F Grollau

C Pothier

M A Gaveau

M Briant

N Shafizadeh

B Soep

Action spectroscopy of spin forbidden states in the gas phase: a powerful probe for large non luminescent molecules

Triplet action spectra of two similar copper porphyrins, copper Tetraphenylporphyrin (CuTPP) and copper Octaethylporphyrin (CuOEP) have been studied in the gas phase at low temperatures in absence of external perturbations by a resonant pump and a 193 nm probe ionizing the 3 * orbital localized on the porphyrin cycle. The molecules were prepared by laser desorption in a disk source, then cooled in a helium supersonic expansion and finally excited in the Q band system (S1 ← S0). This type of experiment allows the spectroscopic characterization of large non luminescent molecules in absence of solvent perturbations. The two copper porphyrins exhibit a broad electronic origin Q00 absorption spectrum, partly caused by the short lifetime of the excited (S1) state. The two porphyrins differ strongly with a strong Q00 band for CuOEP and a weak one for CuTPP, in agreement with M. Gouterman's 4 orbital model. Also, the two molecules exhibit different solvent shifts: CuOEP is blue shifted in non polar solvents owing to its alkyl substituents, while CuTPP is red shifted as for regular transitions to * orbitals. The decay dynamics of the triplet state exhibit a collision free lifetime of 70±7 ns for CuTPP atop a microsecond decay. This non exponential decay can be viewed as an evidence of time evolution of two states combining: the state with spin 1 borne by the porphyrin ring and that by the Cu atom, ½. Therefore, this method allows solvent free spectrodynamics of large molecules in the short microsecond time range.

Introduction

Determining the electronic absorption spectra of neutral molecules in the gas phase is essential since spectra characterize uniquely the electronic structure of these molecules in the absence of solvent perturbations. While electronic absorption spectroscopy in large molecules is performed routinely in solutions by direct transmission measurements, the equivalent measurements in solvent free large molecules are difficult since these molecules have to be evaporated intact and in sufficient number density to provide absorption. This was nonetheless achieved in large stable molecules at the expense of spectral broadening due to high temperatures maintaining a sufficient number density 1 . As an indirect method, fluorescence excitation spectroscopy has been extensively demonstrated for large molecular ions 2,3 , however for large neutral molecules it is of limited interest, since generally these molecules have a low fluorescence yield due to fast electronic relaxation. Resonance multiphoton ionization stepping on the electronic or vibrational state investigated, is an interesting solution and can be achieved with short lasers whose time and spectral profiles are adapted to the excited state lifetimes and their spectral diffuseness. Many groups have thus studied short lived states [4][5][6][7] of molecules or clusters by resonant two photon ionization, yielding resolved electronic or vibrational spectra in vacuo.

Another alternative for the investigation of short lived states is action spectroscopy, where the outcome of a tunable excitation is probed as a function of excitation wavelength, revealing the absorption spectrum. This is often practiced in the spectroscopy of molecular ions where the formation of fragment ions monitors the absorption of the parent during a wavelength scan 8 .

The general difficulty is maintaining a constant branching ratio for this dissociation channel to obtain the unperturbed absorption spectrum. This is achieved by adding a weakly interacting atomic or molecular tag that immediately evaporates after excitation 9 , yielding a quasi-unit dissociation yield at frequencies away from the 0-0 transition, where 1 photon direct dissociation is energetically forbidden. The resulting spectrum is only weakly shifted, especially in the infrared, due to the tagging. The detection of electronic relaxation to metastable states, generally triplet states in organic molecules, is an interesting alternative, since triplet state formation is quite universal among organic molecules and phosphorescence excitation spectra are often used to characterize spectrally large molecules. 10 Also this action spectroscopy does not exclude the origin 0-0 transition.

Here, instead of monitoring the long lived phosphorescence of molecules, we detect the formation of a local triplet state by its selective ionization. This has the advantage of instantly probing the desired electronic state without relying on the phosphorescence emission yield and with a linear intensity dependence with the excitation and ionization lasers. This could be called triplet tagging.

The presence of a transition metal in the porphyrin pocket causes in excited states, an ultrafast electronic relaxation, generally by a charge transfer (CT) process 11 . This electronic relaxation flowing electrons through the metal in the CT state leads specifically in copper porphyrins (CuTPP, CuOEP, Figure 1) to a 3 * state localized on the porphyrin cycle after the decay of the CT state within picosecond times 12,13 , an exceptionally short time for intersystem crossing (scheme in Figure 2). This stems from the existence of a spin on the Cu atom (s=1/2) and its coupling with the local electronic spin on the excited porphyrin (s=1) 14 . The consideration of the local spin on the porphyrin ring is essential since the optical absorption of the Cu porphyrin is conditioned by the porphyrin  delocalized orbitals, while the electronic dynamics of the porphyrin is powered by the coupling of the porphyrin and the Cu atom. Indeed, fast evolution can occur without change of the total initial spin S=1/2 [Cu(s=1/2); TPP (s=0) ] to the final state S=1/2 [Cu(s=1/2) ; TPP (s=1) ] in copper metalloporphyrins. 7 The latter final state is labeled as tripdoublet 2 T (T from the local s=1 spin on the porphyrin and doublet from the total spin S=1/2) 14 . Thus, the local spin of the electrons of the  cloud of the porphyrin can evolve rapidly while no spin orbit cross over is needed, through an internal electron transfer mechanism 12,13 . This situation applies identically to CuOEP 12 13 , but is in sharp contrast with that of metal free H2TPP where 3 * triplet formation occurs within nanoseconds. This slow evolution is due to the need of spin orbit coupling for 1 * intersystem crossing 15,16 .

CuTPP, CuOEP have been prepared by laser evaporation in a special source, cooled in a supersonic expansion of helium and excited in their Q band system by a tunable ns laser in the 450-600 nm region. Finally, the excited porphyrin is probed by ionization with an ArF laser at 193 nm. The 6.43 eV photons provided by the ArF laser is at threshold for the ionization of CuTPP (IP =6.49 eV 17 ) and CuOEP in their electronic ground state, while it efficiently ionizes their triplet state. The delay of the ArF laser allows an investigation of the local 3  porphyrin triplet population decay as a function of time on the ns time scale. It yields a solvent free triplet action spectrum at a vibrational temperature of ≈70 K (temperature achieved in the supersonic beam).

CT

Experimental

The source set-up An improved evaporation source has been used for this experiment, following the original design of Levy and coworkers 18 . It is constructed from a disk evaporation source associated with a long, 2 cm channel. The latter arrangement has been used mostly for metal ablation and the formation of their dimers or clusters 19,20 . The disk source offers a large surface to evaporate thanks to a mechanical asymmetrical rotation system, allowing the complete exploration of its surface, see Figure 3. It ensures a longer average stability of the sample product. The channel design contributes to the mixing and thermalization of the laser-evaporated molecules, before the supersonic expansion. We have used this set-up to evaporate porphyrin molecules and observe their visible spectrum by resonance induced sequential two photon ionization.

The laser evaporation source represented in Figure 3 is very similar in its principle of a rotating -translating disk, to that described in ref 18 . The helium gas pulse is created by a solenoid valve with a backing pressure of 2.5 bar (General Valve / Parker) operating at 10 Hz with a Vespel poppet and an opening time of ~170 µs. The valve orifice (0.7 mm) is opened on a channel with a 1 mm diameter and 2 cm long, created just below the surface side of a rectangular block of stainless steel fixed on the pulsed valve. At the end of the cylindrical channel, the gas undergoes supersonic expansion in the source vacuum chamber. An unfocussed 532 nm, frequency doubled Nd:Yag laser (Continuum Minilite) is fired across the gas channel onto the target, a graphite-sample disk (12 mm diameter) placed on the side of the block; this leads to an easy adjustment of this laser.

The disk is pressed from graphite powder (50%) and CuTPP or CuOEP molecules (50%) in a mold under 10 Tons/cm². The assembled disk is gently pressed over the rectangular block surface on its side, as sketched in Figure 3, by a lamellar spring. The ensemble slides on the smooth surface with a rotation translation movement that allows the unfocussed 532 nm laser to scan the whole surface. The mechanical device is composed of a wheel rotating at ~0.5 turn/second fitted with a silicon O-ring, see Figure 3. The wheel axis is off-centered by 3 mm from the center of the disk, this provides a 6 mm total lateral displacement after a rotation of the wheel over. This amplitude corresponds to the graphite disk radius, allowing the coverage of the surface. The impact of the laser on the surface of the disk thus moves along an unclosed cycloidal curve that covers it. The cycloid is generated by the two rotating circles with of noncommensurate dimensions and a radius ratio close to 0.9. The springs ensuring the lateral contact of the two wheels and the contact on the surface are chosen to be as gentle as possible, in order to provide contact but no wobbling of the rotation axis perpendicular to the surface.

