H Fleurbaey

A Campargue

Y Carreira

Mendès Da Silva

R Grilli

S Kassi

D Mondelain
email: didier.mondelain@univ-grenoble-alpes.fr

Characterization of the H 2 O+CO 2 continuum within the infrared transparency windows

Keywords: Water vapor, carbon dioxide, continuum absorption, CRDS, OFCEAS, planetary atmosphere

1 . All these spectral ranges are situated in transparency windows of both H 2 O and CO 2 . The binary absorption coefficients are retrieved from low pressure spectra (<1atm) recorded with different molar fractions of water vapor in CO 2 after subtracting the H 2 O and CO 2 local monomer contributions and the self-continuum contribution of each species (i.e. H 2 O-H 2 O and CO 2 -CO 2 ). Experimental room temperature binary coefficients are then compared to the only available model based on line shape profiles with -factors. This model well reproduces our experimental values on the low-and high-frequency edges of the 1.6 µm window and gives a relatively good agreement for the 2853 cm -1 data point. Larger differences are observed in the 2.3 µm window where the calculated values are underestimated by a factor of 3.

Around 6000 cm -1 , an additional absorption peak is observed which is tentatively interpreted as a collision induced absorption band of the H 2 O and CO 2 molecules.

Introduction

The knowledge of the absorption of a gas mixture of water and CO 2 is required to better model the atmospheres of rocky planets with stronger constraints on the radiation budget. In addition to the contribution of the H 2 O and CO 2 rovibrational lines which are proportional to their respective partial pressures, the mixture absorption includes binary contributions varying smoothly with the frequency: (i) the self-H 2 O and self-CO 2 continua, with quadratic partial pressure dependence and (ii) the H 2 O+CO 2 continuum which is the sum of two contributions, proportional to the product of the partial pressures: one due to H 2 O molecules as absorbing molecules interacting with CO 2 colliding molecules and the other one due to CO 2 absorbing molecules interacting with H 2 O colliding molecules.

During the early stage of their life, telluric planets, that have a liquid mantle, are supposed to outgas large quantities of volatiles mainly made up of H 2 O and CO 2 [START_REF] Marcq | Thermal radiation of magma ocean planets using a 1-D radiativeconvective model of H 2 O-CO 2 atmospheres[END_REF][START_REF] Abe | Evolution of an impact-generated H 2 O-CO 2 atmosphere and formation of a hot proto-ocean on Earth[END_REF][START_REF] Elkins-Tanton | Linked magma ocean solidification and atmospheric growth for Earth and Mars[END_REF][START_REF] Hamano | Emergence of two types of terrestrial planet on solidification of magma ocean[END_REF]. A modeling, including accurate continua absorption, will give key insights on the rocky planets' evolution from this early stage and will help to understand, for example, why the evolution of Venus and Earth has been so different. It is also a key element in the modeling of the extra-solar planets' atmosphere where CO 2 dominates (Refs) leading to the generation of better observables for future observations, such as the innermost planets of the TRAPPIST-1 system with the forthcoming James Webb Space Telescope.

The accurate determination of the H 2 O+CO 2 continuum is also of first importance to test the proposition according to which the formation of the Martian valley networks could be due to episodic precipitations trigged by meteoritic impacts injecting large quantities of H 2 O and CO 2 in the planet's atmosphere [see for example 5,6].

