immediate proximity with H2O and its proton transfer reaction has a lifetime much shorter than the time required for diffusion.

Therefore, the only possibility of reaction for this radical is the in situ reaction. That is to say, this radical must be in contact with a target molecule in a concentration of similar magnitude to that of water. Thus the diffusion is no longer necessary and it can possibly oxidize the molecule which is in its immediate vicinity. The verification of the occurrence of the ultrafast electron transfer of H2O •+ has initially been made in aqueous NaX (X = Cl, Br) solutions by picosecond pulse radiolysis. [40][41][42][43] In these systems, the energy in proportion to the electrons fraction, is also deposited on the solutes, so the direct ionizing of the solute itself cannot be avoided, resulting in the additional yield of secondary radicals (X • or X2 •-). To make it easier to be understood, it is worth to note that in radiation-induced chemical reactions, the amount or concentration of radicals generated is associated with the radiation dose. The radiolytic yield or a more specific name G value is defined as the number of mole produced per joule of absorbed dose. Halide and concentrated acidic aqueous solutions are proper system to probe the reactivity of H2O •+ .

As noted above, there is additivity between direct and indirect effects of ionizing radiation in the sample. The energy deposited in the solute is related its electron fraction. Consequently, for a given concentrated aqueous solution the observed radiolytic yield can be expressed as follow:

obs obs A G l Dose F      EQ.4-2 s = + (1-) obs dir s indir G f G f G EQ.4-3
Here, obs G is the observed radiolytic yield , dir G is the yield of the direct effect expressed in the molecules formed per Joule absorbed by the solute. It is assumed to be independent of the concentration of the solute S. indir G is the maximum yield of the indirect effect. It depends essentially on the yields of the primary species formed by water radiolysis which react with S give the considered transformation. s f is the electron fraction of the solute, and (1s f ) is the electron fraction of water. F is the dose corrected factor which is related to the density and the electron concentration of the solution. 44 It could be determined as follows:

2 2 2 2 2 100 ( ) ( ) ( ) ( ) () 100 () () (
)

ee e pp N Solute N D O M solute M D O d solution F d H O N H O M H O          EQ.4-4
Where d is the density, e N  is the electron number, p is the weight of the solute in 100 g of solution,

(100-p) is the weight of the solvent, i.e. D2O in 100 g of the solution, and M is the molecular weight.

Accordingly, the indirect effect of water cation radical could be deduced.

Reactivity of H 2 O •+ in halide solution

Several studies in the highly concentrated Br -and Cl -solutions have been performed on the purpose of capturing H2O •+ . [40][41][42][43] As an example in Cl -aqueous solution, the products of radiationinduced oxidation of Cl -ions are observed at 370 nm as shown in

Cl Cl Cl

    8.5 ×10 9 R8 22 Cl H O Cl H O       R9 Reactions Cl -/H3O + 2 ClOH H Cl H O       R10
The reaction R5 is due to the direct effect that means one-electron oxidation of Cl -by ionizing radiation. It occurs when the number of electron of the solute in the solution is not negligible compared to that of water (Figure 4.5). It happens when the concentration is higher than around 0.5 M. The reaction R6 is issued from the water radiolysis. That reaction is not complete because the reaction R7 takes place. The reaction R8 is controlled by diffusion and finally, the electron transfer reaction R9 is ultrafast and can occur if the radical cation of water is formed in contact of Cl -. The initial absorbance reported in Therefore, it is concluded that the precursor of the OH • radical, i.e., H2O •+ radical, forms Cl • atom within the electron pulse and the Cl • atom reacts subsequently with Cl -to form 2 Cl  on very short time scales. It is important to note that the reduction potential of Cl -is very high and only a powerful oxidizing species can transform Cl -into Cl • (E° Cl • /Cl -=2.41 V, E° OH • /OH⁻ = 1.9 V in neutral solution) [START_REF] Stanbury | Reduction Potentials Involving Inorganic Free Radicals in Aqueous Solution[END_REF] .

Reactivity of H 2 O •+ in concentrated acidic solution

As presented in Figure 4.4, H2PO4 -, NO3 -and SO4 2-can be oxidized by H2O •+ according to their redox potential. Unfortunately, as the yield of the secondary radicals from direct ionizing is not quantitatively known, the exact radiolytic yield and reactivity of H2O •+ in various concentrations still remain to be elucidated. Recently, several studies have performed experiments by picosecond pulse radiolysis to probe the chemistry of H2O •+ in the aqueous solutions containing various concentration of SO4 2-, H2PO4 -and NO3 -. [START_REF] Ma | Picosecond Pulse Radiolysis of Highly Concentrated Sulfuric Acid Solutions: Evidence for the Oxidation Reactivity of Radical Cation H2O •+[END_REF]

I. H3PO4/H2O

In analogy to H2SO4 system, H3PO4 solutions also have the above-mentioned advantages.

Firstly, the oxidation of phosphoric acid by OH • is more difficult than that of sulfuric acid. The rate constant of the reaction between OH • and the phosphoric acid is low (10 4 L mol -4 s -1 , as shown in the Table 4.3). Second, the acidic form of phosphate radicals (H2PO4 • ) exhibiting an absorption peak at 520 nm with a molar extinction coefficient of 1850 Lmol -1 cm -1 is the indicator of direct and H2O •+ oxidation. Last and worthy to be noted, the phosphate group ubiquitously occurs in bio systems.

For example, if a biomolecule such as DNA is under the exposure of ionizing radiation, the sugar -phosphate backbone can be damaged either by direct detachment or by indirect oxidation from the water radicals and the phosphate centered radicals are expected to be formed with appreciable yields, leading to subsequent biological effects. [START_REF] Sevilla | Radiation Chemistry from Basics to Applications in Material and Life Science[END_REF][START_REF] Steenken | Photoionization of Organic Phosphates by 193 Nm Laser Light in Aqueous Solution: Rapid Intramolecular H-Transfer to the Primarily Formed Phosphate Radical. A Model for Ionization-Induced Chain-Breakage in DNA?[END_REF] The Phosphate radical reacts with numerous organic or inorganic compounds by abstraction or addition and also by electron transfer.

It is therefore important to estimate the yield of damage through the direct radiation effect and through oxidation by H2O •+ . Consequently, the picosecond pulse radiolysis measurements were further performed in the phosphate system. [START_REF] Ma | Picosecond Pulse Radiolysis of Highly Concentrated Phosphoric Acid Solutions: Mechanism of Phosphate Radical Formation[END_REF] Chapter IV: Ultra-fast Charge Migration Competes with Proton Transfer in the Early Chemistry of H2O As shown in Figure 4.8, the overall trend is found to increase with the electron fraction of solute and reaches the maximum in the neat phosphoric acid. As the oxidation of H3PO4 by OH • radical is too slow to occur on the picosecond time scale, the supplementary yield that could be deduced by EQ. 4-2 and 4.3, indir G , observed for each concentration is considered solely due to the scavenging of H2O

II. HNO3/H2O

HNO3 is also a suitable acid to study the chemistry of H2O •+ as it could generate nitrite radical, 3 NO  that exhibits a broad absorption band in the visible range. As shown in Table 4.4, it is worth noting that the rate coefficient of the reaction OH • and HNO3 is 4 times higher than that with H2SO4. For H3PO4, this reaction cannot be taken in account in picosecond radiolysis measurements because the reaction occurs in millisecond range. Furthermore, concentrated HNO3 is able to decompose to NO2, O2 and H2O at room temperature so it is necessary to pay particular attention to the concentration of HNO3. To date, several investigations have been performed to study the formation pathways of 3 NO  with different time scale pulse radiolysis and simulations. 10,40,[START_REF] Balcerzyk | Picosecond Pulse Radiolysis Study of Highly Concentrated Nitric Acid Solutions: Formation Mechanism of NO3• Radical[END_REF][START_REF] Garaix | Decay Mechanism of NO3•Radical in Highly Concentrated Nitrate and Nitric Acidic Solutions in the Absence and Presence of Hydrazine[END_REF] and the maximum scavenged yield of H2O •+ in sulfuric acid, (4.75± 0.2) ×10 -7 mol•J -1 , a value NO OH NO HO

       EQ.4-5
This reaction is in competition with the reaction R23, which is dominant at the high concentration of undissociated HNO3. Other decay reactions are rather slow to occur on a nanosecond time-scale.

III. H2SO4/H2O

Sulfuric acid solution constitutes an ideal system to verify the hypothesis and to investigate the reactivity of H2O •+ due to several reasons. First, it is a homogenous phase without molecules clustering, which guarantees the sufficient close encounters (H2O •+ … SO4 2-). [START_REF] Kameda | Hydrogen-Chapter IV: Ultra-fast Charge Migration Competes with Proton Transfer in the Early Chemistry of H2O •+ 160 Bonded Structure in Aqueous Sulfuric Acid Solutions[END_REF] Second, the yield of SO4 •-from the direct effect can be measured from the almost neat sulfuric acid (18 M) and it is possible to correlate this contribution to various concentrations by using the factor of the electron fraction. Third, as shown in Table 4.2, the rate constant of the reaction of OH • radicals with HSO4 - (or H2SO4) is relatively low (10 8 L -1 mol cm -1 ) and the reaction time is several nanoseconds at even the highest concentration. [START_REF] Tang | Kinetics and Spectroscopy of The SO4 -Radical in Aqueous Solution[END_REF] Fourth, the counter ion of acid is H + without valence electron. Thus, unlike the salt ions such as Na + or Mg 2+ ,the direct ionizing of H + itself does not yield to any oxidizing species. The last reason is that the spectrum and the molar extinction efficiency 1600 M - 1 cm⁻¹ on the maximum of secondary radicals SO4 •-are well-known, so the yield is ready to be deduced. 10,[START_REF] Ma | Picosecond Pulse Radiolysis of Highly Concentrated Phosphoric Acid Solutions: Mechanism of Phosphate Radical Formation[END_REF][START_REF] Jiang | Pulse Radiolysis Study of Concentrated Sulfuric Acid Solutions[END_REF] nm. The shape of this band does not depend to the concentration and no significant change in extinction coefficient was found in the concentrated sulfuric acid as previous reported. [START_REF] Kameda | Hydrogen-Chapter IV: Ultra-fast Charge Migration Competes with Proton Transfer in the Early Chemistry of H2O •+ 160 Bonded Structure in Aqueous Sulfuric Acid Solutions[END_REF] It only slightly shifts to higher wavelength in response to higher concentration owning to the acid equilibrium of SO4 •-to HSO4 • . As it is known the SO4 •-formation does not correspond to OH • radical at this observed timescale, only two possible pathways accounts for producing SO4 •- radicals, that is direct ionizing and ultrafast electron transfer from H2O •+ . From Figure 4.12, it can be seen that after dose correction, the absorbance of SO4 •-in relation with its radiolytic yield is increasing with the sulfuric concentration from 1 M to 12 M. This can be understood because the direct ionizing becomes more important as more solutes are present.