After the laser hits the surface of the disk, the evaporated molecules are entrained into the gas channel mixing with a synchronous helium gas pulse.

Operation of the source

The evaporation source is installed in a molecular beam machine already described in reference 21 . In brief, the source chamber is pumped by a 2000 l/s turbomolecular pump at 10 -4 mbar and the resulting molecular beam is introduced through a 2 mm skimmer in the second chamber operated at 5.10 -7 mbar. Triplet action spectra are performed in a pump probe configuration of two counter propagating lasers: the pump tunable between 450 and 700 nm (nanosecond dye lasers pumped at 355 or 532 nm) and the probe, an ArF (193 nm) laser electronically synchronized with a 5 ns precision. The ionization laser is separated by ~25 cm from the tip of the laser evaporation nozzle.

The ions are detected by either a time of flight mass spectrometer 22 on axis with the molecular beam, or by a perpendicular velocity map imaging spectrometer 23 . This latter device allows the characterization of the cooling within the beam and of the actual group velocity of the porphyrin molecules within the helium beam.

The synchronization of the firing of the valve and the laser pulses is the key adjustment procedure and is achieved by an electronic synchronization module constructed at ISMO (Paris-Saclay University, France). Essentially, the pulsed valve is fired first by a 170 µs width electrical pulse, then the evaporation laser is fired with a delay of 200 µs. The pump and ionization laser have a global delay of ~300±1 µs after the evaporation laser, after the evaporated bunch has arrived within the spectrometer ionization region. They are triggered independently with a final variable delay of -50 to 1000 ns, with a 5 ns precision.

Cooling in the expansion

Monitoring the translational cooling of molecules diluted in the expansion is the most direct means of observing and characterizing the freezing of the various degrees of freedom of large molecules seeded in a molecular beam. This can be easily achieved using velocity map imaging.

Here CuTPP has been ionized at 532 (I)+ 266 nm (I'), while the background gas is ionized by 2*266 nm photons (I'), as shown in velocity distribution of the ions, thus the translational energy distribution of the corresponding neutrals. This reveals two species, since no mass gating has been applied. The broad spot can be assigned to the background gas with a thermal distribution, since it exists in absence of porphyrin evaporation. The narrow spot on the left corresponds to a bunch of CuTPP molecules with an average translational velocity of 900 m/s, from its displacement from the still background molecules. The translational velocity of the porphyrin is acquired by entrainment of the laser-evaporated molecules due to collisions with the expanding helium gas. This large molecule is accelerated along the helium flow and is cooled by heat exchange (collisions) with helium. The measured translational distribution of the porphyrin is fitted to a ~20 K Boltzmann translational energy distribution (red narrow curve in Figure 4). A rather cold distribution can be thus obtained with a moderate nozzle pressure of helium (2.5 bar). The corresponding vibrational temperature was evaluated as 70 K with the equivalent ZnTPP molecule and monitoring the intensity of a low frequency hot band at 33 cm -1 as compared to the origin Q00.

The important cooling achieved for a molecule of mass 676 amu (CuTPP) is also apparent through the observation of multimers, easily detected here when the sample concentration is sufficient, i.e. with a larger evaporation laser intensity. These multimers could not exist if the cooling could not dispose of their binding energy.

Results and discussion

Two types of experiments probing the copper porphyrins triplet population are performed:

 Triplet action spectra: the tunable pump laser scans the Q band absorption spectrum of the molecule (500-600 nm), while the 193 nm probe laser ionizes selectively triplet CuTPP or CuOEP after ns delay.

 Triplet lifetime measurements: The lifetime of this state is measured by varying the delay (-50, +1000 ns) of the probe laser (193 nm) with respect to the tunable pump.

Several measurements where performed at different pump wavelengths in the 450-560 nm domain for CuTPP.

Triplet action spectra

CuTPP

A series of action spectra have been recorded with 3 different dyes (coumarin 503, 540 A and fluorescein 550) in overlapping regions and combined in Figure 5a. In this figure, two regions are distinguished, the Q00 band origin domain at 558 nm and the vertical excitation of the Qv bands at 530 nm. For comparison, a room temperature spectrum of CuTPP dissolved in toluene recorded at ISMO is also represented as a dashed curve. The main maximum of the asymmetric band at 532 nm (18 800 cm -1 ) is red shifted by ~280 cm -1 in the solution spectrum. Since the corresponding band at 540 nm (18 520 cm -1 ) is symmetric in solution due to spectral broadening by the solvent, in order to compare with solution data, one can also consider the center to center distance of both bands, 430 cm -1 . The reddest band is at 558 nm (17 920 cm -1 ) and the corresponding band is red shifted in solution by 550 cm -1 , this red shift is regular for a * transition and due to the greater solvation of the diffuse excited  state. It is noted that the 558 nm band is very weak compared to that at 532 nm and the same is true in solution, as discussed later.

CuOEP

The same type of spectrum is represented in Figure 5b for CuOEP and exhibits a notably different shape as compared to CuTPP, with a broad absorption (16 nm) maximum at 568 nm (17600 cm -1 ) and a very weak maximum at shorter wavelength. It is compared with the toluene absorption (dashed line) and also with the gas phase absorption at 654 K 1 (blue) that is clearly broadened by hot bands. The CuOEP absorption has its maximum on the Q00 band as assigned by Gouterman et al., as the result of many experimental 1 and theoretical studies 25 .

Gouterman, in a simple model derived from cyclic polyenes and based on Hückel type LCAO combinations (nearest neighbor perturbation) has given principles to rationalize the detailed behavior of B and Q bands. The B bands, at 400 nm, also known as Soret bands, are generally strong and the Q bands such as those here are much weaker. In a simple four conjugated orbital model, the visible Q and near UV B bands of porphyrins have been described as originating from two configurations in D4h symmetry, a1u and a2u -accidentally degenerate in many porphyrins. Electronic excitation from a1u or a2u to a degenerate eg orbital leads in two states of Eu symmetry of configurations a1u eg or a2u eg. Due to configuration interaction, these two configurations are mixed generating two states, the higher energy two electron state corresponding to the symmetric combination of a1u eg and a2u eg, while the lower energy one is due to its antisymmetric combination. These two states exhibit transition moments from the ground state that are symmetric and antisymmetric combinations of two equivalent moments.

Thus a strong absorption is obtained for the symmetric combination, sum of the a1u eg and a2u eg moments, while the corresponding difference for the antisymmetric combination has a reduced absorption intensity. This explains the strong B and weak Q bands. However this is resting on symmetry and on the accidental degeneracy of the a1u and a2u configurations. Such is the case for CuTPP. In addition, due to the low intensity of the Q00 transition, vibronic effects are manifest where a more intense Qv band (vibrationally excited) at higher energy borrows intensity to the B state and overwhelms the Q00 transition.

In turn, when the degeneracy of the lower configurations a1u or a2u is diminished, the Q band gains intensity since the configuration mixing is less efficient. Indeed, in CuOEP, the alkyl groups attached to the pyrolle may inject electrons into the adjacent  orbitals of the pyrolle ring to the a1u, configuration, thus raising its energy relative to the unaffected a2u 26 . This lifts the degeneracy of the a1u and a2u configurations, hence changes the sum and differences of the transition moments leading to a less intense B band and a more intense Q band. In this situation, the Q00 transition being augmented, the relative intensity of the Qv band is diminished as observed for CuOEP. In both CuOEP and CuTPP , the Q00 -Qv distance is ≈1200 cm -1 as for the basic copper porphyrin 10 .

In CuTPP and CuOEP, the Q00 bands at respectively 558 and 568 nm are broad. For CuOEP the breadth is ~390 cm -1 at half maximum (~10 nm) and CuTPP is equally broad. This width is very similar in the toluene solutions of both compounds. Two factors contribute to this width: i) the 70 fs lifetime 12 , causing a homogenous width of 80 cm -1 , ii) a spectral congestion at the ~70 K vibrational temperature. In CuTPP, for instance, many low frequency modes are populated, among them phenyl ring torsions identified at 22 and 45 cm -1 in tetraphenyl porphyrins, those are optically active and generate hot band progressions 27 . Thus they should contribute to the origin Q bandwidth trough Q11 and Q22 torsion sequences (=0) close to Q00, either blue or red shifted depending on their frequency change on excitation. A similar broad and diffuse Q00 band was observed for copper protoporphyrin prepared in similar conditions 28 .

The temperature evolution of the Q00 transition appears in Figure 5b in blue at 654 K with the vapor absorption spectrum of CuOEP twice as broad as it is at 70 K, and shifted to the blue.

This blue shift of the sequence bands results from the low frequencies being higher in the excited state. The CuOEP spectrum in Figure 5b is very similar in position and diffuseness to that of copper protoporphyrin obtained in the gas phase by resonant two photon ionization 28 .