Pioneer theoretical calculations of the far wing line shapes were performed by Ma and Tipping [START_REF] Ma | A far wing line shape theory and its application to the foreign broadened water continuum absorption[END_REF] based on a quasi-static and binary collision approach providing the H 2 O continuum in CO 2 at 296 K in the 0-10 000 cm -1 spectral range. About 25 years later, the first experimental measurements of the H 2 O+CO 2 continuum were reported in [START_REF] Baranov | On the significant enhancement of the continuum-collision induced absorption in H 2 O+CO 2 mixtures[END_REF] for the 1100-1350 and 2450-2750 cm -1 spectral ranges at four temperatures (between 294 K and 339 K) with a Fourier transform spectrometer (FTS). This work showed that (i) near 1100 cm -1 , the measured H 2 O continuum in CO 2 is about twenty times larger than in N 2 , (ii) in the high frequency wing of the  3 band of CO 2 (around 2500 cm -1 ), the continuum of CO 2 with H 2 O as collision partner is an order of magnitude larger than the CO 2 self-continuum. These experimental data have been confirmed recently, by FTS at temperatures up to 367 K in the 2400-2600 cm -1 [START_REF] Tran | Measurements and modeling of absorption by CO 2 +H 2 O mixtures in the spectral region beyond the CO 2 ν 3 -band head[END_REF] and 100-1500 cm -1 [START_REF] Tran | The CO 2 -broadened H 2 O continuum in the 100-1500 cm -1 region: Measurements, predictions and empirical model[END_REF] intervals. The latter reference validated the theoretical results of [START_REF] Ma | A far wing line shape theory and its application to the foreign broadened water continuum absorption[END_REF] in the considered range. In addition, an empirical modeling using a sub-Lorentzian line-shape with temperature-dependent correction factors  was proposed to better reproduce the experimental data [START_REF] Tran | Measurements and modeling of absorption by CO 2 +H 2 O mixtures in the spectral region beyond the CO 2 ν 3 -band head[END_REF]. Recently, classical molecular dynamic simulations were carried out allowing generation of the absorption spectrum of the H 2 O broadened CO 2 spectrum in the high frequency wing of the  3 band [START_REF] Hartmann | Effect of humidity on the absorption continua of CO2 and N2 near 4 μm: Calculations, comparisons with measurements, and consequences for atmospheric spectra[END_REF]. Good agreement with the measurements of Refs. [START_REF] Tran | Measurements and modeling of absorption by CO 2 +H 2 O mixtures in the spectral region beyond the CO 2 ν 3 -band head[END_REF][START_REF] Tran | The CO 2 -broadened H 2 O continuum in the 100-1500 cm -1 region: Measurements, predictions and empirical model[END_REF] was achieved without any adjusted parameters.

The aim of the present contribution is to provide additional experimental data points in different CO 2 and H 2 O transparency windows (see Figure 1). These windows play a major role in radiative transfer as spectral regions corresponding to strong H 2 O and CO 2 absorption bands are generally saturated. Due to the expected weak absorptions, highly sensitive spectroscopic techniques, namely cavity ring-down spectroscopy (CRDS) and optical feedback cavity enhanced absorption spectroscopy (OFCEAS) have been used to record spectra of CO 2 +H 2 O mixtures. The experimental setups are described in the next section (Section 2). The procedures adopted to record spectra and retrieve the binary coefficients are detailed in Section 3 as well as the error budget. Comparison between these experimental binary coefficients and the available models is presented and discussed in Section 4 before concluding remarks (Section 5).

Experimental setups

The CRDS and OFCEAS techniques have proven their ability to measure weak absorption continua for pure H 2 O (see for example [START_REF] Lechevallier | The water vapour selfcontinuum absorption in the infrared atmospheric windows: new laser measurements near 3.3 and 2.0 μm[END_REF][START_REF] Richard | Water vapor selfcontinuum absorption measurements in the 4.0 and 2.1 μm transparency windows[END_REF][START_REF] Ventrillard | Accurate measurements and temperature dependence of the water vapor self-continuum absorption in the 2.1 μm atmospheric window[END_REF]), O 2 [START_REF] Mondelain | Accurate laboratory measurement of the O 2 collision-induced absorption band near 1.27 µm[END_REF][START_REF] Kassi | Temperature dependence of the collision-induced absorption band of O 2 near 1.27 µm[END_REF], CO 2 [START_REF] Kassi | High pressure Cavity Ring Down Spectroscopy: Application to the absorption continuum of CO 2 near 1.7µm[END_REF][START_REF] Mondelain | The CO 2 absorption spectrum in the 2.3 µm transparency window by high sensitivity CRDS: (II) Self-absorption continuum[END_REF][START_REF] Mondelain | The CO 2 absorption continuum by high pressure CRDS in the 1.74 µm window[END_REF] and H 2 O in air [START_REF] Vasilchenko | The water vapour self-and foreign-continua in the 1.6 μm and 2.3 μm windows by CRDS at room temperature[END_REF] gas samples. In this work, we applied our CRDS and OFCEAS spectrometers to the determination of the H 2 O+CO 2 continuum in the 3.5 µm, 2.3 µm and 1.6 µm transparency windows. The spectral ranges together with the technique used and corresponding laser sources and experimental conditions are summarized in Table 1. All the studied spectral regions are located in transparency windows where both CO 2 and H 2 O weakly absorb (Figure 1).