Interestingly, from 12 M to 18 M as water molecules are not abundant, the absorbance continuously drops. These observations clearly present an evidence that direct ionizing alone cannot be the only effect, interpreting the formation of SO4 •-is due to at least two pathways, otherwise the absorption would keep increasing. There must exist an ultrafast process of H2O •+ that occurs on the timescale of femtosecond, accounting for the supplementary yield of SO4 •-. The radiolytic yield of SO4 •-is directly deduced to be (3.75 ± 0.2) × 10 -7 mol J -1 from the 18 M sulfuric acid. Based on the electron fraction of solutes in solutions, the yield contribution of direct and indirect effect can be obtained according to EQ. 4-2 and 4-3. M is considered to be the direct effect yield and is linearly extrapolated to the zero concentration. The supplementary yield is considered to be due to the electron-transfer reaction with radical cation H2O •+ .

Performed by Ma et al. [START_REF] Ma | Picosecond Pulse Radiolysis of Highly Concentrated Sulfuric Acid Solutions: Evidence for the Oxidation Reactivity of Radical Cation H2O •+[END_REF] From the above analysis, the yield of H2O •+ , which participates in ultrafast oxidation is deduced and their values are plotted as a function of the concentration correlated with the electron fraction. The analyzed results in Figure 4. 13 show four striking characteristics. First, at very low concentration or in dilute medium, neither direct effect nor H2O •+ oxidation is important as the formation of SO4 •-is not observed. Second, the yield of H2O •+ is highly dependent on the concentration, particularly below 12 M, where we observed a continuously increasing trend with the rising of the solute concentration. This is in agreement with our predications and it suggests that the probability of electron transfer of H2O •+ is greater at higher concentration. Third, after 12 M, the yield is saturated to (4.75 ± 0. 5) × 10 -7 mol J -1 , which is found to be similar with the value of OH • radical yield reported at picosecond timescale in neat water. [START_REF] El Omar | Time-Dependent Radiolytic Yield of OH • Radical Studied by Picosecond Pulse Radiolysis[END_REF] In these cases, it can be concluded that after certain concentration, all of the precursors of OH • react with sulfuric solutes. The scavenged yield of H2O •+ in sulfuric acid solutions is compared with that in phosphate acid (Figure 4.14). It clearly shows that the saturated yield of H2O •+ is significantly lower compared with that in sulfuric acid solutions. The reasons why the water hole trapping in phosphoric acid is not efficient are not clear. However, on the basis of microscopic structure of phosphate solutions from X-rays diffraction measurements, [START_REF] Elmore | Dissociation of Phosphoric Acid Solutions at 25° 1[END_REF][START_REF] Caminiti | Phosphate-Water Interactions in Concentrated Aqueous Phosphoric Acid Solutions[END_REF] we may propose that the existence of aggregations of phosphates should be taken into account for this. Because of the formation of dimer or trimer at higher concentrations, the amount of close encounters between water and phosphate is relatively less than those in sulfuric acid under identical molar concentration, and thus the efficiency of in situ H2O •+ capturing reaction is lower.

In spite of studies of H2O •+ reactivity towards HNO3, H2SO4 and H3PO4, as yet there are two additional acids (perchlorate acid and hydrofluoric acid) remaining to be of interest in aqueous systems. Unfortunately, direct ionizing of perchlorate acid gives rise to two components of radicals (ClO4 • and ClO3 • ) with very low value of extinction coefficient, so it might not be easy to correctly deduce the secondary radical yield from H2O •+ . The fact that the element fluorine has a high electron affinity may result in a high value of redox potential of one electron couple F • /F -, which has not been known yet. It is of great interests to testify on the oxidizing ability of H2O •+ by observing the formation of F • or F2 •-in pulse radiolysis of highly concentrated HF solution. This measurement has not been performed so far because it requires a very specific fused cell and solution cycling system to avoid the corrosions from HF samples and safety issues when handling with HF.

One may assume that H2O Besides, we are carrying out pulse radiolysis measurements of highly concentrated DNA subunits solutions in order to extend our findings obtained in inorganic solutions to a situation that is biologically relevant.

Moreover, the absence of information on the yield of a direct effect of radiation on the solute was also another barrier in making definitive conclusions about the reactivity of H2O •+ . Meanwhile, in biological systems, it was shown by electron spin resonance at low temperatures that hole transfer occurs from the inner hydration layer to DNA after γ rays irradiation. 13 However, there has been no direct observation of electron transfer involving the radical cation of water. The dose per electron pulse was 55.3 grays (Gy). The radiolytic yield of s e  is not affected even in 50 mM G solution within 1% uncertainty. The solid line is the theoretical fit; we have obtained it using timedependent rate constants. DO.D.900nm, optical density at 900 nm. By Ma et al. [START_REF] Ma | Reactivity of Prehydrated Electrons toward Nucleobases and Nucleotides in Aqueous Solution[END_REF] Despite these achievements, conclusions could not be made about the actual mechanism of H2O •+ reactivity. The purpose of this study is to elucidate the detailed mechanism of electron transfer from the combination of new experiments in hydrogenated and deuterated solutions along As an example, at 15 ps in 4 M D2SO4, the transient absorbance could be expressed as

          EQ. 4-7
The same procedure was performed for the concentrations from 2 to 6 M. As the presolvated electron has reacted with D3O + fully when the concentration is higher than 8 M, the observed absorbance at 15 ps solely arises from An absorption band with a maximum at 450 nm is clearly observed in solutions from 2 to 12 M, which is identical with the spectrum of SO  at 450 nm is also taken as most reliable one as 1600 M -1 cm -1 reported in the literatures. 10,[START_REF] Ma | Picosecond Pulse Radiolysis of Highly Concentrated Phosphoric Acid Solutions: Mechanism of Phosphate Radical Formation[END_REF][START_REF] Jiang | Pulse Radiolysis Study of Concentrated Sulfuric Acid Solutions[END_REF] Moreover, it is assumed that this value remains constant and is not dependent on the concentration of the sulfuric acid solution.

Analysis of the kinetics

( ) ( ) ( ) ( )              s F e SO measur S d O e FC A t A t A t A t EQ.4-9
The time dependent absorbance of FSOFC has been achieved carefully in Chapter III. As the transient absorbed spectra of the fused silica is already known (Figure 3.21 in Chapter III), so the signal contributed from FSOFC can be obtained as:

450 450 260 260 ( ) ( )           FSOFC FSOFC FSOFC FSOFC I A t A t I EQ.4-10
The fast decay just after electron pulse results from the reaction between s e  and D3O + in the concentration of 2 to 8 M. Additionally, D3O + could also scavenge the presolvated electron with a higher rate constant. When the concentration is larger than 8 M, the decay remains stable and shows

For the contribution of the pair ( s e  , D3O + ), it can be deduced from the absorbance at 600 nm by the ratio of the molar extinction coefficient (Figure 3

HO

D is the dose deposited in H2O under the same experimental condition, and F is the correction factor to estimate the dose deposited in the solution. F is related to the density and the electron concentration of the solution. 44 It could be determined according to EQ. 4-4.

The experimental yields of Therefore, it can be considered that the amount of ionized water by irradiation at 10 -15 s is no less than the value of (4.75×10 -7 ) mol J -1 within the experimental uncertainty. The radical SO4 •-(or its protonated form HSO4 • ) is observed at 450 nm, 7 ps after the electron pulse. Observing on this timescale guarantees that the sulfate radicals have not been formed upon oxidation by the hydroxyl radical since it reacts very slowly.

Results -Isotopic effect on the chemistry of H2O

The formation yield of the sulfuric acid radical is obtained in hydrogenated and deuterated solutions, respectively. Isotopic effects on the competitive reactions will be discussed later. The yield of the radical HSO4 • is 25% higher in the deuterated solution than in hydrogenated one.

For 10 M solutions, the signature of the solvated electron is absent because at this concentration the pre-solvated electron is totally scavenged by H3O + (D3O + ). Therefore, only the HSO4 • radical is observed at 450 nm and again the absorbance is higher in D2O than in H2O solution but only by At a concentration higher than 10 M, the difference between the amplitude of the radical absorbance in the two solutions almost vanishes. Finally, the yield of HSO4 • radical in pure sulfuric acid is the same for deuterated and hydrogenated solutions (Figure 4

.23b).

In both deuterated and hydrogenated solutions, the yield of the sulfate radical increases with the acid concentration up to a plateau; then at higher concentration and in neat sulfuric acid it decreases slightly. The maximum yield of the sulfate radical is found for a concentration around 12 M (Figure 4.24). In the case of neat sulfuric acid, the mechanism of sulfate radical formation is by the direct effect of energy absorption by SO4 2 ⁻ (or acidic forms HSO4⁻ and H2SO4, and DSO4⁻ and D2SO4, respectively) presented as the following reaction R26: [START_REF] Jiang | Pulse Radiolysis Study of Concentrated Sulfuric Acid Solutions[END_REF]

SO₄²⁻→ SO₄˙⁻+ e⁻

The yield of reaction R26 is the same in deuterated and hydrogenated neat sulfuric acid and is equal to Gdir = (3.75 ± 0.05) × 10 -7 mol J -1 . In sulfuric acid solutions, the sulfate radical can also be formed by the electron transfer from solute to the primary radical cation D2O •+ in aqueous systems.

Knowing the value of Gdir (Figure 4.20), it is possible to deduce the yield of the scavenged radical cation (H2O •+ and D2O •+ ) according to EEQ. 4-2 and 4-3. The scavenging yield of the radical cation in two solutions, deuterated and hydrogenated, versus concentration of acid is shown on Figure 4.24. It is important to note that at these relatively low concentrations of the solute, a significant part of the energy is deposited in water following the electron density of water. A difference between the yield of sulfate radical in deuterated and hydrogenated solutions is observed.

Analysis and discussion

For example, the scavenged yield of the radical cation in deuterated water is 2.0, 1.5 and 1.3 times higher for fs equal to 0.08, 0.16 and 0.3, respectively (Figure 4.25). As the concentration increases, the difference of the yield between the two solutions decreases and both reach almost the same plateau where all radical cations are scavenged. Obviously, there is an isotopic effect on the competitive reactions. To explain these experimental results, we carried out theoretical simulations of electron transfer from DSO4⁻ (HSO4⁻) to D2O •+ (H2O •+ ). Different mechanisms can govern the electron transfer (ET) reaction depending on the amplitude of the quantum-mechanical coupling between the H2O •+ /HSO4⁻ and H2O/HSO4 • electronic states. We evaluated the non-adiabatic ET rate constants arising from the hypothesis of weak coupling (electronic perturbative case) by the Jortner-Bixon formulation of ET theory. [START_REF] Kestner | Thermal Electron Transfer Reactions in Polar Solvents[END_REF] This theory combines a quantized treatment of the innersphere vibrations with a classical electrostatic treatment of the outer-sphere response to ET. It turns out that an ET mechanism governed by nuclear motions cannot be competitive with a femtosecond proton transfer reaction because of the associated free energy barriers. Actually, traditional electron transfer theories are not likely to apply in the present case. In fact, the sudden ionization of a water molecule caused by the interaction with the accelerated electron beam, leaves the electron density in an out-of-equilibrium state. The excess of electronic energy may propel charge migration with surrounding molecules. To assess this possibility, we carried out real-time electron dynamics simulations at fixed nuclear positions. The almost +1 charge on water that results from ionization oscillates back-and-forth on the femtosecond time scale between the water and the HSO4⁻ ion (Figure 4.25a, simulations 2 and 3). Ionization does not systematically lead to charge migration.