The latter intense Q00 band of CuOEP appears also in solutions but blue shifted in toluene by ≈300 cm -1 . This shift generally identifies n* transitions through their excited state less stabilized in solution by their loss of their non-bonding n electron. The blue shift is however unusual for a * transition such as for the Q transition, showing generally a solvent red shift, as for CuTPP in Figure 5a where the solvent spectrum (dashed line) is red shifted from the red gas phase curve. As can be seen in Table 1 for an ensemble of metalloporphyrins, the Q00 bands of tetraphenylprophyrins and phtalocyanins are red shifted in non coordinating solvents, while those of octaethyl porphyrins are blue shifted like in the case of ZnOEP. Triplet states of CuTPP refers to electronic states of the porphyrin ring bearing a s=1 spin on a ring centered delocalized 3 * orbital. Triplet states are created through an ultrafast process 7 with an intermediate CT state conserving the total spin ½ 12 33 . This ultrafast intersystem crossing is mediated by a charge transfer state with a 350 fs lifetime as observed for CuTPP (or CuOEP) following B state excitation 7 and equivalently 260 fs in benzene solution 13 .

However, by interaction with the spin on the Cu atom, this state is split in two final states: 2 T (S=1/2 ; Cu s=1/2, TPP s=1) directly formed by the above process and 4 T (S=3/2 ; Cu s=1/2, TPP s=1) 33 . The latter quartet 4 T state, which is not formed directly, has been characterized in the case of copper porphyrin CuP by its Zeeman splitting at 4.2 K 34 . 4 T is the lowest state for all studied copper porphyrins and for its total spin change with the ground state, it is expected to exhibit a longer radiative lifetime than 2 T. The 2 T -4 T splitting also differs through the different copper porphyrins: CuOEP (octaethylporphyrin) has a smaller gap ~400 cm -1 than CuTPP, with a 800-1000 cm -1 gap 35 . It is possible to link the present observation of two decays to that of two states in CuTPP: 2 T formed directly by the ultrafast process and decaying with the 70 ns lifetime into 4 T that further evolves with the observed longer lifetime >5 µs. On the other hand, CuOEP displays only a single decay time, 300±30 ns. In this case, the 4 T decay time may be shorter than this value. Both molecular beam decay values 70 ns (CuTPP) and 300 ns (CuOEP) are in the same range as that of 300 K solutions (100 ns range 35 ), but are very different from those in frozen solutions at 77 K, in the order of 100 µs 33 , close to the radiative lifetime. The populated 2 T state decays either directly or indirectly (via 4 T as in CuTPP) to the ground state in a much faster than the radiative decay rate.

Conclusion

We have observed the action spectra of the short lived S1 (Q) excited state of CuOEP and CuTPP in a cooled molecular beam using the triplet tagging method with a pump probe arrangement combining a 500 -600 nm resonant pump and a 193 nm delayed probe. Solvent free spectra are obtained for these porphyrins bands that accord with the 4 orbital model with respect to the intensities of the origin Q transitions. On the other hand, these solvent free spectra, rather rarely obtained for large neutral molecules, can be compared with solution spectra revealing the nature of the solvent shifts. These gas phase spectra are naturally broad though their short lifetime and are further broadened by sequence bands close to the origin. The triplet decay time measurements confirm the potential of triplet action spectra in the gas phase detected a few ns after excitation by ionization. The ns delay time ensures a good sampling of the triplet population even for weakly emitting systems. The sensitivity of the method could be greatly improved with a suitable choice of the probe, here close to the ionization energy of the system.

Using a 4.6 eV (266 nm) probe that ionizes selectively the triplets 2 T and 4 T should increase by an order of magnitude the detection. This should allow a generalization of this methodology for solvent free measurements.

Figure 1 :

 1 Figure 1: Chemical structures of CuTPP (left) and CuOEP (right).

Figure 2 :

 2 Figure 2: Schematic representation of the transient energy levels of CuTPP after a 2 S1 ←-2 S0 excitation. The doublet and quartet superscripts refer to the total porphyrin+metal spin, while the S and T refer to the local multiplicity of the porphyrin cycle as in ref 12 .

Figure 3 :

 3 Figure 3 :Sketch of the evaporation source. A cut has been made along AA' to visualize the gas channel. The bottom wheel actuates with an off axis rotation the sample holder pressed upon it, sliding across the surface represented by a dashed rectangle. Therefore, the evaporation spot in red moves along an unclosed cycloid on this sample surface. The translation and rotation movements of the driving wheel are indicated in green.

Figure 4 .Figure 4 :

 44 Figure 4 :1+1' REMPI of CuTPP with 532+266 nm, left spot and corresponding translational energy distribution (Ibeam), full black trace with 20 K Boltzmann fit in red. On the right, a broad spot due to ionization of the background chamber impurities with 2*266 nm distribution shown by the dashed curve, a 300 K Boltzmann fit in red.

Figure 5a :

 5a Figure 5a: triplet action spectrum of CuTPP in the Q band region with a tunable pump 490-570 and an ArF 193 nm probe preferentially ionizing 2 T CuTPP (red). Dashed line, room temperature toluene solution spectrum. b) triplet action spectrum of CuOEP in the same conditions (red). Dashed line, absorption in toluene solution, green line absorption in CH2Cl2 solution and blue line, vapour absorption at 654 K ( adapted from ref 1 ).

  at 525 nm close to the Qv absorption maximum is represented in Figure6a. It can be fitted by an exponential lifetime, 70 ±7 ns with a Gaussian convolution of 1/e width=10 ns. This decay is superimposed on a long-lived component, > 5 µs (10%). A series of decays has been recorded from the second maximum at 532 nm to the 558 nm Q00 origin and no increase of the lifetime was detected with decreasing the excess energy, comparing Figure6a and b with 532 and 558 nm excitations, and using the same fit parameters (with a lesser precision due to a 10 times smaller signal at 558 nm).

Figure 6 :

 6 Figure 6 : a) Decay of CuTPP pumped at 532 nm and probed with 193 nm as a function of pump/probe delay. b) Same as a) but CuTPP pumped at 558 nm and probed at 193 nm. The dashed lines indicate the two sequential components, 70 ns in red and >>5µs in blue.

Table 1 :

 1 The blue solvent shift of Q00 in CuOEP could have the same origin as the ground state configuration splitting, through electron donation from the solvent to the pyrolle orbitals that would stabilize more its ground state relative to excited states. It should be noted that in large nonfluorescent molecules, measuring the solvent shift is difficult by lack of data from gas phase unperturbed molecules, therefore these unexpected values may be more common than observed. Q and B band absorption maxima in different environmentsTriplet lifetime measurementsThe lifetime of triplet state can be measured by electronically delaying the probe laser(193 nm) with respect to the tunable pump in the 430-560 nm domain. The probe only weakly ionizes CuTPP or CuOEP in absence of the pump since its ionization potential is 6.49 eV17 , therefore the corresponding background is small at negative delays. The triplet decay of CuTPP pumped

	Compound	Medium		Absorption		References
			B	Q(1,0)	Q(0,0)
	Cu II TPP	Molecular beam	-	532	558	This work
	Cu II TPP	Vapor T=714K	405	544	-	29
	Cu II TPP	Toluene Solution	-	540	578	This work
	Cu II TPP	Benzene Solution	416.3	539	578	29
	Cu II OEP	Molecular beam	-	-	568	This work
	Cu II OEP	Vapor T=654K	387.5	531	566	1, 29
	Cu II OEP	Toluene Solution	400	525	563	30
	Cu II OEP	C2H2Cl2 Solution	397	523	560	This work
	Cu II PP	Molecular Beam	-	-	565	28
	Cu II PP	Pyridine solution	387	538	575	31
	Zn II TPP	Molecular beam He			571.8	27
	Zn II TPP	Vapor T =718K	406	550	594	29, 32
	Zn II TPP	Solution benzene	425	550	589	29
	Zn II OEP	Vapor T=652K	385	535	572	29 , 1
	Zn II OEP	Solution CH2Cl2	400	530	566.7	1
	Zn II OEP¨	Solution Toluene	404	532	570	30
	TPP :TetraPhenyl Porphyrine; OEP: OctaEthylPorphyrine, PP : ProtoPorphyrine ,

Acknowledgments:

This work has been financially supported by "Investissements d'Avenir du LabEx PALM (ANR-10- LABX-0039-PALM)"


W R M Rocha

S Pilling

A Domaracka

H Rothard

P Boduch

Infrared Complex Refractive Index of N-containing astrophysical ices free of water processed by cosmic-ray simulated in laboratory

Keywords:

Several nitrogen containing species has been unambiguously identified in the Solar System and in the Interstellar Medium. It is believed that such rich inventory of species is a result of the energetic processing of astrophysical ices during all stages of the protostellar evolution. An intrinsic parameter of matter, the complex refractive index, stores all the "chemical memory" triggered by energetic processing, and therefore might be used to probe ice observations in the infrared. In this study, four N-containing ices have been condensed in ultra-high vacuum chamber and processed by heavy ions (O and Ni) with energies between 0.2 and 15.7 MeV at the Grand Accélérateur National d'Ions Lourds (GANIL), in Caen, France. All chemical changes were monitored in situ by a Infrared Absorption Spectroscopy. The complex refractive index was calculated directly from the absorbance spectrum, by using the Lambert-Beer and Kramers-Kroning relations. The values containing the values will be available in a online database: https://www1.univap.br/gaa/nkabs-database/data.htm. As result, other than the database, it was observed that non-polar ices are more destroyed by sputtering than polar ones. Such destruction and chemical evolution leads to variation in the IR albedo of samples addressed in this paper.