Spectral Range (cm

The CRDS setup

The CRDS setups use as light sources a set of distributed feedback (DFB) laser diodes, from NTT Electronics Corporation and Eblana Photonics, allowing covering the entire 1.6 µm window (the gaps between the spectral ranges reported in Table 1 for this window (see also Fig. 1) being due to the too strong absorption of CO 2 ) and three additional spectral intervals centred near 4250, 4430 and 4525 cm -1 (from Nanoplus GmbH). These latter sources are narrowed by an optical feedback scheme adapted from [START_REF] Lin | Long-external-cavity distributed Bragg reflector laser with sub kilo hertz intrinsic linewidth[END_REF] to increase the injection efficiency into the high finesse cavity and improve the absorption sensitivity. Note that each laser diode can be tuned over a few tens cm -1 . The cavity ring down spectrometers use 142 cm long high finesse cavities fitted with different sets of highly reflective dielectric mirrors optimized for the 4000-4550, 4550-5130, 5300-6000 and 5700-6670 cm -1 spectral ranges depending of the laser diode used. To achieve resonances between the laser emission and a longitudinal mode of the optical cavity, the output mirror of the cavity is placed on a PZT tube supplied by a triangular voltage allowing for dithering the cavity length and periodically obtaining build-up events. For build-up event with a photodiode signal higher than a user-defined threshold, I threshold , an acousto-optic modulator switched off the laser light leading to a purely exponential decay signal on the photodiode corresponding to a ring down (RD) event expressed by:

     t I t I threshold   exp (1)
where  is the ring-down (RD) time.

The absorption coefficient at the laser frequency, ν, is then derived from the following equation:

            0 c n c n Ar   (2)
where c is the speed of light, τ 0 the ring down time for the cell filled with argon (which is a non absorbing gas used here to determine the baseline absorption n Ar /cτ 0 ) and n and n Ar , the refractive index of the absorbing gas and of argon, respectively.

The absorption spectra are recorded by tuning step-by-step the laser diode temperature and thus the emitted frequency, keeping unchanged the injection current. Typically, ten RD times are averaged at each spectral step. A part of the laser light is also sent into a wavelength meter (model 621-A IR from Bristol) to monitor the laser frequency. The intra-cavity pressure is monitored by a pressure sensor from Edwards (1000 mbar full scale, accuracy 0.15 % of reading). The cavity temperature is not stabilized and is close to room temperature (see Table 1).

OFCEAS setup

The application of the OFCEAS technique to continua measurements has been described in detail in [START_REF] Ventrillard | Accurate measurements and temperature dependence of the water vapor self-continuum absorption in the 2.1 μm atmospheric window[END_REF]. Basically, this high-sensitivity technique exploits optical feedback, from a V-shape highfinesse cavity, towards the diode laser source. This feedback appears at cavity resonances and leads to laser emission width narrowing and to a temporary frequency locking to the cavity mode which ensures an optimized cavity injection by the laser radiation. The laser frequency is scanned with fast laser current ramps (at a typical frequency of 5 Hz). During these ramps, the laser is successively locked to the different cavity modes leading to transmission spectra after dividing the cavity output signal by the reference laser power signal at each cavity-mode maximum. The transmission spectra are then converted into cavity-loss spectra with an absolute absorption scale after calibration with a ring-down measurement [START_REF] Kerstel | A water isotope ( 2 H, 17 O, and 18 O) spectrometer based on optical feedback cavity-enhanced absorption for in situ airborne applications[END_REF]. In this technique, the spectrum data points are equally spaced by the cavity free spectral range. Note that the phase and the coupling rate of the optical feedback are controlled respectively to improve the injection of the cavity modes and increase the time during which the laser remains locked.