It is dependent on the identity of the water molecular orbital that is initially depopulated. Subsequent charge migration is possible if the relative orientation and distance between the partners permit strong electronic coupling. Interestingly, since charge migration takes place at fixed position of atom nuclei, the process is not sensitive to isotopic substitution. This contrasts with proton transfer that de facto is dependent on nuclear vibrations and leads to kinetic isotopic effects. 6

Conclusions

In this study, the radiolytic yield of D2O •+ in D2SO4/D2O system is found to be (4.75 ± 0.2) ×10 -7 mol J -1 . Accordingly, the geminate recombination between presolvated electron and D2O •+ occurs within 1 ps and reduces the yield of hydrated electron. Besides, with the observed kinetics, our results show clearly that in deuterated solutions, the electron transfer is favored over proton transfer in competition reactions of the radical cation of water. In addition, numerical simulations show that electron transfer can take place within a few fs via a mechanism that is not governed by nuclear dynamics. Therefore, it is not sensitive to the isotopic substitution. Hence, we can conclude that the observed isotopic effects are solely due to the modulation of the proton transfer rates. Taken together, our experimental and theoretical data permit to elucidate the mechanism of direct oxidation triggered by H2O •+ (D2O •+ ) in competition with proton transfer in the condensed phase.

According to the simulation, charge migration is propelled by the excess energy present in the electronic cloud just after ionization and not by nuclear motions as in standard chemical theories of electron transfer. The present study is a rare example of a competition between two chemical reactions whose branching ratio is driven by a faster electronic dynamic ( Our study demonstrates that the radical cation of water is engaged not only in proton transfer reactions but also in redox reactions when H2O •+ (D2O •+ ) is formed in the vicinity of molecules different from water. Future studies will have to unravel if alternative reactions such as H2O •+ additions or H • abstraction may also be competitive with proton transfers. Finally, we remark that due to lack of information, the simulation codes, used by important communities working in the nuclear energy domain or in the radiobiology domain, do not take into account the oxidation reactivity of the radical cation of water. This makes little sense and should be reconsidered. the spurs of ionization and the yield of secondary radical which can be produced when the solutes are present in the solutions. For this objective, it is necessary to obtain two independent parameters of the hydrated electron in neat D2O: the initial yield and the absorption spectra with molar absorption coefficient. In Chapter III careful and systematic measurements, by taking into account the absorption induced in the windows of the optical cell and the radical hydroxyl, allowed us to determine precisely the absolute absorption spectra of the hydrated electron in D2O from 250 to 1500 nm. The shape and its amplitude are different from those reported previously in the literature.

The amplitude of the absorption band as found here is different by 10% from that reported in the literature. 4 Additionally, the shape of the absorption spectra of the solvated electron reported by Freeman often used is not correct. For example, the ratio of max () s Ge   between D2O and H2O in Freeman's work is 1.25, but in this work it is 1.15, and the extinction coefficient value of the hydrated electron in D2O at 300 nm is almost 20% higher than that reported by Freeman. 2,5 The use of the isosbestic method for the determination of the extinction coefficient with a specific scavenger is more appropriate because it is based only on the knowledge of the molar absorption coefficient of the reaction product. We found that the value of the initial yield of the hydrated electron and the hydroxyl radical, observed at 7 ps, are equal (within the experimental errors) to those found in H2O, but the kinetics of the solvated electron are slower in D2O than in H2O meanwhile that of hydroxyl radical presented no obvious different in D2O and H2O. The isotopic effect was explained initially by the fact that the initial spatial distribution between electron and hole in D2O is 1.2-1.4 times larger in D2O than in H2O. 6 Then laser photolysis measurements 7 showed that the main reason of slower decay of the solvated electron in D2O is due to the lower rate constant of the two main reactions responsible for the decay of the solvated electron in water at short time: But the yields are not comparable when the solvated electrons are produced by laser photolysis or by ionizing radiation. In our work, thanks to the determination of the initial yields (7 ps) of the hydrated electron and of the hydroxyl radical that are found to be same in neat water and in neat

Chapter V: Conclusions and perspectives 165 deuterated water, it can be concluded that the geminate recombination reactions are not affected significantly in D2O. Therefore, the slower decay can be explained well by the change of reaction rate of EQ.5-1 and 5-2. The reaction EQ.5-2 is diffusion controlled and as the diffusion coefficient is lower in D2O than H2O, the slower decay of solvated electron in D2O is understandable. The reaction EQ.5-1 is not fully limited by diffusion. In D2O, however, it needs higher reorganization energy and it can be considered slower in D2O than in H2O. Another point should be considered:

due to the isotopic effect and the change of the vibration frequency which is responsible of hydroxyl radical formation through proton transfer, the formation of OD • through deuterated of D2O •+ should be slower than that of OH • and we had expected a lower yield of OD • and 2 DO e  at picosecond range. 8 Because, the geminate recombination between electron and the radical water hole (D2O •+ ) should be favored when the proton transfer is slowed down by isotopic effect. However, the initial yields are found to be the same within the measurements precision. Therefore, our conclusion is that the change of water hole lifetime does not affect significantly the geminate reaction.

In Chapter VI, the reactivity of water cation radical in D2O has been investigated by picosecond pulse radiolysis. In this study, by using the scavenging method at picosecond range the radiolytic yield of D2O •+ in D2SO4/D2O system is found to be (4.75 ± 0.2) ×10 -7 mol J -1 . Accordingly, the geminate recombination between presolvated electron and D2O •+ occurs within 1 ps and reduce the yield of hydrated electron. Besides, with the observed kinetics, our results show clearly that in deuterated solutions, electron transfer is favored over proton transfer in competition reactions of the radical cation of water. In addition, numerical simulations show that electron transfer can take place within a few fs by a mechanism that is not governed by nuclear dynamics. Therefore, it is not sensitive to the isotopic substitution. Hence, we can conclude that the observed isotopic effects are solely due to the modulation of the proton transfer rates. Taken together, our experimental and theoretical data permit to elucidate the mechanism of direct oxidation triggered by H2O •+ (D2O •+ ) in competition with proton transfer in the condensed phase. According to the simulation, charge migration is propelled by the excess energy present in the electronic cloud just after ionization and not by nuclear motions as in standard chemical theories of electron transfer. The present study is a rare example of a competition between two chemical reactions whose branching ratio is driven by a faster electronic dynamic.

Perspectives

Water plays an indispensable role in radiation chemistry. Better understanding of reaction mechanism in water after irradiation is essential for further application. However, the initial excitation and ionization process in water just after radiation is not confirmed due to the limitation of the resolution of time resolved spectroscopy. In my thesis, the reactivity of the solvated electron, In addition, our study demonstrates that the radical cation of water is engaged not only in proton transfer reactions but also in redox reactions when H2O •+ (D2O •+ ) is formed in the vicinity of molecules different from water. The proton transfer reaction is slowed down by isotopic effect.

Abstract:

Picosecond pulse radiolysis measurements were performed in neat D2O and D2SO4/D2O system in order to study the isotopic effect on the reactivity of the solvated electron( s e  ), the hydroxyl radical (OH • ) and its precursor-water cation radical (H2O •+ ). In D2O system, the molar absorption coefficient spectra of spectrum in D2O was determined between 250 to 1500 nm using the isosbestic point method. Based on this extinction coefficient, the radiolytic yield of s e  in D2O just after the 7 ps electron pulse was determined to be (4.4 ± 0.2) × 10 -7 mol J -1 , which coincides with the one for in H2O. The yield of the radical OD • just after the 7 ps electron pulse was found to be (5.0 ± 0.2) × 10 -7 mol J -1 . In the spurs of ionization, the decay rate of s e  is slower in D2O than that in H2O, whereas the decay rate of OD • is similar to the one of OH • . In D2SO4/D2O system, the reactivity of and D2O •+ was probed in deuterated sulfuric acid solutions of various concentrations and compared with that of H2O •+ . The oxidized solute sulfate radical was observed at 7 ps and remarkably higher yields are found in deuterated solutions. The isotopic effects reveal the competition between two ultrafast reactions: proton transfer toward H2O (D2O) and electron transfer from 4 HSO  to H2O •+ (D2O •+ ). Density functional theory simulations decipher the electron transfer mechanism: it proceeds via sub-femtosecond charge migration and is not affected by isotopic substitution.

Figure 1 . 2

 12 Figure 1.2 Initial non-homogeneous spatial distribution of ionization events in spurs along the track, and



  Z is the charge of the incident ion. V is the velocity of the incident ion.

Figure 1 . 3

 13 Figure 1.3 Scheme of reactions of transient species produced by irradiation in water without or with a

( 31 )

 31 McCracken, D. R.; Tsang, K. T.; Laughton, P. J. Aspects of the Physics and Chemistry of Water Radiolysis by Fast Neutrons and Fast Electrons in Nuclear Reactions; Canada, 1999. (32) Lin, C. C. Radiochemistry in Nuclear Power Reactors; The National Academies Press: Washington, DC, 1996. (33) Katsumura, Y. Applications of Radiation Chemistry to Nuclear Technology, Y. Katsumura Charged Particle and Photon Interactions with Matter. In Charged Particle and Photon Interactions with Matter: Chemical, Physicochemical, and Biological Consequences with Applications; Mozumder, A., Hatano, Y., Eds.; CRC Press: New York, 2003.

3

 3

Figure 2 . 1

 21 Figure 2.1 Neutron induced fission.

  can travel great distances and have the ability to pass through the human body and interact with living cells. Lead, concrete, and other dense materials are used to attenuate (not to stop) gamma rays. Radioactive gamma sources based on 60 Co or 137 Cs isotopes are two basic types of sourcebased irradiators for chemical research. They are commonly used in continuous radiation mode for radiolysis product studies and competition kinetics measurements.

Figure 2 . 2

 22 Figure 2.2 The mechanic diagram of generation of X-rays.

Figure 2 . 3

 23 Figure 2.3 Schematic depictions of electrostatic (a) and oscillating electromagnetic (b) field for charged particle acceleration.4

Figure 2 . 4

 24 Figure 2.4 Schematic of a Van de Graaff Accelerator. Made by J. Wishart.4

Figure 2 .

 2 Figure 2.5 a) Schematic disgram of high voltage generation in Febetron Model 706. 5 b) Transient absorbance recorded at 500 nm with resolution of 50 ns by Febetron 706. Performed by J-L. Marignier in 1985.

Figure 2 . 6

 26 Figure 2.6 Diagram of LINAC.

Figure 2 . 7

 27 Figure 2.7 Schematic of a general detection system for picosecond pulse radiolysis. By A. Saeki. 4

Figure 2 . 8 A

 28 Figure 2.8 A picture of an overview in ELYSE

Figure 2 . 9

 29 Figure 2.9 Principle of the laser-driven RF electron accelerator. BSlaser beam splitter. LMElaser entrance mirror. VCvirtual cathode. EA-1, EA-2, EA-3experimental areas.13

Figure 2 .

 2 Figure 2.10 A schematic of femtosecond laser chain in ELYSE.

Figure 2 . 11

 211 Figure 2.11 The principle of chirped pulse amplification for femtosecond laser.

Figure 2 .