Introduction

Astrophysical ices consist of frozen molecules in extraterrestrial environments inside the Solar System and beyond. Regarding to the early stages of Young Stellar Objects (YSOs), H 2 O is the major contributor to the entire inventory of ices in Space [START_REF] Öberg | The Spitzer Ice Legacy: Ice Evolution from Cores to Protostars[END_REF][START_REF] Boogert | Observations of the icy universe[END_REF]. Ammonia (NH 3 ), on the other hand, is the fourth most abundant molecular specie in the solid phase as also pointed out by Öberg et al. [START_REF] Öberg | The Spitzer Ice Legacy: Ice Evolution from Cores to Protostars[END_REF].

Inside our Solar System, N-containing environments were identified in several places, such as the Saturn's moons (e.g. Titan, Iapetus, Triton), Pluto, and Ultima Thule [START_REF] Cruikshank | The surface compositions of Pluto and Charon[END_REF][START_REF] Stern | Initial results from the New Horizons exploration of 2014 MU 69 , a small Kuiper Belt object[END_REF]. All these objects have in common a dark/red coloration that have been attributed to "tholins", namely, extended heterogeneous aromatic and aliphatic species with various degrees of nitrogen incorporation. They are synthesized when simple molecules containing, N, C, H or O are energetically processed by ionizing agents, such as ultraviolet (UV) and cosmic rays (CRs). Due to their extensive molecular conjugation, the tholins are known by their low albedo. Other than these peculiar material, simple nitrogen bearing molecules, such as N 2 has been identified in the coma of comet 67P/Churyumov-Gerasimenko, after analysis carried out by the ROSINA (Rosetta's Orbiter Spectrometer for Ion and Neutral Analysis) mass spectrometer on board the Rosetta spacecraft [START_REF] Rubin | Molecular nitrogen in comet 67P/Churyumov-Gerasimenko indicates a low formation temperature[END_REF].

From a physical chemistry perspective, nitrogen exists both in reactive (e.g. N 2 O, NO - 3 , NO - 2 , NH 3 , NH + 4 ) and * Principal corresponding author will.rocha@nbi.ku.dk (W.R.M. Rocha) ORCID(s): 0000-0001-6144-4113 (W.R.M. Rocha) 1 Current address: Niels Bohr Institute Centre for Star and Planet Formation, University of Copenhagen, Øster Voldgade 5-7, DK-1350 Copenhagen K., Denmark non-reactive form (N 2 ), giving its strongest triple molecular bond (N N). Laboratory experiments have shown [START_REF] Satorre | Infrared spectra of N 2 -rich ice mixtures[END_REF][START_REF] Boduch | Chemistry induced by energetic ions in water ice mixed with molecular nitrogen and oxygen[END_REF][START_REF] Cruz-Diaz | Vacuum-UV spectroscopy of interstellar ice analogs. II. Absorption cross-sections of nonpolar ice molecules[END_REF][START_REF] Augé | Irradiation of nitrogen-rich ices by swift heavy ions. Clues for the formation of ultracarbonaceous micrometeorites[END_REF][START_REF] Almeida | Processing of N 2 O ice by fast ions: implications on nitrogen chemistry in cold astrophysical environments[END_REF][START_REF] Vasconcelos | Radiolysis of N2-rich astrophysical ice by swift oxygen ions: implication for space weathering of outer solar system bodies[END_REF][START_REF] Vasconcelos | Energetic Processing of N 2 :CH 4 Ices Employing X-Rays and Swift Ions: Implications for Icy Bodies in the Outer Solar System[END_REF][START_REF] Fedoseev | Cosmic ray processing of N 2 -containing interstellar ice analogues at dark cloud conditions[END_REF], however, that energetic processing of N 2 -containing ices opens several channels for new chemical reactions, allowing the formation of N-reactive species. In addition, ⋅NO radicals have been considered an important precursor of prebiotic molecules, and therefore might be linked to the origin of life on earth as discussed in [START_REF] De Barros | Formation of nitrogen-and oxygen-bearing molecules from radiolysis of nitrous oxide ices -implications for Solar system and interstellar ices[END_REF][START_REF] Fulvio | Ion irradiation of N 2 O ices and NO 2 :N 2 O 4 ice mixtures: first steps to understand the evolution of molecules with the N-O bond in space[END_REF].

Apart of the chemical interest in N-containing ices, the complex refractive index (henceforth called CRI) is an important parameter, as it determines how the material interacts with electromagnetic radiation. In addition, it records the chemical variation triggered by the ionizing radiation such as UV, X-rays and Cosmic-rays [START_REF] Hudgins | Mid-and far-infrared spectroscopy of ices -Optical constants and integrated absorbances[END_REF][START_REF] Ehrenfreund | Infrared spectroscopy of interstellar apolar ice analogs[END_REF][START_REF] Moore | Infrared Spectra and Optical Constants of Nitrile Ices Relevant to Titan's Atmosphere[END_REF][START_REF] Luna | Density and refractive index of binary CH 4 , N 2 and CO 2 ice mixtures[END_REF][START_REF] Rocha | Infrared complex refractive index of astrophysical ices exposed to cosmic rays simulated in the laboratory[END_REF][START_REF] Luna | Densities, infrared band strengths, and optical constants of solid methanol[END_REF]. On this subject, the aim of this paper is not to provide a detailed description of the chemical synthesis after ion bombardment of the ices, but instead, to provide a new CRI database, calculated from different N-containing ice samples, and contribute to reduce the huge lack of this kind of data in the astronomical community. Moreover, the data provided will be useful in astrophysical modelling of mid-IR spectrum of ices, as well as some clue about the role of polarization level in the ice destruction by radiation.

This papers is structured as follows: section 2 described the laboratory experiments, and the methodology employed to calculate the ice thickness and the complex refractive index of the samples probed in this work. Sections 3 and 4 show the results and astrophysical implications focusing on the role of polarity and albedo of irradiated ices. The conclusions are summarized in section 5.

Methodology

In this work we employ a Lambert-Beer and Kramers-Kronig based code [START_REF] Rocha | Determination of optical constants n and k of thin films from absorbance data using kramers-kronig relationship[END_REF] in four sets of infrared spectra of Ncontaining peculiar astrophysical ice analogs (virgin and irradiated) obtained in the lab to determined their complex refractive index.

Laboratory experiments

The experiment simulating bombardment of cosmic rays and energetic particles on four peculiar N-containing ices was performed by using the high-vacuum chamber mounted at the IRRSUD (IR Radiation SUD) beamline at the Grand Accelerateur National d'Ions Lourds (GANIL) located in Caen, France. Briefly, the incoming ions impinge perpendicularly the solid samples, previously produced from gas mixture and deposited over a ZnSe substrate coupled to the helium closedcycle cryostat inside the vacuum chamber. In-situ Fouriertransformed infrared (FTIR) spectra of the sample were recorded before irradiation and at different ion fluences, using a Nicolet FTIR spectrometer (Magna 550). The ion flux was around 7 × 10 9 cm -2 s -1 . During the experiments, the chamber pressure was roughly 2 × 10 -8 mbar. More details of the experimental set up is given in Vasconcelos et al. [START_REF] Vasconcelos | Energetic Processing of N 2 :CH 4 Ices Employing X-Rays and Swift Ions: Implications for Icy Bodies in the Outer Solar System[END_REF] and Pilling et al. [START_REF] Pilling | Radiolysis of ammoniacontaining ices by energetic, heavy, and highly charged ions inside dense astrophysical environments[END_REF].

This work presents data from 4 different experiments: 1) N 2 :CO (1:1) ice at 14 K irradiated by 5.8 MeV O 2+ , 2) N 2 :CH 4 (19:1) ice at 19 K irradiated by 15.7 MeV O 5+ , 3) NH 3 :CO(1:1) ice at 14 K irradiated by 5.8 MeV O 2+ and, 4) NH 3 :CH 3 OH (1:1) at 14K irradiated by 0.2 MeV Ni 24+ . The absorbance infrared spectra of these ice mixtures are shown in Figure 1, and some assignments taken from [START_REF] Caro | Comparison of UV and high-energy ion irradiation of methanol:ammonia ice[END_REF] and [START_REF] Vasconcelos | Energetic Processing of N 2 :CH 4 Ices Employing X-Rays and Swift Ions: Implications for Icy Bodies in the Outer Solar System[END_REF] are indicated. Table 1 presents additional parameters of the addressed N-containing samples.