In this work, two OFCEAS setups have been used. The first one is based on an interband cascade laser (ICL) source centred at 2853 cm -1 from Nanoplus GmbH and high reflectivity mirrors from Layertec GmbH providing a reflectivity of 99.99 % at this wavenumber. Ring-down times of 13 µs are measured for an unfolded V-shaped cavity length of 80 cm. Variations of the baseline of 5×10 -9 cm -1 are observed for that setup requiring averaging typically 100 spectra to determine the mean baseline. In the second setup, a DFB laser diode (from Nanoplus GmbH) emitting near 4302 cm -1 and a set of three high reflectivity mirrors with a reflectivity of 99.997 % at this wavenumber were used, all the rest being identical to the first setup. This latter setup allows reaching RD times of 45 µs, and a baseline stability of 2×10 -9 cm -1 . An averaging over 100 spectra was also used. Spectral point spacing was equal to the free spectral range of the cavity (FSR = 187.5 MHz) while the absolute frequency calibration relied on comparing fitted values of line centres to reference line positions [START_REF] Gordon | The HITRAN2016 Molecular Spectroscopic Database[END_REF].

In both cases, it is possible to stabilize the cell within the temperature range 303-323 K with ±0.3 K accuracy by using platinum thermistances and a heating ribbon. The intra-cavity pressure is monitored by a pressure sensor (STS ATM1ST, 1 bar full scale, accuracy 0.1 % of full scale).

Humidity generator and determination of the water vapor partial pressure

For all the CRDS and OFCEAS measurements, series of spectra are recorded for H 2 O+CO 2 mixtures with different molar fractions of H 2 O (between 3000 and 15000 ppmv) and total pressures (between 300 and 750 Torr) (see Table 1). For that, a gas flow of H 2 O+CO 2 is produced thanks to a commercial humidity generator (from Omicron Technologies) based on a temperature regulated membrane system to generate pure water vapor. Water vapor is then mixed with the dry carrier gas (here CO 2 )

with a proportional control solenoid valve with bypass to achieve the desired humidity level. The gas flow (300 sccm) as well as the pressure (1045 mbar) are regulated in the exit line by a mass flow controller (Area model, FC-R7800 series from Hitachi Metals Ltd.) and a back pressure controller (Model IQP-700C from Bronkhorst). Only a limited part of this flow (typically a few tens sccm) is derived into the CRDS or OFCEAS high finesse cavity to avoid dirtying the mirrors by displacing small particles inevitably present on the walls of the cavity. The pressure in the CRDS cavity is maintained at a fixed value with a solenoid valve and a proportional integral software based loop acting on the gas flow. In the OFCEAS setups, the gas flow and intra-cavity pressure were regulated using Bronkhorst modules. In addition, the remote head of a precision chilled mirror hygrometer (Model S8000 from Mitchell) is placed on the exit line to provide a direct measurement of the dew or frost point temperature. After calibration at CETIAT [24], the dew point temperature is known with a 2σuncertainty better than 0.1°C over the -20°C to +20°C range. A PID loop acting on the solenoid valve with bypass of the humidity generator is used to maintain the dew point at the chosen set point. This system is able to produce a flow of CO 2 where the H 2 O molar fraction varies by no more than 0.1% (peak to peak) during several hours for set points in the 5000 -15000 ppmv range.

To retrieve the molar fraction, the dew point temperature is first converted into saturated vapor pressure (corresponding to the water vapor partial pressure, P H2O ) using formulas given by [START_REF] Wexler | Vapor Pressure Formulation for Water in Range 0 to 100 °C. A Revision[END_REF][START_REF] Wexler | Vapor Pressure Formulation for Ice[END_REF] for liquid water and ice. The partial pressure values are then multiplied by an enhancement factor (calculated from [START_REF] Hyland | A Correlation for the Second Interaction Virial Coefficients and Enhancement Factors for Moist Air[END_REF][START_REF] Hyland | Formulations for the thermodynamic properties of dry air from 173[END_REF]) to take into account non-ideal gas effects and the small departures from ideal solution behaviour with regard to equilibrium between the gas mixture and condensed phase located at the sensing surface of the chilled-mirror hygrometer. For our pressure and temperature conditions, this factor is lower than 1.004. Knowing the total pressure, P tot_EL , in the exit line (EL)

(measured by a Wika pressure gauge; 1.6 bar full scale; 0.1% accuracy), we deduce the molar fraction of water vapour X H2O = P H2O_EL /P tot_EL . After having flushed during several hours the high finesse cavities with a flow of CO 2 with constant water vapor molar fraction, we make the hypothesis that this latter quantity is the same in the high finesse cells than in the exit line, so that we can also derive the water vapor partial pressure in the cell as P H2O = X H2O P tot where P tot is the intra-cavity pressure.