 2 Figure 2.12 A general schematic of the ELYSE accelerator. IP: ion vacuum pump; CPC: cathode preparation chamber; VV: vacuum valve; SOL: solenoid; D: dipole; TR1 and 2: triplets; Q: quadrupole; WCM: wall current monitor; FC: Faraday cup; PIQ: Ion pump; T: translator for Cherenkov light emitter and visualization screen; LME: laser entrance mirror; LMEx: laser exit mirror; VC: virtual cathode; HS: horizontal slit; VS: vertical slit.Vacuum.

Figure 2 . 13

 213 Figure 2.13 Oscillograms showing (left to right): the RF filling of the gun, the laser pulse as measured by the fast photodiode and the electron pulse as measured by a wall current monitor (WCM) and a Faraday cup.

Figure 2 . 14

 214 Figure 2.14 Horizontal section through the vacuum chamber for the Cs2Te photo-cathode preparation.16

Figure 2 .

 2 Figure 2.15 Time-resolved photocurrent signals with the Cs2Te photocathode. Pulse duration of the Cerenkov light measured by the streak camera at 500nm. Deviated beam location EA-3, 9 MeV.

Figure 2 .

 2 scheme of the set-up.

Figure 2 . 16

 216 Figure 2.16 Schematic representation of picosecond pulse-probe pathway in ELYSE.

Figure 2 . 17

 217 Figure 2.17 The optical path of pulse-probe detection in the direct line of ELYSE. The red line indicates the fundamental laser light of 780 nm used to generate the probe light and the pump electron beam. The rainbow color line indicates the supercontinumm light from 350 to 780 nm meanwhile the purple line represents the 260 nm probe light.

Figure 2 . 18

 218 Figure 2.18 Time dependent absorbance in water at 600 nm.

Figure 2 . 19

 219 Figure 2.19 The diagram of the system configuration of a streak camera.

Figure 2 . 20

 220 Figure 2.20 Operating principle of streak tube.

Figure 2 .

 2 Figure 2.21 Up: the mechanic scheme of streak camera in ELYSE. On the left are different signals extracted from a 2D experimental image. Bottom: The optical path of the optical detection by streak camera.

Figure 2 .

 2 Figure 2.22 a) Image recorded by streak camera with analysis light and electron beam, b) Image recorded by streak camera with analysis solely, c) The background noise recorded by streak camera and d) the analyzed result.

Figure 2 . 22

 222 Figure 2.22 The cycling system in the optical detection by streak camera.

Figure 2 .

 2 23 b shows the decay of the solvated electron recorded at 660 nm on an average image averaging from 400 acquisitions.For example, the decay of the solvated electron presented below (Figure2.23 b), the value of the optical density at 3 ns: At=3 ns = 0.34, the dose deposited in the water could been given by:

Figure 2 . 23

 223 Figure 2.23 The decay of solvated electron in H2O recorded by streak camera. a) Image in 2D form, and b) Kinetics at 660 nm.

Figure 2 . 24

 224 Figure 2.24 Schematic representation of 60 Co source (Laboratoire de Chimie Physique, Université Paris-Sud).

Figure 2 . 25 )

 225 , related to the different absorbing species (intermediate and product states), weighted by their respective time-dependent concentration, as expressed by the bilinear model as follow: D  C  S T EQ. 2-where the columns of matrix C, sized m × k, are the time-dependent profiles of the k components at the m time delay values and the rows of matrix S T , sized k × n, are the related k difference transient spectra measured at n spectroscopic variables. The elements of D are denoted dij, where the time delays ti design the rows and the wavelengths j design the columns. It should be noted that the situation described above corresponds to noise-free data. The term "component" should also be clarified since it can sometimes be quite misleading. It would refer, in an ideal case, to pure physical or chemical factors, i.e., photochemical species involved in the transient multi-species system studied.

Figure 2 . 25

 225 Figure 2.25 Bilinear model related to a three-component process monitored by time-resolved spectroscopy.27

  have also demonstrated that global analysis of multisets reduces the variance of the parameters in model-based analysis. In other words, the combination of several experiments (different time scale, different probe wavelength or different molecular environment, for example) involving the same molecule or complementary species will always help in a better description of the system.

Figure 2 . 26

 226 Figure 2.26 Examples of multiset structures: (a) column-wise appended data sets sharing the pure spectra for all data sets; (b) row-wise appended data sets sharing the same time-dependent profiles for all components, (c) column-wise and row-wise appended data sets, with subsets of experiments sharing timedependent profiles or pure spectra.27

59 Chapter

 59 absorption extinction coefficient of s e  in D 2 O (250 to 1500 nm) obtained in the present work Chapter III: Time Dependent Yield of the Solvated Electron and the OD • Radical in Deuterated Water at Low LET Studied by Picosecond Pulse Radiolysis subject of radiation reaction of many experimental and theoretical investigation for more than 70 years. The ionizing radiation reaction impact on neat water has great importance in biology, medicine and nuclear energy industries. It is well known that the radicals s e  , OH • , H • , and the molecules H2, and H2O2 are produced during the decomposition of water under ionizing radiation with linear energy transfer (LET, such as electron beam or γ ray). For the determination of the time dependent radical yield in non-homogenous step (shorter than 100 ns in water) of the radiolytic process, the parameters involved in the simulation code of water radiolysis were adjusted by fitting the data obtained by scavenging method during the 70's and 80's, except a few works performed with picosecond pulse radiolysis time resolution. 1 . Important progress made during the last decade in the pulse radiolysis techniques allowed obtaining an acceptable model for the water radiolysis with an accurate determination of the initial yield of the hydrated electron and the hydroxyl radical in H2O.Their time dependent yield are now well known from the picosecond range, and the simulations can at least fit well the new experimental data in the case of the hydrated electron. 2-7

Figure 3 . 1 A

 31 Figure 3.1 A schematic of CANDU nuclear reactor.

Figure 3 . 2

 32 Figure 3.2 Simulation of the time dependent decay of solvated electron in H2O and D2O under low LETradiolysis at 25 °C and 300 °C, respectively.9 The inset was experimental results from C. D. Jonah et al.2, 10

Figure 3 . 3

 33 Figure 3.3 Simulation of the time dependent decay of hydroxyl radical in H2O and D2O under low LET radiolysis at 25 °C and 300 °C by G.V. Buxton et al., respectively.9 The inset is the experimental results from C. D. Jonah et al.10

Figure 3 . 4

 34 Figure 3.4 Absorption spectra of OD • by S. O. Nielsen (in blue)20 and G. V. Buxton (in pink).21 The solid

2 DOe

 2  and OD • simultaneously. In the present work, we measured the absorption spectrum of the hydrated electron in D2O from 250 to 1500 nm for the first time (Figure 3.5). The time dependent yield of 2 DO e  and OD • were determined and compared with those in H2O. For the study of OD • , with its low molar absorbance coefficient, the pulse radiolysis detection configuration was adapted to measure precisely in situ the dose per pulse via the solvated electron in the visible and the absorbance of OD • radical in the UV at 260 nm in the same volume irradiated by electron beam.

Figure 3 . 5

 35 Figure 3.5 Scheme of objective in this work

  Therefore, the dose deposited in D2O is same with that in H2O. Obviously, to determine the time dependent yield of 2 DO e  by measuring the time dependent absorbance, it is essential to confirm the molar absorption coefficient of 2 DO e  first of all. Considering the evolution of the molar absorption coefficient of 2 DO e  , there were scarce information and all the measurements were Chapter III: Time Dependent Yield of the Solvated Electron and the OD • Radical in Deuterated Water at Low LET Studied by Picosecond Pulse Radiolysis 66 performed by indirect methods. 8,11,22 In this work, a stoichiometric reaction between 2 DO e  and an oxidizing species giving a product with a very well-known absorption spectrum which overlaps the spectrum of 2 DO e  will be used. Thus, one or several wavelengths corresponding to the intersection between the spectra of 2 DO e  and the oxidizing species used are in accordance with the wavelength where the molar absorption coefficients of both species are identical. If there exists only one reaction of 2 DO e  with the acceptor, then these wavelengths where an optical density remains constant are going to be observed during the reaction. And these wavelengths are the isosbestic points of 2 DO e  and the product. It is important to identify precisely an isosbestic point and confirm that the reaction of 2 DO e 

2 DOeFigure 3 . 6

 236 Figure 3.6 Spectra of MV •+ radical following γ radiation of 10 -4 M MV²⁺, 1% 2-propanol, and 1 % acetone at pH 7.

Figure 3 . 7 ,

 37 Figure 3.7, the absorption band of MV •+ is not affected by the isotopic change. However, the third

Figure 3 . 7

 37 Figure 3.7 Left axis: UV-Visible absorption spectra of MV • ⁺ in two solutions (containing 5⨯10 -4 M MV 2+, 0.2 M isopropanol and 2% acetone in H2O and D2O, respectively) after irradiation with γ source. Isopropanol was used to scavenge hydroxyl radical and Acetone was used to slow down the reduction of MV • ⁺. The dose is 600 Gy. Right axis: The spectra of MV •+ detected by streak camera for a D2O sample containing 5.5 × 10 -4 mol L⁻¹ methyl viologena and 0.5 M tert-butanol (to scavenge OD • ).The dose was 54 Gy per pulse. It is clear that the spectra of MV •+ in D2O observed by streak camera at ELYSE from 400 to 650 nm fits well with that obtained by γ source. The intensity of the absorption band from 350 to 400 nm measured by Streak

Figure 3 . 8

 38 Figure 3.8 Up: Image obtained by streak camera after electron pulse (dose is 71 Gy per pulse). Bottom: Transient spectra in 5.5 × 10 -4 mole L⁻¹ methyl viologen / D2O (Containing 0.5 M tert-butanol to scavenge OH • ).

  Chapter III: Time Dependent Yield of the Solvated Electron and the OD • Radical in Deuterated Water at Low LET Studied by Picosecond Pulse Radiolysis 70 concentration of the hydrated electron of a few 10 -5 mol L -1 only. The reduction of MV 2+ by

3 . 8 .Figure 3 . 9

 3839 Figure 3.9 The kinetics of the species involved in the transient absorption analyzed by MCR-ALS with the matix of the time-resolved measurements. The left is the matrix measured within 200 ns and the right within 500 ns.

Figure 3 . 10 1 MFigure 3 . 11 2 DOe 2 DOe 2 DOe

 3101311222 Figure 3.10 Kinetics at 408, 413 and 420 nm in 5.5 × 10 -4 mol L⁻¹ methyl viologen / D2O (Containing 0.5 M tert-butanol to scavenge OH • ) (Dose=71 Gy).

2 DOe  can be deduced as shown in Figure 3 Figure 3 . 12

 23312 Figure 3.12 Transient spectra of s e  in D2O at 200 ns after subtracting the contribution of OD • radical.

2 DOeFigure 3 . 13

 2313 Figure 3.13 Absorption spectra of es ⁻ in neat D2O. The solid line is a guide to the eyes.

Figure 3 . 14 2 DOeTable 3 . 3 2 DO e  and 2 HOe 2 DOe  is larger than that of 2 HOeFigure 3 . 15

 3142332222315 Figure 3.14 Molar absorption coefficient spectra of solvated electron in H2O and D2O, respectively. For

Figure 3 . 16 3 is found that the initial yield of 2 DOe 2 DOe  at 10 ps is found to be 7 ( 4 .Figure 3 . 17

 31632274317 Figure 3.16 Kinetics of es -at 600 nm in neat water and neat deuterated water (Dose per pulse is 42.5 Gy).Inset: the normalized kinetics in H2O and D2O.