Initial ice thickness

The thickness of the samples listed in Table 1 before the irradiation was analytically calculated using the equations below:

( m) = × 10 4 (1a) (cm -2 ) = 2.3 2 ∫ 1 (1b)
where is the column density (cm -2 ), the specific density (g cm -3 ), the molar mass, Avogrados's number, the band strength of the of molecular bond and Abs the measured absorbance in the infrared spectral range.

For the N 2 :CO mixture, the band centered at 2139 cm -1 (∼ 4.67 m) corresponding to C-O stretching mode, was used to calculate the initial thickness. The band strength (1.1 × 10 -17 cm molecule -1 ) and density (0.8 g cm -3 ) was taken from Hudgins et al. [START_REF] Hudgins | Mid-and far-infrared spectroscopy of ices -Optical constants and integrated absorbances[END_REF] and Luna et al. [START_REF] Luna | Density and refractive index of binary CH 4 , N 2 and CO 2 ice mixtures[END_REF], respectively. As N 2 has no active vibrational mode in IR, the thickness relative to nitrogen was assumed to be the same as that of CO.

In the case of N 2 :CH 4 sample, the combination mode 1 + 4 of CH 4 was used to calculate the initial thickness.

The band strength and density for this mixture were taken from Bouilloud et al. [START_REF] Bouilloud | Bibliographic review and new measurements of the infrared band strengths of pure molecules at 25 K: H 2 O, CO 2 , CO, CH 4 , NH 3[END_REF], respectively, as 3.5 × 10 -19 cm molecule -1 and 0.45 g cm -3 . The initial thickness of NH 3 :CO, was calculated from the bands centered at 1070 cm -1 (∼ 9.35 m) relative to NH 3 ( 2 ) and 2139 cm -1 (∼ 4.67 m) belonging to CO. The density and band strength used in this calculation for NH 3 was, respectively, 0.7 g cm -3 and 1.7 × 10 -17 cm molecule -1 , as taken from Giuliano et al. [START_REF] Giuliano | Interstellar ice analogs: band strengths of H 2 O, CO 2 , CH 3 OH, and NH 3 in the far-infrared region[END_REF], whereas for CO, the parameters of Section 2.2.1 have been used.

For the last sample, NH Absorbance infrared spectra of the four ice mixtures addressed in this paper. In order to assure a better readability, an arbitrary offset has been applied. The colours black, blue and red refer to the different fluences used in each experiment. Some vibrational modes associated to the bands are indicated by the vertical dashed grey line and their assignments is shown, as taken from [START_REF] Vasconcelos | Energetic Processing of N 2 :CH 4 Ices Employing X-Rays and Swift Ions: Implications for Icy Bodies in the Outer Solar System[END_REF][START_REF] Caro | Comparison of UV and high-energy ion irradiation of methanol:ammonia ice[END_REF]. In the panel d, some bands are associated to different functional groups of molecules in literature, and are indicated by the chemical formula inside the parenthesis.

4 , 5 , 6 and 7 at 1461 cm -1 (∼ 6.84 m) of CH 3 OH, has been used to calculate the methanol thickness in this sample. As described in Giuliano et al. [START_REF] Giuliano | Interstellar ice analogs: band strengths of H 2 O, CO 2 , CH 3 OH, and NH 3 in the far-infrared region[END_REF], the band strength and density of methanol ice are, respectively, 1.2 × 10 -17 cm molecule -1 and 1.02 g cm -3 . For the ammonia thickness, the parame-ters described in the above paragraph were used.

Thickness variation

During the ice irradiation by heavy ions, the sample thickness decreases due to compaction and sputtering as widely discussed in literature [START_REF] Palumbo | Cosmic ion irradiation and UV photolysis of solids in star forming regions[END_REF][START_REF] Dartois | Swift heavy ion irradiation of water ice from MeV to GeV energies. Approaching true cosmic ray compaction[END_REF][START_REF] Baragiola | Water ice on outer solar system surfaces: Basic properties and radiation effects[END_REF][START_REF] Dartois | Cosmic ray sputtering yield of interstellar H 2 O ice mantles. Ice mantle thickness dependence[END_REF].

In this paper, the compaction and sputtering model introduced in Dartois et al. [START_REF] Dartois | Cosmic ray sputtering yield of interstellar H 2 O ice mantles. Ice mantle thickness dependence[END_REF], has been applied to estimate the sputtering yield ( ) for each experiment. Briefly, the model assumes that if a chemical species is radiolytically resistant, their column density evolution at each fluence ( ) can be modelled by the following equation:

( ) = × ln ⎛ ⎜ ⎜ ⎜ ⎝ e - ∞ +ln e 0 -1 + 1 ⎞ ⎟ ⎟ ⎟ ⎠ , ( 2
)
where is the ice column density where the sputtering effect ends, and 0 is the initial column density. The term ∞ is the sputtering yield at an asymptotic regime, calculated from:

( ) = ∞ 1 -e - (3)
It is worth to note, however, that a radiolytically resilience property do not belong to all chemical species, and therefore, the Equation 2 applied to these situation, provides the upper limit for ∞ .

The compaction effect, on the other hand, dominates for fluences below 10 11 ions cm -2 , as pointed out in Dartois et al. [START_REF] Dartois | Swift heavy ion irradiation of water ice from MeV to GeV energies. Approaching true cosmic ray compaction[END_REF]. As the data used in this paper only take into account fluences above 10 12 ions cm -2 , the measured absorbances probe the real thickness evolution.

Figure 2, shows the sputtering model applied to column density variation as the fluence increases. The fitted values for and ∞ are shown inside each panel. The ices used to calculate the column densities of the Figures 1a-1d where CO, CH 4 , CO and NH 3 , respectively. As shown in Pilling et al. [START_REF] Pilling | Radiolysis of ammoniacontaining ices by energetic, heavy, and highly charged ions inside dense astrophysical environments[END_REF] and Vasconcelos et al. [START_REF] Vasconcelos | Energetic Processing of N 2 :CH 4 Ices Employing X-Rays and Swift Ions: Implications for Icy Bodies in the Outer Solar System[END_REF], these molecules are not radiolytically resistant, as their column densities drastically change with the fluence, and and ∞ must be taken as upper limits.

Theoretical methodology for the complex refractive index calculation

In order to calculate the Complex Refractive Index (CRI) given by m = n + ik, the NKABS code [START_REF] Rocha | Determination of optical constants n and k of thin films from absorbance data using kramers-kronig relationship[END_REF] has been used. It is an interactive code, that uses the Lambert-Beer and Kramers-Kroning equations to find accurate values of CRI from the Absorbance data ( ) at different wavenumbers -often called frequency.

To start the code, a few parameters must be provided, such as the sample thickness ( ), the refractive index at 670 nm ( 0 ), and for the substrate used during the experiments ( 2 ). The Mean Average Percentual Error (MAPE) is used to estimate the accuracy of each iteration.

The imaginary term ( ) is calculated from the Lambert-Beer absorption coefficient ( ), given by:

= 1 2.3 × Abs + | | | | | t01 t12 ∕ t02 1 + r01 r12 2 x | | | | | 2 (4a) = 4 (4b)
where t and r, are the transmission and reflection Fresnel's coefficients, respectively. The sub-indexes 01, 12 and 02 are related to the interfaces vacuum-sample, sample-substrate and vaccum-substrate, respectively. The complex number in the power is directly proportion to CRI, and is given by x = 2 d m.

Once the imaginary term has been determined, the real part ( ) is calculated using the Kramers-Kroning relations:

( ) = 0 + 2 ℘ ∫ 2 1 ′ ( ′ ) ′2 -2 ′ ( 5
)
where ℘ is the Cauchy Principal Value, used to solve integrals with singularity, that is this case occurs at ′ 2 = 2 . As this integral is numerically calculated, the Maclaurin's method [START_REF] Ohta | Comparison among several numerical integration methods for kramers-kronig transformation[END_REF] is used to avoid such singularity. The adopted values for n 0 were 1.20 in the N1# and N2# cases and 1.30 taken from Luna et al. [START_REF] Luna | Density and refractive index of binary CH 4 , N 2 and CO 2 ice mixtures[END_REF], and Hudigins et al. [START_REF] Hudgins | Mid-and far-infrared spectroscopy of ices -Optical constants and integrated absorbances[END_REF]. In all cases the substrate refractive index was 2.54.