Figure 3 . 18 .Figure 3 . 18 Figure 3 . 19

 318318319 Figure 3.18. It is found that in relation to the calculated curve by Trumbore34,35 and with respect to the experimental points given by Fanning, the curve measured by Torche showed a faster decay of the solvated electron. The difference is due to the higher dose rate in ELYSE.

Figure 3 . 5 ,

 35 the region where OD • radical absorbs is in UV. The extinction coefficient spectrum has already been reported by S. O. Nielson (210 ~ 260 nm) and G. V. Buxton

For the contribution of the hydrated electron 2

 2 Chapter III: Time Dependent Yield of the Solvated Electron and the OD • Radical in Deuterated Water at Low LET Studied by Picosecond Pulse Radiolysis 82 possible to use the kinetics in the visible part of spectra, for example at 600 nm (Figure 3.16). And taking the ratio of the extinction coefficients at 260 and 600 nm into account, the kinetics of the hydrated electron at 260 nm can be determined

Figure 3 . 20

 320 Figure 3.20 Transient absorption spectra in irradiated Suprasil sample of 1 mm thickness. Measured immediately after 3 µs electron pulse by Nielson et al. 20 Dose per pulse is 240 Gy per pulse.

(Figure 3 .

 3 20).The spectrum was found to be scaled in proportion to the dose and Suprasil path length. In order to determine correctly the ultraviolet absorption spectra of s e  , OH • , H • , OD • and D • , the Chapter III: Time Dependent Yield of the Solvated Electron and the OD • Radical in Deuterated Water at Low LET Studied by Picosecond Pulse Radiolysis 83contribution induced by the fused silica cell is necessary to be subtracted.In 2006, Janata reported the transient absorption in fused silica induced by 3.8 MeV electron pulses using a 2 ns electron accelerator. He observed the main absorption was in the UV more than the visible.41 The absorption bands with varying decay kinetics indicated that different color centers or lattice defects were formed (Figure3.21).

Figure 3 . 21

 321 Figure 3.21 Transient absorbed spectra at various time after the beginning of irradiation (Left) and longtime decay of absorption at 260 nm (Right) in fused silica by E. Janata. 41

Figure 3 . 22

 322 Figure 3.22 Transient absorbed spectra normalized at 260 nm in fused silica recorded at 50 ps after excited by electron pulse.42

Figure 3 .

 3 Figure 3.23 Top: Simulations of electron beam propagation and its scattering by the mirror, air, cell windows and the sample. Bottom: Scheme of the setup for measuring the yield of OD ▪ at 260 nm by insuring that the supercontinuum probe light and 260 nm probe the same irradiated volume. For that purpose, a pinhole with 300 m diameter is placed 1 cm after the sample.

Figure 3 .

 3 Figure 3.23 shows the experimental configuration of pump and probe beams (bottom) and in particular the electron beam propagation (top). The latter was simulated by EGSnrc (electron gamma shower by national research council Canada) code and considers the electron scattering by the mirror located just after the exit of the accelerator line, by air, by the windows of optical cell and also by the sample. For the precise quantitative determination of the different transient contributions, the probe beams have to measure the same irradiated volume in the optical cell. If

  Chapter III: Time Dependent Yield of the Solvated Electron and the OD• Radical in Deuterated Water at Low LET Studied by Picosecond Pulse Radiolysis 86

  At ) placed at the same position with the entrance window of FSOFC (as demonstrated in Figure 3.24).

Figure 3 . 24

 324 Figure 3.24 Measurement with an empty FSOFC and a plate of 200 µm under the same condition.

Figure 3 . 25

 325 Figure 3.25 Decays obtained at 260 nm for the same dose (45 Gy per pulse) in the empty fused silica optical flow cell (containing two plate of 200 m thickness) and in a fused silica plate with 200 m thickness. The lines serve as guides to the eyes.

  be directly used to compare with the results measured in the experiments.

Figure 3 . 26

 326 Figure 3.26 Simulation of the radial dose distribution in 1 st and 2 nd window of FSOFC obtained by EGSnrc code for FSOFC filled with air or water. The color presents the dose intensity scale.

Figure 3 .Figure 3 . 27

 3327 Figure 3.26 shows a dosimetry distribution in both windows of the sample cell filled with air and water, respectively. The palette displays the relative dose. Due to the scattering in the cell, electrons are more spread outward from the beam axis in the second window than in the first window. As the diameter of the laser probe used in our experiment is around 200μm, the concerned area of () r 

3. 4 . 2 Figure 3 . 28 2 DOe 2 DOeFigure 3 . 29 Figure 3 . 30

 4232822329330 Figure 3.28 Absorption measured at 260 nm in the FSOFC (with a thickness of 200 µm for each window)containing neat D2O. Absorbance due to the species formed in solid state in the windows of the optical cell at 260 nm (according to EQ. 3-10 and the contribution of the solvated electron at 260 nm (according to EQ. 3-9). The dose per pulse is 42.5 Gy.

  Chapter III: Time Dependent Yield of the Solvated Electron and the OD• Radical in Deuterated Water at Low LET Studied by Picosecond Pulse Radiolysis 93

Finally, the present

  experimental work giving the time dependent yield of the hydrated electron and hydroxyl radical in neat deuterated water can also contribute to improve the simulation codes usually used in radiation chemistry by treating the energy deposition in the spurs and the kinetics Chapter III: Time Dependent Yield of the Solvated Electron and the OD • Radical in Deuterated Water at Low LET Studied by Picosecond Pulse Radiolysis 95 in the spurs. The simulation codes are very important for the chemistry of the nuclear reactors such as CANDU in which D2O is used. Chapter Ⅳ: Ultra-fast Charge Migration Competes with Proton Transfer in the Early Chemistry of H 2 O •+

Figure 4 . 1 .

 41 Figure 4.1. a) Time dependence of the induced absorbance at 460 and 410 nm following the femtosecond photoionization of pure liquid H2O by ultraviolet pulse (310 nm). b) Influence of an H/D isotope substitution on the time resolved induced absorbance (410 nm) following the femtosecond photoionization of water molecules (X2O with X=H or D) by ultraviolet pulses (310 nm). The computed fits of the experimental curves yield a relaxation time of 100 fs in H2O against 170 fs in D2O. By Gauduel et al.5

  Ultra-fast Charge Migration Competes with Proton Transfer in the Early Chemistry of H2O •+ 110

Figure 4 . 2

 42 Figure 4.2 (a) The time-frequency spectrogram for the anisotropic signal shows the disappearance of the 150 and 500 cm -1 frequency components at early times and the concomitant rise of the frequency component at 310 cm -1 . (b)The FFT amplitude at 310 cm -1 reveals an oscillatory rise (symbol), from which a fit to first-order kinetics (solid line) yields a lifetime of 196 ± 5 fs for the aqueous H2O radical cation. This work was presented by J. Li et al.11

  ,18

Figure 4 . 3

 43 Figure 4.3 Schematic description of the reactions occurring in solutions containing a solute M at high concentration. At subpicosecond time scales involving proton transfer (t. p + , black), direct ionizing of solutes M (red), ultrafast electron transfer (t.e -, blue) and electron relaxation. Picosecond electron pulse radiolysis is ready to observe the formation of M •+ within the pulse. The oxidation of M by OH • radical takes place at longer time, which does not affect our observation.19

Figure 4 . 4

 44 Figure 4.4 Scale of the values of the redox potential of certain transient radicals with respect to the standard hydrogen electrode (V/NHE).19

Figure 4 . 5 .

 45 The following reactions are considered:

Figure 4 . 5

 45 Figure 4.5 The electron fraction of NaCl in aqueous solution as function of concentration. The red solid line is a guide for eyes. Data adapted from ref 41 .

Figure 4 . 6 2 Cl 4 . 6 Figure 4 . 6 4 . 7 .Figure 4 . 7

 46246464747 Figure 4.6 Transient kinetics observed at 370 nm in a wide range of NaCl aqueous solutions to show the products of oxidation of Cl -. The absorbance of hydrated electron and transient signal from irradiated fused silica were subtracted from the raw data. The solid lines are drawn to guide the eye. The absorbed dose (22.5

Figure 4 . 8

 48 Figure 4.8 Radiolytic yield of phosphoric acid radical versus the electron fraction of the solute. The obtained yield at 19 M is considered as the direct effect yield that is linearly extrapolated to the zero (dash line). The superliner contribution (green bars) is due to the charge transfer reaction involving water radical cation (revised) H2O •+ .

  Balcerzyk et al. succeeded in separating the direct and indirect effect of radiation on HNO3 , and found that the direct effect on the formation of 3 NO  is not negligible when the concentration of HNO3 is higher than 1 M. Very recently, Musat et al. presented a systematic investigation of the radiolysis of HNO3 acid from picosecond to nanosecond both under room temperature and 80°C conditions.

  Chapter IV: Ultra-fast Charge Migration Competes with Proton Transfer in the Early Chemistry of H2O

3 NO 3 NO

 33  recorded just after the electron pulse with increasing concentration keeps the same trend with the one observed in H3PO4 (Figure 4.9). Figure 4.10 presents the recorded kinetics at 640 nm on picosecond and nanosecond range in the solutions containing different concentrations of HNO3. The time evolution absorption of the 3 NO  at 640 nm for solutions at different concentration shows no decay up to 400 ps after subtracting the contribution of the solved electron. At longer time, the yield of  increases due to the reaction of OH • and HNO3.

Figure 4 . 9

 49 Figure 4.9 Transient absorption spectra at 22.5 °C of NO3 • recorded just after the passage of the 7 ps electron pulse for HNO3 solutions >2 M. The transient spectra for 0.5, 1, 1.5, and 2 M are recorded afte the decay of the solvated electron signal (300, 200, 200, and 100 ps, respectively, after the electron pulse). The spectra are shifted by 1.5 mOD for clarity. The inset presents the variation of the measured absorbance at 640 nm as a function of the solution concentration. By Musat et al.10

Figure 4 .11 shows that the observed yield of 3 NO

 43 Figure 4.11 shows that the observed yield of

Figure 4 . 10

 410 Figure 4.10 Recorded kinetics at 640 nm in solutions containing different concentrations of HNO3 at 22.5 °C. By Musat et al. 10

Figure 4 . 3 NOreaction of 3 NO

 433 Figure 4.11 (A) Observed radiolytic yields of

Figure 4 . 3

 43 Figure 4.3 presents the schematic description of the possible reactions occurring in sulfuric acid solutions at subpicosecond time scales involving proton transfer (t. p + , black), direct ionizing of solutes SO4 2-(red), ultrafast electron transfer (t. e -, blue) and electron relaxation. In order to clarify all of these processes, we performed a picosecond pulse radiolysis investigation of solutions containing a wide range of sulfuric acid concentration. 46 Figure 4.12 displays the absorption spectrum of SO4 •-in a variety of sulfuric acid solutions ranging from 1 M up to 18 M observed on the ps timescale (immediately after the 7 ps electron pulse). It has a typical absorption band at 450

Figure 4 . 12

 412 Figure 4.12 Transient absorption spectrum of SO4 •-or HSO4 •-at maximum of absorption versus concentration of acid observed immediately after 7 ps electron pulse. Insert figure shows the corresponding transient kinetics of SO4 •-or HSO4 •-. The contribution of solvated electron in 1 to 6 M H2SO4 is subtracted from the signal on the basis of the known extinction coefficient at various wavelengths. The dose deposited in the solutions has been corrected with F value. The dose was 31.3 Gy per pulse. Adapted from 46 .

Figure 4 . 13

 413 Figure 4.13 Yield of sulfuric acid radical formation versus electron fraction of the solute. The value at 18

Figure 4 . 14

 414 Figure 4.14 End of pulse radiolytic yield of scavenged radical cation H2O •+ in aqueous sulfuric and phosphoric acid solutions(Revised). Performed by Ma. et al 49 .

Figure 4 . 15

 415 Figure 4.15 Transient absorption kinetic traces of s e  in liquid water containing Guanine, Adenine, Cytosine, and Thymine compounds with different concentrations (10 to 150 mM). To increase the solubility of purine nucleobases, the solutions were prepared in 90 mM NaOH followed by the work of Wang et al. 58 . Radiolytic yields of s e  are shown in the left axes, and these correspond to the absorbance in the right axes.

1 61 Table 4 . 6 Figure 4 . 4 SO  (or 4 HSO

 16146444 Transient absorption spectra A series of experiments in D2SO4/D2O from 0 to 18 M were performed. The transient absorption spectra at 15 ps after electron pulse are presented in Figure 4.16a. The large absorption band located in the red regions are mainly due to the solvated electron pair ( s e  , D3O + ), and it decreases with increasing the sulfuric acid concentration from 0 ~ 6 M and produce D • atom (R4) Chapter IV: Ultra-fast Charge Migration Competes with Proton Transfer in the Early Chemistry of H2O •+ 138which is known to absorbs far in UV. The blue shift with increasing sulfuric acid concentration is as the result of the formation of ( s e  , D3O + ) , as reported by Ma et al. The wavelength where solvated electron absorbs maximum in 0 to 6 16b, the absorption maximum of s e  moves to lower wavelength with increasing the concentration of D2SO4. s e  has been totally scavenged within the pulse when the concentration of D2SO4 is over 8 M. Table4.6 presents the wavelength where the s e  appears to have the maximum absorbance in the concentration range of 0 to 6 M D2SO4.As more and more presolvated electron is scavenged by D3O + with the concentration increasing, another absorption band became more significant in concentrated solutions with a maximum around 370 to 520 nm. The absorbing signal of solvated electron in the visible completely vanishes for concentrations higher than 8 M.The absorption measured by pump-probe in our experiments from 370 nm to 720 nm contains not only the contribution of  ), but also that of non-scavenged solvated electron in less concentrated sulfuric solutions as well as the transient species induced in the fused silica optical flowing cell (FSOFC) (as shown in Figure3.21 Chapter III).

Figure 4 .

 4 17 is the transient absorbed spectra from 2 to 18 M obtained after subtracting the contribution due to the solvated electron and the transients in FSOFC carefully.

Figure 4 . 4 SO

 44 Figure 4.16 a) Transient spectra recorded at 15 ps after electron pulse in 0 to 18 M D2SO4/D2O solutions. The dose per pulse was 42.5 Gy. b) Normalized absorbance at 15 ps after electron pulse inn 0 to 6 M D2SO4/D2O solutions.

4 SO

 4  . The results are shown in Figure4.17.

Figure 4 . 17

 417 Figure 4.17 Example of the transient absorption spectra of the sulfate radical (HSO4 • and SO4 • ˉ) recorded at 10 ps in D2SO4 at different concentration. The contribution of solvated electron and the transient induced in the optical cell are subtracted from the observed signal. The dose was 42.5 Gy per pulse.

Figure 4 .contribution of 4 SO

 44 Figure 4.18(a) describes the time dependent decay measured at 450 nm in the sulfuric solutions from 2 to 18 M. As noted above, the absorption measured at 450 nm consisted not only the contribution of

4 SO  is presented in Figure 4 . 4 SOFigure 4 . 19

 444419 Figure 4.19 Kinetics of sulfate radical (SO4 •-or HSO4 • ) on absorption maximum wavelength. The contribution of solvated electron has been subtracted from the signal. The dose per pulse was 42.5 Gy.

4 SO 4 SO 4 SOFigure 4 . 20 4 SO 4 SO 4 ]Figure 4 . 21

 444420444421 Figure 4.20 Yield of SO4 • ⁻ versus the electron fraction of the solute D2SO4 in D2O. The value at 18 M is considered as the direct effect yield and is linearly extrapolated to zero concentration. The supplementary yield is due to the electron transfer reaction to D2O •+ .

Figure 4 .

 4 Figure 4.21 presents the yield of D2O•+ scavenged by sulfate anion. It is notable that the

Figure 4 .

 4 Figure 4.22 a) Ultrafast absorbance kinetics after the electron pulse in neat deuterated and hydrogenated sulfuric acid solutions in 4 M solutions in H2O and D2O. b) The kinetics at 450 nm after subtraction of the absorbance of solvated electron and the transient induced in fused silica cell for 4 M solutions.

Figure 4 . 23

 423 Figure 4.23 Ultrafast absorbance kinetics observed at 450 nm after the electron pulse in deuterated and hydrogenated sulfuric acid solutions in 10 M (a) and 18 M (b) solutions in H2O and D2O.

10% (Figure 4 Figure 4 . 24

 4424 Figure 4.24 Yields of H2O •+ scavenging versus concentration case of H2SO4 and D2SO4. The difference between H2SO4 and D2SO4 are zoomed in insert figure to better present the isotope effect in proton transfer.

Figure 4 . 25

 425 Figure 4.25 Yield of scavenged radical cation (H2O •+ and D2O •+ ).

  Chapter IV: Ultra-fast Charge Migration Competes with Proton Transfer in the Early Chemistry of H2O •+ 151

Figure 4 .

 4 Figure 4.25 Electronic Dynamics Simulations. a: evolution of the charge of the water molecule hydrogen bonded to HSO4⁻ Ionization takes place at t = 0. Each plot corresponds to a different electronic dynamics generated by depopulating different valence MOs, the representations of which are shown in panel b (for isosurfaces of ± 0.08 bohr -3/2 ). The two green curves (2 and 2') correspond to ionization from the same MO but with different values of the hydrogen bond length between H2O and HSO4⁻, namely 1.8 and 2.4 Å, respectively. c: isosurfaces of the difference charge density with respect to the initial time for dynamics 2 and for different times spanning the first half period of charge migration. Blue and green isosurfaces indicate accumulation and depletion of electron density, respectively. Performed by A. De La Lande.

Figure 4 .

 4 26).

Figure 4 . 26

 426 Figure 4.26 Schematic of probing the competition reactions of the radical cation D2O •+ . 7 ps is the time resolution of the setup for probing the sulfate radical in this work.

  hydroxyl radical and water cation radical in D2O have been studied and compared with those in H2O. The time dependent radiolytic yields of solvated electron and hydroxyl radical in D2O have been determined by direct measurements after 7 ps. And the scavenged yield of water cation radical in D2O also has been estimated by indirect measurements.

Figure 5 . 1

 51 Figure 5.1 Diagram of the radiolytic yield of hydrated electron, hydroxyl radical and water cation radical at 7 ps by direct or indirect measurements.

Future

  studies will have to unravel if alternative reactions such as H2O •+ additions or H • abstraction may also be competitive with proton transfers. Université Paris-Saclay Espace Technologique / Immeuble Discovery Route de l'Orme aux Merisiers RD 128 / 91190 Saint-Aubin, France Titre : Réactivité de l'électron solvaté, du radical hydroxyle et de son précurseur dans l'eau lourde, etudiée par la radiolyse pulsée picoseconde Mots clés : électron solvaté, radical hydroxyle, radical cationique de l'eau, eau lourde, radiolyse pulsée Résumé : Des mesures de radiolyse impulsionnelle avec des paquets d'électron de 7 picosecondes ont été effectuées dans des systèmes D2O et D2SO4 / D2O purs afin d'étudier l'effet isotopique sur la réactivité de l'électron solvaté ( s e  ), du radical hydroxyle (OH • ) et de son radical cation cationique (H2O •+ ). Dans le système D2O, le spectre d'absorption molaire de l'électron solvaté dans D2O a été déterminé entre 250 et 1500 nm en utilisant la méthode du point isosbestique. Sur la base de ce coefficient d'extinction, le rendement radiolytique de s e  dans D2O juste après l'impulsion d'électrons de 7 ps a été déterminé à (4,4 ± 0,2) × 10 -7 mol J -1 , ce qui coïncide avec celui dans H2O. Le rendement du radical OD • juste après l'impulsion d'électrons s'est révélée être (5,0 ± 0,2) × 10 -7 mol J -1 . Dans le cas de l'électron solvaté la vitesse de déclin dans D2O est plus lent que dans H2O, tandis que la vitesse de déclin de OD • est similaire à celui de OH • . Dans le système D2SO4 /D2O, la réactivité de D2O •+ a été sondée dans des solutions d'acide sulfurique deutérées de diverses concentrations et comparée à celle de H2O •+ . Le soluté oxydé, le radical sulfate a été observé à 7 ps et des rendements plus élevés ont été trouvés dans les solutions deutérées. Les effets isotopiques révèlent la compétition entre deux réactions ultra-rapides : le transfert de protons vers H2O (D2O) et le transfert d'électrons de H2O •+ (D2O •+ ). Les simulations par la théorie de la densité fonctionnelle décryptent le mécanisme du transfert d'électrons : elles se font par migration de charge sub-femtoseconde et ne sont pas affectées par la substitution isotopique. Title: Reactivity of the Solvated Electron, the Hydroxyl Radical and its Precursor in Deuterated Water Studied by Picosecond Pulse Radiolysis Keywords: solvated electron, hydroxyl radical, water cation radical, heavy water, pulse radiolysis

Table 1 .

 1 1 LET value for radiation.29

	Radiation	LET (keV/µm)	Radiation	LET ( keV/µm)
	1 MeV electrons				0.3	1 H + 300eV/amu	0.3
	60 Co γ-rays				0.3	4 He 2+ 6 Mev/amu	26
	137 Cs γ-rays				0.9	56 Fe 26+ 1 GeV/amu	150
	200 kV X-rays				1.7
	1.2.2 Definition of radiolytic yield
	( G molecule	/ 100	eV	)		100 Number of species formed or disappeared eV absorbed

Radiolytic yields presents the results of quantitative research on the chemical effects of ionizing radiation. The yields are expressed in terms of G: a number of molecules, of ions, of radicals, and of bonds etc., either formed or destroyed by 100 eV of absorbed energy at certain time. The formula of calculation of the G value is defined as:

  •+ , and e  , react further to produce chemically reactive species: OH• , H • , aq e  1.3.3 Non-homogenous chemical stage (~10 -12 -10 -6 s) Three of the new species created are radicals: OH • , H • , aq e  . These species now begin to migrate randomly about their initial positions. As this diffusion proceeds, individual pairs may come close enough together to react with each other. A variety of reactions are possible in the track of the charged particle.