The experimental parameters , and 0 are the main source of error in calculating the CRI. In this sense, the error propagation was calculated by using the equations below:

2 = 2 0 + 2 (6a) 2 = 2 2 (6b) 2 = 2 2 + 2 2
(6c)

2 = 2 2 (6d)
As such experimental parameters used in this paper are very sensitive to the chemical composition as shown in Luna et al. [START_REF] Luna | Density and refractive index of binary CH 4 , N 2 and CO 2 ice mixtures[END_REF][START_REF] Luna | Densities, infrared band strengths, and optical constants of solid methanol[END_REF], the values of , and 0 , were estimated from the confidence limits (See Appendix). Due to uncertainties involved in the initial parameters, the error for and were around 12%.

Results and Discussion

Refractive Index in infrared

Figures 3 and4 show the real and imaginary parts of the complex refractive index in the infrared (5000 -600 cm -1 ), corresponding to 2.0 -16.6 m. The values obtained from different irradiation levels are shown by the colours black, blue and red, respectively.

The decreasing and increasing of the band areas in Figures 3 and 4 are, respectively, related to the destruction and formation of chemical species during the irradiation process, which consequently changes the values of and , for virgin and processed ices.

Using accurate values for 670 and thickness ( ), is important to determine and . However, the instrumentation required to calculate these parameters might not be available in some laboratories. In such cases, an analytical method using averaged values has been used [START_REF] Pilling | Radiolysis of ammoniacontaining ices by energetic, heavy, and highly charged ions inside dense astrophysical environments[END_REF][START_REF] Dawes | Using the C-O stretch to unravel the nature of hydrogen bonding in low-temperature solid methanol-water condensates[END_REF] instead. In order to address the impact of approximated values in the Complex Refractive Index calculation, a statistical analysis was performed.

As result, the confidence limit of 1 allows a variation of 30% of thickness around the mean, but only of 7% of n 0 . The density and band strength allows variations around 20% and 10%, respectively.

Relative destruction

The electronic sputtering yield ( ), assumed here to be the main trigger of ice destruction, roughly increases quadratically with the electronic stopping power ( ). Furthemore, is a function of energy, mass and charge of a particle [START_REF] Seperuelo Duarte | Heavy ion irradiation of condensed CO 2 : sputtering and molecule formation[END_REF][START_REF] Boduch | Radiation effects in astrophysical ices[END_REF][START_REF] Mejía | Compaction of porous ices rich in water by swift heavy ions[END_REF][START_REF] Rothard | Modification of ices by cosmic rays and solar wind[END_REF][START_REF] Dartois | Cosmic ray sputtering yield of interstellar H 2 O ice mantles. Ice mantle thickness dependence[END_REF]. For instance, such dependence might be observed in the yield values in Figure 2, where of sample bombarded by Ni 24+ is higher than for samples bombarded by O 2+ or O 5+ .

The effects of electronic sputtering is shown in Figure 5 in terms of the relative destruction, i.e. Δd∕d and fluence for all samples in Table 1. This figure also shows that two linear fits are required as can be seen from the solid red lines. For comparison, the black dashed lines indicates the relative destruction of H 2 O-rich ices taken from Rocha et al. [START_REF] Rocha | Infrared complex refractive index of astrophysical ices exposed to cosmic rays simulated in the laboratory[END_REF] both for polar-containing ices and non-polar containing ices, as the van der Walls force are different. The blue and green shaded areas were also taken from Rocha et al. [START_REF] Rocha | Infrared complex refractive index of astrophysical ices exposed to cosmic rays simulated in the laboratory[END_REF]. One can observe, in fact, that such relation with the polarity found for H 2 O-containing ices is also observed for N-rich ices. As described in Table 1, the samples N1a and N2a are composed by two non-polar molecules, whereas in N3a and N4a polar ices are present. However, if the ice is very thick, such effect cannot be observed, as it is the case of N2#. If samples sharing common characteristics are compared, instead (e.g. N1# and N3#), such polarity effects become more evident as they have almost the same initial thickness and were processed by the same ionizing agents. As N3# is more polar than N1#, the destruction behavior is more similar to polar ices than with non-polar species.

By comparing the linear fittings at fluence of 10 13 ions cm -2 , of N-rich and H 2 O-rich ices, the relative destruction for water-containing cases is about 30% higher compared to nitrogen-containing samples. However, for a non-polar situation, the N-rich ices are almost three times more rapidly destroyed than water-rich ices, which strength the idea that polar molecules are important to keep processed molecules in the same site to allow the radicals to react again and enrich the chemical complexity.

Astrophysical implications

Polarity in astrophysical ices

Astrophysical environments at very low temperatures (T < 20K), and n > 10 4 cm -3 , allow the formation of a nonpolar layers of ice, mainly composed by CO and N 2 as shown by Boogert et al. [START_REF] Boogert | Observations of the icy universe[END_REF], given their low desorption temperature [START_REF] Collings | A laboratory survey of the thermal desorption of astrophysically relevant molecules[END_REF]. Such layers, however, are formed onto a previous ice mantle made up of H 2 O, CH 4 and NH 3 , as consequence of the hy- 1 at different fluences. In order to assure a better readability, an arbitrary offset has been applied. drogenation mechanism of small grains in Molecular Clouds [START_REF] Tielens | Model calculations of the molecular composition of interstellar grain mantles[END_REF].

Cosmic rays, on the other hand, are an important agents to enrich the chemistry in Molecular Clouds, as many endothermic reactions are forbidden due to low temperature [START_REF] Waddington | The ultra heavy elements in the cosmic radiation[END_REF]. As such energetic particles penetrate deeper than UV or Xrays photons [START_REF] Glassgold | X-Ray Ionization of Protoplanetary Disks[END_REF] in high density regions, they reach regions where the ices are formed, leading to many physicochemical processes, including electronic sputtering, as described in Sections 2 and 3.

In such context, and assuming that non-polar ices are at least 20% more susceptible to electronic sputtering effects, as shown in Figure 5, one could expect a reduced lifetime of icy layers containing non-polar molecules. In fact, the most abundant ices in ISM are composed by H 2 O, CO 2 , CH 3 OH, and NH 3 as shown by Öberg et al. [START_REF] Öberg | The Spitzer Ice Legacy: Ice Evolution from Cores to Protostars[END_REF]. CO ice, on the other hand, is the most abundant non-polar molecule, as also pointed out by Öberg et al. [START_REF] Öberg | The Spitzer Ice Legacy: Ice Evolution from Cores to Protostars[END_REF]. Its IR feature, however, has been attributed to CO diluted in polar-matrices of H 2 O, CO 2 and CH 3 OH.

Other aspects relating polarity to astrophysical ices, have also been reported by Müller et al. [START_REF] Müller | O 2 signature in thin and thick O 2 -H 2 O ices[END_REF], associating the band strength of O 2 with the chemical environment. In addition, Knez et al. [START_REF] Knez | Laboratory IR Studies and Astrophysical Implications of C 2 H 2 -Containing Binary Ices[END_REF], point out that the acetylene's 5 feature is much more resolved in ices dominated by non-polar or lowpolarity molecules.

Opacities and albedos for astrophysical models

Since the complex refractive index records the chemical evolution of astrophysical ices, they can be used to derive opacity table of dust models, which are the input of radiative transfer models dominated by dust. In fact, previous works by [START_REF] Pontoppidan | Ices in the Edge-on Disk CRBR 2422.8-3423[END_REF][START_REF] Robinson | The water-ice librational band: radiative transfer model for AFGL 961[END_REF][START_REF] Rocha | Computational Modeling of the Class I Low-Mass Protostar Elias 29 Applying Optical Constants of Ices Processed By High Energy Cosmic Ray Analogs[END_REF][START_REF] Min | The abundance and thermal history of water ice in the disk surrounding HD 142527 from the DIGIT Herschel Key Program[END_REF][START_REF] Kamp | Diagnostic value of far-IR water ice features in T Tauri disks[END_REF], have shown that the IR spectrum of YSOs are better reproduced by grain model combining dust and ice opacities. Other than ISM, where the standard ice model is dominated by H 2 O[1, 2], some places in the Solar System are dominated by N-containing ices, such as Saturn's moons (e.g. Titan, Iapetus, Triton), Pluto, and Ultima Thule [START_REF] Cruikshank | The surface compositions of Pluto and Charon[END_REF][START_REF] Mckinnon | Convection in a volatile nitrogen-ice-rich layer drives Pluto's geological vigour[END_REF][START_REF] Stern | Initial results from the New Horizons exploration of 2014 MU 69 , a small Kuiper Belt object[END_REF] Another direct astrophysical implication of cosmic ray irradiation of N-containing ices, is related to the albedo changes with the chemistry. The generic albedo, can be given by the 1 at different fluences. In order to assure a better readability, an arbitrary offset has been applied.

following equation as defined in Hanner et al. [START_REF] Hanner | On the definition of albedo and application to irregular particles[END_REF]:

( , ) = ( ) 2 (7)
where ( ) is the dimensionless scattering function, and is the size parameter. Both equations are defined by:

( ) = 4 11 ( ) (8a) = 2 (8b)
where is the scattering opacity in cm 2 g -1 , 11 ( ) are the values from the scattering matrix in cm 2 g -1 sr -1 calculated from a full scattering Muller matrix for all angles of scattering (see Bohren & Huffman[50] for details), and is the phase angle. The subindex '11' means the intensity from the Stokes parameter 2 . The geometric albedo, on the other hand, is defined as the ratio between the scattered light at zero phase angle and the scattering of a idealized lambertian surface of same cross section. In another approach, the 2 As the polarization effect is not addressed in this study, the other Stokes parameters Q, U, and V are not being considered. geometric albedo is also defined as two thirds of the singlescattering albedo, given by:

= + (9)
where is the scattering opacity in cm 2 g -1 . The sum + is known as extinction opacity ( ). In order to address the geometric albedo variation due to the chemical changes induced by the energetic processing of ices, a computational code in Python programing language was used3 . Briefly, this code uses the Mie theory [START_REF] Mie | Beiträge zur Optik trüber Medien, speziell kolloidaler Metallösungen[END_REF] to solve Maxwell's equation for the light dispersion in a medium containing spherical particles. Figure 6 show the geometric albedo of the samples addressed in this paper, as function of the wavelength, fluence, and grain size. Different particle radius are used to highlight the scattering effects from small to large grains. The trend between geometric albedo d,h,l), since the geometric albedo at Fluence 3 is higher than in Fluence 2 between 2.9 and 4.2 m. Such a relationship between geometric albedo and fluence is given by the synthesis of complex molecules, including several saturated or non-saturared aliphatic compounds [START_REF] Vasconcelos | Energetic Processing of N 2 :CH 4 Ices Employing X-Rays and Swift Ions: Implications for Icy Bodies in the Outer Solar System[END_REF], and heteropolymers [START_REF] Cleaves | Amino acids generated from hydrated titan tholins: Comparison with millerâĂŞurey electric discharge products[END_REF]. In the former case, however, the disarranged polymer-like structure and the complex combinations of functional groups has been called "tholins", as coined by Sagan & Khare [START_REF] Sagan | Tholins: organic chemistry of interstellar grains and gas[END_REF], and are responsible for the low albedo and reddish colour associated to these compounds [START_REF] Cruikshank | and New Horizons Composition Team. The Colors of 486958 2014 MU69 ("Ultima Thule"): The Role of Synthetic Organic Solids (Tholins)[END_REF]. The existence of tholins in our samples, however, has not been addressed yet, and will be investigated in a future manuscript. The grain size dependence, in general, decrease the geometric albedo in all wavelengths, although this effect is more prominent for particle radius of 100 m (panels i,j,k,l). Particularly, both O H bonds and ammonia hydrates affects the bands in the spectral range shown in Figure 6. Additionally to the chemistry, nevertheless, large grains cause an extra extinction due to scattering for > 3.1 m as reported by Léger et al. [START_REF] Leger | Properties of amorphous H2O ice and origin of the 3.1-micron absorption[END_REF], that's particularly the case of particle radius of 10 m (middle column), compared to 1 m (left column). For very large grains, however, it worth to describe the limts of the Mie theory applied to scattering effects. This limit is given by the non-dimensional size parameter given by Equation 8b. In terms of the single scattering albedo and the CRI calculated in this paper, > 200 characterizes the geomet-ric optics limit, whereas 0.2 < < 200 the Mie theory is fully considered [START_REF] Moosmãijller | Small and large particle limits of single scattering albedo for homogeneous, spherical particles[END_REF]. For < 0.2 the Rayleigh regime is adopted. In the case of 100 m icy grain radius, the range below 3 m is in the geometric optics limit, and must be used with caution. The range above 3 m, however, can be fully considered.

Figure 9 show the the geometric albedo as function of the wavelength, fluence, and grain size for the range between 5 and 8 m. This spectral range contains contributions of several functional groups associated to complex species such as alcohols, aliphatic ethers (R1-OCH 2 -R2) and related molecules [START_REF] Boogert | Observations of the icy universe[END_REF]. As observed in Figure 6, both the fluence and grain size increasing, reduce the icy grain geometric albedo, except in specific spectral intervals in the case of NH 3 :CH 3 OH. For example, the geometric albedo in the the range between 6.5 m and 7.2 m for grain radius of 1 m and 10 m, is higher than the samples at Fluence 0 and Fluence 1. Nevertheless it is an unexpected result, since one could expect an albedo reducing due to the synthesis of more complex species with the irradiation exposure, this suggest that the production of complex molecules do not increases with all levels of irradiation in some cases, but rather, high fluences might destroy the chemical bonds of synthesized species formed earlier in the same experiment. In terms of scattering regime, the interval between 5 and 8 m for large particle radius (100 m) ramains in the Mie regime, although very close of the geometric optics domain. The extinction caused by scattering in this case still very prominent, and reduces the geometric albedo by a factor of 4-6 for 5.8 < < 7.2 . The relative and integrated geometric albedo changes with the chemical processing and particle size for the cases between 2.0-5.0 m and 5.0-8.0 m is shown in Figures 8 and9, respectively. The ice mixtures N 2 :CO, NH 3 :CO and NH 3 :CH 3 OH are shown in the upper panels, whereas N 2 :CH 4 is shown in the bottom panels, since it was irradiated in a different fluence regime, and also because this sample do not contains Oxygen atoms in its structure. In Figure 8, the geometric albedo of all samples decreases with the fluence and grain size, except in the case of NH 3 :CH 3 OH. Apart of the small decreasing observed in panels a and b (less than 2%), its geometric albedo increases with the fluence. Differently of the O-containing ice mixtures, the normalized and integrated geometric albedo of N 2 :CH 4 decreases linearly with the fluence. On the other hand, some differences are observed in the normalized and integrated geometric albedo between 5-8 m, as shown in Figure 9. The decreasing albedo is observed for the O-containing ice mixtures, except in the case of NH 3 :CH 3 OH, that increases for large fluences, independent of the grain size. The role for N2:CH4 is no longer linear, as observed for the case between 2-5 m, but show a small increasing at low fluences, and then decreases significantly for small (1 m) and large grains (100 m). Intermediary grain sizes, such as 10 m, the normalized and integrated albedo remains constant, taken into account the error bars.

As a test case, the NH 3 :CO geometric albedo is compared to the reflectance profile of two regions of Iapetus, a Saturn's icy moon. The Iapetus's surface has a unique bimodal albedo distribution as shown in the top panel of Figure 10. The white hemisphere is rich in H 2 O, and show a geometric albedo of around 60%, whereas the dark region (white dashed box) is dominated by nitrogen-rich tholin ( C N ) [START_REF] Brown | Observations in the Saturn system during approach and orbital insertion, with Cassini's visual and infrared mapping spectrometer[END_REF]. The origin of this dichotomy is beyond the scope of this work, but detailed explanations can be found in [START_REF] Grundy | Near-Infrared Spectra of Icy Outer Solar System Surfaces: Remote Determination of H 2 O Ice Temperatures[END_REF][START_REF] Owen | Decoding the Domino: The Dark Side of Iapetus[END_REF]. Between these two hemispheres, a mixed region (green dashed box) has also been studied [START_REF] Buratti | Cassini Visual and Infrared Mapping Spectrometer Observations of Iapetus[END_REF]. The temperature variation as seen between 9-16 m is shown to highlight the high temperature associated to the low albedo in the dark region, compared to the H 2 O-rich water. The spectrum as observed by the Visible and Infrared Mapping Spectrometer (VIMS) instrument on-board of Cassini spacecraft between 2-5 m for the dark [START_REF] Clark | The surface composition of Iapetus: Mapping results from Cassini VIMS[END_REF] and mixed [START_REF] Buratti | Cassini Visual and Infrared Mapping Spectrometer Observations of Iapetus[END_REF] region is shown in the bottom panel by the solid black line. Both blue and red lines are the scaled geometric albedo (also called reflectance) taken from panel g in Figure 6. Only the particle radius of 10 m is shown since it provided the best comparison by eye. The NH 3 at 3 m and CO 2 at 4.25 m features are prominent in this comparison, whereas other bands such as CO 2 at 2.65 m, OCN -at 4.62 m and CO at 4.67 m are not observed by VIMS, which can be caused by a resolution issue. Another feature observed in the experiments is the 13 CO mode at 4.73 m, but only observed in the mixed region during the mission at 2004 July 13, but not observed at another flyby at 2004 October 7. Buratti et al. [START_REF] Buratti | Cassini Visual and Infrared Mapping Spectrometer Observations of Iapetus[END_REF], however, do not comment about this band in their work. Some chemical differences are strickly between 2.5 and 4.0 m. Since H 2 O is also present in the low albedo region, the band centered in 3 m is more broader than the transitions for NH 3 :CO even at high fluences. Two regions are hatched in this papen, namely, = 2.8 m and = 3.6 m, since they are strongly affected by (C N) and (N H) bonds, although the grain size effect at = 3.6 m is also known [START_REF] Leger | Properties of amorphous H2O ice and origin of the 3.1-micron absorption[END_REF].