	OH OH  	22 H O	EQ.1-15
		aq OH e   	OH		EQ.1-16
	OH H  	2 H O	EQ.1-17
		aq H e   	H		EQ.1-18
	aq e 	aq e   	22 22 H O H OH  		EQ.1-19
	aq e 		22 H H O H OH    		EQ.1-20
			H H		H	2

 

EQ.1-21

  Ferradini, C.; Jay-Gerin, J.-P. La Radiolyse de l'eau et Des Solutions Aqueuses : Historique et Actualité. Can. J. Chem. 1999, 77, 1542-1575.

	141-147.
	(25) (26) Zimbrick, J. D. Radiation Chemistry and the Radiation Research Society: A History from
	the Beginning. Radiat. Res. 2002, 158, 127-140.
	(27) Spotheim-maurizot, M.; Mostafavi, M.; DOUKI, T.; Belloni, J. Radiation Chemistry-From
	Basics to Applications in Material and Life Sciences; Spotheim-maurizot, M., Mostafavi,
	M., DOUKI, T., Belloni, J., Eds.; EDP SCIENCES: Les Ulis Cedex A, France, 2008.
	(28) Bethe, H. Zur Theorie Des Durchgangs Schneller Korpuskularstrahlen Durch Materie. Ann.
	Phys. 2006, 397, 325-400.

(29) Watt, D. Quantities For Generalized Dosimetry Of Ionizing Radiations in Liquid Water; CRC Press: London, 1996. (30) Pimblott, S. M.; LaVerne, J. A. Production of Low-Energy Electrons by Ionizing Radiation. Radiat. Phys. Chem. 2007, 76, 1244-1247.

Table of Content

 of

	2.1 Sate of art
	2.1.1 High LET
	2.1.2 Low LET
	2.1.3 Detection technique for ultrafast radiolysis
	2.2 ELYSE -A picosecond pump-probe radiolysis platform
	2.2.1 EA-1 -Picosecond pulse-probe setup
	2.2.2 EA-3 -Streak camera
	2.2.3 γ-source
	2.3 SK-Ana -MCR-ALS analysis method
	2.4 Chemicals
	References

Table 2 . 1

 21 The chemical products used in the thesis

	Product	Chemical formula	State	Role	Molecular mass (g mol -1 )	Density (g mL -1 )	Purity (%)
	Deuterium oxide	D 2 O	Liquid Solvent	20.03	1.107	99.9
	Sulfuric acid-d 2	D 2 SO 4	Liquid Solute	100.09	1.88	99.5
	Sulfuric acid	H 2 SO 4	Liquid Solute	98.08	1.84	99.99
	Methyl
	viologen dichloride	C 12 H 14 Cl 2 N 2‧ xH 2 O	Solid	Solute	257.16	-	98
	hydrate
	tert-Butanol	(CH 3 ) 3 CO H	Liquid Solute	74.12	0.775	≥ 99.5

Table 3 . 1 .

 31 Comparison of the physical properties of H2O and D2O

		Molecular weight (g/mol)	Density (g mL‾¹)	Viscosity (mPa) at room temperature	Dielectric constant at 25°C
	H2O	18.02	1.000	1.00	78.54
	D2O	20.03	1.107	1.25	78.3

Heavy water (D2O) was discovered by Dr. H. C. Urey in 1932 and attracted enormous interest in industry, biology and medicine. D2O exhibits different physical properties compared to normal water (H2O). As shown in Table

3

.1, there are higher molecular weight, density and viscosity in D2O than that in H2O. The deviations are expected to result in different behavior of the radicals s e 

Table 3 . 2

 32 The radiolytic yield value of

			2 DO e  at various stage and	2 DO e   reported in literatures at room
	temperature.
	G( (10‾⁷ mol J‾¹) 2 DO e  )	2 DO e   (M‾¹cm‾¹)	Description	Dose (Gy)	Time of measurement	Reference
	2.9	20200 at 700 nm	X-ray	0.05	Homogenous stage	Fielden 11
	3.06 ± 0.05	--	SF₆ scavenger method	--	Homogenous stage	Chase 18
	3.1	--	SF₆ scavenger method	--	Homogenous stage	Schmidt 12
	4.5	19100 at 705 nm	radiolysis Picosecond pulse	30 ~ 50		30 ps	Nielsen 19
	--	--	Radiolysis and calculations	--			Jonah* 15
	3.24	--	Radiolysis and calculations	--	Homogenous stage	Singh 13
	4.25	--	Calculations	--		1 ps	Chernovitz 9
	4.4	--	Picosecond pulse	--		60 ps	Muroya 5
			radiolysis

.2. It's obvious that there are controversial results on the time dependent yield and the absorption peak of 2 DO e  both in the spur reaction and in the homogenous stage.

Chapter III: Time Dependent Yield of the Solvated Electron and the OD • Radical in Deuterated Water at Low LET Studied by Picosecond Pulse Radiolysis 63

  Chapter III: Time Dependent Yield of the Solvated Electron and the OD • Radical in Deuterated Water at Low LET Studied by Picosecond Pulse Radiolysis 68 the spectrum obtained in this work and the extinction coefficient value presented by K. Honda to process the isosbectic method.

								0.7
								0.6
	Relative absorbance		510 -4 M MV 2+ in D 2 O 510 -4 M MV 2+ in H 2 O	Spectra of MV  in D 2 O observed by pulse radiolysis	0.2 0.3 0.4 0.5	Absorption / 1 cm
								0.1
								0.0
	350	400	450	500	550	600	650
				Wavelength (nm)

•+ is not affected by the isotopic change. However, the third maximum of the absorption band in this work is at 603 nm different from Honda's work. We used

  Time Dependent Yield of the Solvated Electron and the OD • Radical in Deuterated Water at Low LET Studied by Picosecond Pulse Radiolysis 71

	Chapter III:
	200 ns	500 ns

2+

. It is clear that

Table 3 . 3

 33 Comparison of the molar extinction coefficient of solvated electron in H2O and D2O at various wavelength, respectively.

	λ (nm)	Species	2 () HO e   (M -¹cm -¹) 27	2 () DO e   (M -¹cm -¹)
	260		723	707
	400		2510	2611
	500		6183	6653
	600		13482	16270
	705 (maximum)	19634	22900
	713 (maximum)	19687	22851
	800		16749	17894
	900		10460	9845
	1000		5085	4558
	1100		2512	2053
	1200		1538	1145
	1300		1082	758
	3.3.3 Determination of the time-dependent yield of	2 DO e 

  Chapter III: Time Dependent Yield of the Solvated Electron and the OD• Radical in Deuterated Water at Low LET Studied by Picosecond Pulse Radiolysis

	260  OD   A	260 measured   t A ( ) 	200 / 260    m Fused silica t A ( ) 	( ) t		22 2 600 260 600 ()          D O D O DO ee e At	EQ. 3-12
			90

Table 3 . 4

 34 The radiolytic yield of solvated electron and hydroxyl radical in the spur reaction of H2O and

	D2O.
		2 HO e 	2 DO e 	OH • radical	OD • radical
	max  (nm)	713	704	-	-
	max  (M -1 cm -1 )	19700	22900	-	-
	7 t ps G  (10 -7 mol J -1 )	4.5	4.5	5.0	5.0
	1 t ns G  (10 -7 mol J -1 )	3.83	3.96	3.17	3.41
	3 t ns G  (10 -7 mol J -1 )	3.46	3.65	2.91	3.04
	9 t ns G  (10 -7 mol J -1 )	3.00	3.17	2.91	3.02
	The use of the isosbestic point for the determination of the extinction coefficient with a specific

scavenger is more appropriate because it is based only on the knowledge of the molar absorption coefficient of the reaction product. We found that the value of the initial yield of the hydrated electron and the hydroxyl radical, observed at 7 ps, are equal (within the experimental errors) to Chapter III: Time Dependent Yield of the Solvated Electron and the OD • Radical in Deuterated Water at Low LET Studied by Picosecond Pulse Radiolysis 94

Ultra-fast Charge Migration Competes with Proton Transfer in the Early Chemistry of H 2 O •+ 4.1 State of the art

	Chapter IV: Ultra-fast Charge Migration Competes with Proton Transfer in the Early Chemistry of H2O •+
	Table of content 4.1 State of the art 4.1.1 General introduction 4.1.2 Oxidation reactions by radicals Chapter IV: 4.1.1 General introduction
	4.1.3 Reactivity of H 2 O •+ in halide solutions
	4.1.4 Reactivity of H 2 O •+ in concentrated acidic solutions
	4.2 Results -Reactivity of radical cation of water in D 2 SO 4 / D 2 O system
	4.2.1 Transient absorption spectra
	4.2.2 Analysis of the kinetics
	4.2.2 Yield of scavenging H 2 O •+
	4.3 Results -Reactivity of radical cation of water in D 2 SO 4 / D 2 O system
	4.3.1 Experiment on isotopic effect
	4.3.2 Discussion and analysis
	4.4 Conclusion
	Reference
	108

  •-couple is -0.33 vs NHE.35

Table 4 . 1

 41 The primary radiation induced reactions involved in Cl -/H2O solution

	Primary radiation induced reaction	Rate coefficient (L mol -1 s -1 )
	22 H O H O e  		R1
	2 e H O   	2 H O*	R2
	2 H O H O H O OH 2 3     		8.0 × 10 12	R3
	s e H O 3  		2 H H O   	2.3 × 10 10	R4
	Reactions Cl -/H2O
	Cl		Cl e  		R5
	OH Cl   	ClOH		4.3 × 10 9	R6
	ClOH		Cl OH  		6.1 × 10 9	R7
							2

Table 4 . 3

 43 The primary radiation induced reactions involved in H3PO4/H2O solution

	Primary radiation induced reaction	Rate coefficient (L mol -1 s -1 )
	22 H O H O  	e		R1
	22 e H O H O*   	R2
	2 H O			2 H O	 	3 H O OH  		8.0 × 10 12	R3
	s e H O 3  		2 H H O   	2.3 × 10 10	R4
	Reactions H3PO4/H2O
	32 44 PO PO    	e		R11
	3 H PO H O 4 2 			2 H PO H O 4 3  		R12
	3 H PO OH 4 	2 H PO H O 4 2   	4.2 × 10 4	R13
	2 H PO OH 4  			HPO	4 		2 H O	2.0 × 10 4	R14
	2 H PO H 4  	3 H PO	4	1.3 × 10 10	R15
	2 H PO H PO 4 2  4 	4 2 8 H P O	2.5 × 10 9	R16
	2 H PO OH 4  	3 H PO	5	4.0 × 10 9	R17
	2 H PO HO 4  2 		3 H PO O 4 	2	3.0 × 10 9	R18
	2 H PO H O 4 2 2  3 4 2 H PO HO   	3.5 × 10 9	R19

•+ 123

  •+ by rapid electron transfer reaction.

						Concentration de H 3 PO 4 (mol L -1 )
		4.0	0	1	2	4	6	8	11	13	14.6	19
	-1 ) Yield of phosphate radical (mol J	1.0 1.5 2.0 2.5 3.0 3.5		H 2 PO  4 generated by Indirect effect Direct effect
	10 7 	0.5
		0.0
		0.0		0.2		0.4	0.6		0.8	1.0
						Electron fraction of H 3 PO 4 (f s )

Table 4 . 4

 44 The primary radiation induced reactions involved in H3NO3/H2O solution

•+ 125

Primary radiation induced reaction Rate coefficient (L mol -1 s -1 )

	22 H O H O  	e		R1
	22 e H O H O*   	R2
	2 H O			2 H O	 	3 H O OH  		8.0 × 10 12	R3
	s e H O 3  		2 H H O   	2.3 × 10 10	R4
	Reactions H2SO4/H2O
	NO	33 NO e   		R20
	NO	2 33 e NO     	4.5 ×10 12	R21
	3 NO H O 2  			3 NO H O 2  	R22
	3 HNO OH 	3 NO H O 2   	5.3 × 10 7	R23
	33 NO H HNO    	1.0 × 10 7	R24

Table 4 . 2

 42 The primary radiation induced reactions involved in H2SO4/H2O solution

	Primary radiation induced reaction	Rate coefficient (L mol -1 s -1 )
	22 H O H O  	e		R1
	22 e H O H O*   	R2
	2 H O				2 H O	 	3 H O OH  		8.0 × 10 12	R3
	s e H O 3  		2 H H O   	2.3 × 10 10	R4
	Reactions H2SO4/H2O
	SO		2 44 SO   	e		R20
	2 H SO H O 4 2 			4 HSO H O 3  		R21
	2 H SO OH 4 			HSO	4 		2 H O	1.4 × 10 7	R22
	4 HSO OH  			4  SO		2 H O	4.7 × 10 5	R8
	44 H HSO  SO   	1.0 × 10 10	R9
	SO	4 	4   SO	2 2 8 S O		7.6 × 10 8	R10
	SO	45 OH HSO    	1.0 × 10 9	R11
	4  SO		22 2 8 2 8 4 S O S O SO    		6.6 × 10 5	R12
	SO	4 		HO	2 		4 HSO O  	2	3.5 × 10 9	R13
	2 2 8 S O		H   	4  SO		HSO	4 	2.5 × 10 7	R14
	5 HSO H    	4  SO		2 H O	2.2 × 10 8	R15

  •+ is, in fact, the acidic form of OH • in aqueous solutions but the pKa value is not clear. It is known that OH • radical will attack guanine molecules, the most easily oxidized DNA bases, through a series of complex processes. It involves H-abstraction, adducting and one-electron oxidation reaction at similar rates, leading to the formation of a variety of guanine based radicals. Interestingly, our recent study of radiation-induced guanine oxidation in highly concentrated phosphoric acid (6 M) shows all of OH • radicals in strong acid condition solely oxidize guanine molecule to guanine cation (G +• G •+ ) (Figure4.15). It is therefore inferred that H2O •+ radical cation in some cases indeed displays a distinguished reactivity from OH • radicals.

Table 4 . 5

 45 The primary radiation induced reactions involved in D2SO4/D2O solution

	Primary radiation induced reaction	Rate coefficient (L mol -1 s -1 )
	22 D O D O  	e		R1
	22 e D O D O*   	R2
	2 D O			2 D O D O OD 3   		-	R3
	s e D O 3  		2 D D O   	9.5 × 10 9 60	R4
	Reactions D2SO4/D2O
	SO			2 44 SO   	e		R26
	SO	2 4			2 D O			SO	4 		2 D O	R27
	2 D SO OD 4 			DSO	4 		2 D O	-	R28
	4 DSO OD   		4  SO		2 D O	-	R29
	44 D DSO  SO   	-	R30
	SO	4 		4   SO	2 2 8 S O		7.6 × 10 8	R31
	SO	45 OD DSO    	-	R32
	4  SO			22 2 8 2 8 4 S O S O SO    		6.6 × 10 5	R33
	SO	4 			DO	2 		4 DSO O  	2	-	R34
	2 2 8 S O		D   	4  SO		DSO	4 	-	R35

Table 6

 6

	) and

   reported in the literatures by radiolysis and photolysis measurements[START_REF] Tang | Kinetics and Spectroscopy of The SO4 -Radical in Aqueous Solution[END_REF] both in diluted and concentrated solutions up to 10 M. It is remarkable that the peak of the absorption band in 18 M shifts to 460 nm. This is in account of the protonation

	of	4 SO  as the non-dissociation D2SO4 molecules are the predominate species in extremely high
	concentration. The reaction is as:
		44 D DSO  SO   	EQ.4-8
	To determine the radiolysis, the molar extinction coefficient of	4 SO  is necessary. In this study, the
	value of the molar extinction coefficient of	4

4 SO

  4.2.2 Yield of scavenging D 2 O •+ in D 2 SO 4 /D 2 O solution

	Since the time dependent absorbance of	4 SO  corresponding to different concentration at 450
	nm is known, the time dependent radiolytic yield of	4 SO  can be deduced by:
				4 450  SO   A	( ) t			4 450  SO  		G	4  SO	( ) t D  sol		l	EQ.4-12
	Where		4 450  SO  	is 1600 M -1 cm -1 as mentioned, sol D is the dose deposited in the solution and l is the
	length of the optical path. Additionally,
								sol D	2 H O D 	F	EQ.4-13

Table 4 . 7

 47 Concentration of sulfuric acid solutions and their corresponding parameters: The electron density fraction of solute 𝑓 𝑠 . b The electron density fraction of D2O 𝑓 𝐷 2 𝑂 . c The absorbed dose factor F. d The Yield of SO4 • ˉ observed after 7 ps electron pulse which is deduced from the kinetics according to the data from Figure 4.19:

	[D 2 SO 4 ]	[D 2 O]	[D 2 O] [D 2 SO 4 ]	𝑓 𝑠 ᵅ	𝑓 𝐷 2 𝑂	b	Density (g ml -1 )	F c (g cm⁻³)	 4 SO G d 7ps (×10⁷ mol J⁻¹ )
	1	53.4	53.4	0.09	0.91	1.17	1.06	0.84
	2	51.4	25.7	0.16	0.84	1.23	1.11	1.44
	4	46.8	11.7	0.30	0.70	1.34	1.21	2.24
	6	42.4	7.07	0.41	0.59	1.45	1.31	2.92
	8	37.9	4.74	0.51	0.49	1.56	1.41	3.50
	10	33.0	3.30	0.60	0.40	1.66	1.50	3.87
	12	27.0	2.25	0.69	0.31	1.74	1.57	3.96
	14	20.4	1.46	0.77	0.23	1.81	1.64	3.95
	16	12.5	0.78	0.87	0.13	1.85	1.67	3.86
	18	2.7	0.15	0.97	0.03	1.88	1.70	3.75
	ᵅ 4 450 SO A   	450 meas ure d t A ( )   	( ) t A e s    	450	450 FSOF C  t A ( )  	( ) t	.
	Where	2

  •+ scavenged by sulfate anion. It is notable that the scavenged yield of D2O •+ increases with the electron fraction of sulfuric acid and reaches a plateau with a value around (4.75×10 -7 ) mol J -1 . Particularly, this behavior is observed on the concentration of 14 and 16 M, where the amount of water is so low that the reaction R2 is almost suppressed and total D2O •+ is scavenged by sulfuric ion in few tens femtosecond. For these solutions, the yield of

	Chapter IV: Ultra-fast Charge Migration Competes with Proton Transfer in the Early Chemistry of H2O •+
	D2O •+ is in close proximity to that generated just after ionizing radiation. Moreover, the value
	obtained is larger than the yield of hydrated electron in D2O (confirmed in Chapter III). That meant
	that the proton transfer (R2) and electron transfer (R27) reactions are in competition with the
	germinate recombination:
	22 D O e D O   	EQ. 4-14
	147

Table 4 .7).

 4

	•+

Acknowledgements

Firstly, I would like to express my sincere gratitude to my advisor Prof. Mehran Mostafavi for the continuous support of my Ph.D study and related research, for his patience, motivation, and

with theoretical simulations. Towards this end, our strategy is to investigate the change of the yield of the oxidative pathway upon isotopic substitution (H by D). We expect to bring new insights into the competition between the two parallel reactions, proton transfer toward another water molecule and electron transfer from the solute, both involving the radical cation of water. 3.1), meanwhile the same electron number as H2O.

Deuterium sulfuric acid (D2SO4), which also contains two hydrogen isotopes deuterium, has larger molecular weight and density that H2SO4. As D2SO4/D2O solution is a homogenous system like H2SO4/H2O, it is a highly appreciated system to study the isotopic effect on water radical cation by H/D. Moreover, the parameters of the solvated electron and hydroxyl radical in D2O in the spur reaction have been studied in Chapter III, thus it will help to learn about the radiation reaction in D2SO4/D2O system The primary reactions occurring in D2SO4/D2O system are presented in detail in Table 4.5. [START_REF] Stuart | Pulse Radiolysis Studies of Liquid Heavy Water At Temperature up to 250[END_REF] Compared with H2SO4/H2O, there is less information about the ionizing reaction in the D2SO4/D2O system. Most rate constants of the reactions involved in D2SO4/ D2O system have not been

Chapter V: Conclusions and perspectives

Conclusions

From the first measurement by Hart et al. 1 during the 1960's, the progress in the knowledge of radiation chemistry throughout is accelerated by the deep-going studies by pulse radiolysis. The importance of the pulse radiolysis technology is undoubtedly underlined. The picosecond pulseprobe facility ELYSE allows us to explore the ultrafast radiolysis process in solutions induced by high energy radiation (6-9 MeV). In ELYSE, short-lived species is generated with a pulse of electrons much shorter than the time constants of the subsequent reactions, and detected by timeresolved optical techniques. Time-resolved absorbance measurements of the short-lived species at picosecond range are achieved by two complementary fast detection systems. One is a pump-probe set-up, which synchronize the laser beam and the electron beam to measure the absorbance. The pump is the electron pulse meanwhile the probe is a part of the laser beam delayed by a variable optical delay (7 ps-12 ns). The probe light originated from the laser source could cover the wavelength range from 360 nm to 1500 nm to detect the intermediates that absorb in these region.

Another system is based on a high dynamic range streak camera (250-800nm) and pulsed laser diodes (absorbance kinetics at given wavelengths) or a Xenon flash lamp. Streak camera could be used to observe from ps to 1 ms sweep time. Both systems display high signal-to-noise ratios suitable for fast transient absorption kinetics studies. The pump-probe and streak camera systems permit to trace the ultrafast reactions or chemical processes induced by the fast electron taking place from the timescale of picosecond to millisecond.

The focus of my thesis is on the solvated electron, hydroxyl radical and water cation radical in deuterated water. Firstly, we studied the radiolytic properties of the solvated electron in D2O. The solvated electron is a unique species that can be produced in most polar and low polar solvents.

The study of its properties is an important key factor to understand the dynamics and kinetics reactions in solutions. Moreover, the hydrated electron in neat D2O is used for the measurements at high temperatures to study the temperature effect on the radical yield and the spur kinetics. 2,3 Then the knowledge of its properties and its reactions in various solvents is a key issue in physical chemistry. In radiation chemistry, the measurements of the absolute value of the radical yields at shortest time is obviously necessary to obtain two important information: the energy deposition in