Conclusions

The Complex Refractive Index of N-containing ices processed by heavy ions in the laboratory is shown in this paper. This contributes to reduce the lack of this kind of data in the astronomical community, and become useful to improve the computational simulations in environments dominated by dust and N-ice.

The relative destruction triggered by sputtering can be associated to the polarity level of ice samples, as it was also observed for H 2 O-containing molecules in Rocha et al. [START_REF] Rocha | Infrared complex refractive index of astrophysical ices exposed to cosmic rays simulated in the laboratory[END_REF]. From a astrochemical perspective, this implies in a short lifetime of non-polar mantles of ices inside Molecular Clouds. Such scenario, however, opposes to layered structure of ices, and strengthen the idea that non-polar species are triggered in a H 2 O-dominated ice matrix.

The geometric albedo calculated in this paper in two wavelength windows has shown a strong dependence with the particle size and chemical processing induced by cosmic-rays analogues. The albedo decreasing observed in both cases suggest the formation of N-rich tholins and aliphatic saturated and non-saturared complex molecules, which is associated to reddish colours in small bodies of the Solar System. As a test case, the geometric albedo of NH 3 :CO at two fluence levels were compared to the geoemtric albedo (reflectance) of the dark and mixed region in Iapetus, a Saturn's moon. Although the aim of this comparison was not provide a conclusive analysis of the Iapetus' surface composition, one can note that a energetic processing of N-rich component containing traces of other materials (carbon and oxygen), seems to be the likely scenario to explain the low albedo region seem by the Cassini's instruments. This becomes a strong motivation for future prospects aiming further chemical constraints of low albedo surfaces inside the Solar System and beyond.

Aknowledgements

The authors acknowledge the anonymous referee's comments that securely improved this manuscript. They also are grateful to the financial support provided by the Brazilian agencies FAPESP (Projects 2015/23054-7, 2016/11334-5, 2017/07283-9, 2016/22018-7 and 2013/07657-5) and CNPq (Projects 304130/2012-5 and 306145/2015-4). WRMR also acknowledge MSc. Giulia Perotti for the fruitful discussions about this paper.

A. Appendix

The degeneracy was addressed by using a 2 statistical analysis. In summary, random values were attributed to , , and 0 in Equations 1a, 1b and 5, which allowed NKABS

where is the laboratory data. At the end of 5000 simulations, a 2 analysis was performed, by using the following equation:

Δ 2 ( , ) = 2 -2 (11)
where and are the degrees of freedom and the statistical significance, respectively. One can observe, that confidence regions do not depend on the accuracy of the fit, but the number of degrees of freedom. Figure 11 shows an example of this investigation for the sample N1, once the other results were similar. The colours

Figure 1 :

 1 Figure1: Absorbance infrared spectra of the four ice mixtures addressed in this paper. In order to assure a better readability, an arbitrary offset has been applied. The colours black, blue and red refer to the different fluences used in each experiment. Some vibrational modes associated to the bands are indicated by the vertical dashed grey line and their assignments is shown, as taken from[START_REF] Vasconcelos | Energetic Processing of N 2 :CH 4 Ices Employing X-Rays and Swift Ions: Implications for Icy Bodies in the Outer Solar System[END_REF][START_REF] Caro | Comparison of UV and high-energy ion irradiation of methanol:ammonia ice[END_REF]. In the panel d, some bands are associated to different functional groups of molecules in literature, and are indicated by the chemical formula inside the parenthesis.

Figure 2 :

 2 Figure 2: Column density as function of fluence for the samples studied in this paper. Each blue dot was estimated from the Absorbance data collected after the experiments (see Section 2.2). The red line is the fitting using a sputtering model detailed in Dartois et al.[30]. N D and Y ∞ S are the column density probed by sputtering, and the sputtering yield at the asymptotic level, respectively.

2 F1: 10 12 ions/cm 2 F2: 10 13 ions/cm 2 a 2 F1: 10 10 ions/cm 2 F2: 10 2 F1: 10 12 ions/cm 2 F2 2 F1: 10 Figure 3 :

 2222210222103 Figure 3: Real part of the Complex Refractive Index for the samples shown in Table1at different fluences. In order to assure a better readability, an arbitrary offset has been applied.

2 F1: 10 12 ions/cm 2 F2: 10 13 ions/cm 2 a 2 F1: 10 10 ions/cm 2 F2: 10 11 ions/cm 2 b) N 2 :CH 4 ( 19 2 F1: 10 12 ions/cm 2 F2: 10 13 ions/cm 2 c 2 F1: 10 Figure 4 :

 22222224192222104 Figure 4: Imaginary part of the Complex Refractive Index for the samples shown in Table1at different fluences. In order to assure a better readability, an arbitrary offset has been applied.

Figure 5 :

 5 Figure 5: Relative destruction variation (in percentage) against fluence. The squares are the values for each sample in Table1. The red solid lines are the fittings assuming a linear approximation for the N-rich ices. The black dashed lines are the fittings for H 2 O-rich ices taken from Rocha et al.[START_REF] Rocha | Infrared complex refractive index of astrophysical ices exposed to cosmic rays simulated in the laboratory[END_REF]. Both polar and non-polar regions are identified by a green and blue shaded region.

  3 :CH 3 OH (1:1) at 14 K

Figure 7 :

 7 Figure 7: Same of Figure 6, but for the range between 5-8 m.

4 fFigure 8 : 4 fFigure 9 :

 4849 Figure 8: Normalized and integrated geometric albedo between 2-5 m against fluence for all samples in Table 1 for 3 icy grain sizes. As the sample N2# was irradiated in a fluence range different of the other samples, it is shown separately at the bottom layer. The lines are to guide the eye only. A light gray dotted line indicate the ratio equal the unity.

Figure 11 :

 11 Figure 11: Confidence limit levels and histogram of the input parameters based on a 2 statistical analysis. The levels 1-3 are shown by red, yellow and white solid lines, respectively. The mean is shown by a green solid line, whereas the upper and lower 1 confidence limits are given by red and blue dashed lines.

Table 1 N

 1 -containing samples addressed in this paper. Non-irradiated species are labelled "a", whereas the letters "b" and "c" refer to the first and second level of irradiation. The samples identification, thickness, and relative destruction are shown in columns 2, 3 and 4, respectively. The laboratory conditions under which the experiments were performed are shown in the columns 5-7.

	Label Samples	d ( m)	Δ ∕	Temp. (K) Energy/Projectile Fluence (10 10 ions cm -2 ) Reference
	N1a	N 2 :CO (1:1)	0.7 ± 0.1	0.0	14	-	0	-
	N1b	N 2 :CO	0.7 ± 0.1	0.0	14	5.8 MeV 16 O 2+	120	-
	N1c	N 2 :CO	0.5 ± 0.1	35.6 ± 1.2 14	5.8 MeV 16 O 2+	1000	-
	N2a	N 2 :CH 4 (19:1)	11.3 ± 1.0 0.0	19	-	0	[53]
	N2b	N 2 :CH 4	11.2 ± 0.7 2.8 ± 0.8	19	15.7 MeV 16 O 5+	1000	[53]
	N2c	N 2 :CH 4	10.5 ± 0.4 16.1 ± 1.4 19	15.7 MeV 16 O 5+	6000	[53]
	N3a	NH 3 :CO (1:1)	0.8 ± 0.2	0.0	14	-	0	-
	N3b	NH 3 :CO	0.8 ± 0.2	0.0	14	5.8 MeV 16 O 2+	120	-
	N3c	NH 3 :CO	0.6 ± 0.1	20.0 ± 1.7 14	5.8 MeV 16 O 2+	1000	-
	N4a	NH 3 :CH 3 OH (1:1) 4.4 ± 0.4	0.0	14	-	0	-
	N4b	NH 3 :CH 3 OH	4.4 ± 0.4	0.0	14	0.2 MeV Ni 24+	100	-
	N4c	NH 3 :CH 3 OH	4.2 ± 0.3	4.6 ± 0.3	14	0.2 MeV Ni 24+	1000	-
	Note: Initial samples according to polarity: N1a [Non-Polar:Non-Polar], N2a [Non-Polar:Non-Polar], N3a [Polar:Non-Polar]
	,	N4a [Polar:Polar]

The database containing these values in ASCII (American Standard Code for Information Interchange) format are available online at: https://www1.univap.br/gaa/nkabs-database/data.htm

Note by C. Dullemond: This is a python version of the famous Bohren Huffman Mie code. It was ported to Python from its original version bhmie.f (written by Bruce Draine) by Cornelis Dullemond, February 2017. Available in: http://www.ita.uniheidelberg.de/ dullemond/software/radmc-3d/

to calculate the and after 10 iterations, and a 2 given by Equation 7a was calculated.