Densities of CO 2 , ρ CO2 , and H 2 O, ρ H2O , are then calculated knowing the cell temperature measured with a Pt 100 sensor calibrated at CETIAT (2σ-uncertainty ±0.06°C) fixed on the external wall of the cavity underneath the enveloping thermal insulation foam.

P H2O can also be retrieved from a fit of a selection of water vapor lines with intensity provided with a high accuracy (1 to 2%) in the HITRAN database. Comparison between those two methods shows that differences can occur. More precisely, in the CRDS setup used for measurements between 5730 and 5950 cm -1 , we obtained lower P H2O values than expected from the chilled-mirror hygrometer due to adsorption on the gas line going to the CRDS cell for all molar fractions considered in this study, with an average relative difference ranging from 2% for a molar fraction of 5000 ppm, to 6% at 15000 ppm. However, at a lower molar fraction of 1000 ppm (not used for continua measurements), P H2O values derived from the fit were larger than expected, due to outgasing. For all the scans recorded with this setup, the P H2O value derived from the fitted line absorption has been adopted. This effect has been minimized for the measurements between 5950 and 6660 cm -1 by replacing the tubes in the gas line with tubes having a Sulfinet surface treatment (from Restek company) limiting water adsorption.

In the case of the OFCEAS measurements near 2 µm and 3.5 µm, this effect was considered to be below the 2 % uncertainty on water molar fraction and the P H2O value derived from the hygrometer dew point temperature was adopted. For the CRDS measurements in the 2.3 µm window, the value of P H2O was determined from fitted line absorption.

Recordings and retrieval of the binary coefficients

CRDS spectra

In the 2.3 µm window, spectra were acquired in the following manner. For each total pressure and molar fraction conditions, a sequence of one argon spectrum, two H 2 O+CO 2 spectra and a second argon spectrum was recorded, emptying the cavity before each change of the gas composition. Such a sequence (Figure 2) allows (i) checking the water vapour stability in the mixture, (ii) accurately determining the spectrum baseline, as argon does not absorb, and (iii) checking the baseline stability after pumping and filling cycles. Baseline stability of the order of 10 -10 cm -1 was achieved for the CRDS spectra near 4525 cm -1 while a stability of a few 10 -9 cm -1 was observed near 4250 cm -1 and 4430 cm -1 . This larger value results from the fact that the coating of one of the mirrors was damaged, making the baseline more sensitive to changes in the mirrors alignment.

In the 1.6 µm window, the cavity alignment was more sensitive to large pressure variations.

Hence, the intra-cavity pressure was kept constant throughout an entire measurement, while argon and H 2 O+CO 2 mixtures were alternatively flowed through the cavity. In this case the observed baseline stability was on the order of a few 10 -10 cm -1 .

Measurements were repeated for several values of water molar fraction in each spectral region (see Table 1) to ensure the linearity of the obtained binary absorption coefficient with respect to the density product.

OFCEAS spectra

For OFCEAS spectra, the recording sequence was as follows. In a first step, dry nitrogen was flowed through the cavity to measure the baseline losses. As spectra were recorded continuously, monitoring the absorption after the gas source was switched to the H 2 O+CO 2 mixture allowed to check the stability of the gas inside the cavity. Once the absorption had become stable, 100 spectra were averaged before modifying the water molar fraction set point on the hygrometer. In total, four values of the water molar fraction were used: 5 000, 10 000, 12 500 and 15 000 ppmv (see Table 1).

Retrieval of the H 2 O+CO 2 binary coefficients

For each series of spectra, the zero-absorption baseline was determined by fitting the argon or nitrogen spectrum with a second order polynomial, after removing the water lines (in traces). The baselines were then subtracted from the H 2 O+CO 2 spectra. In the 1.6 µm window, the Rayleigh scattering losses due to CO 2 differ from those due to argon by a few 10 -9 cm -1 [START_REF] Thalman | Rayleigh scattering cross-section measurements of nitrogen, argon, oxygen and air[END_REF]30] and the corresponding difference was taken into account. In the other spectral windows, Rayleigh losses are much smaller and negligible. As a result, the absorption coefficient, , can be expressed as:

(

where and are the contributions due to the "monomer local lines" of CO 2 and H 2 O, respectively and represents the density. The coefficients (in cm -1 amagat -2 ) correspond to the binary coefficients, where i denotes the absorbing gas and j the perturber.

The binary coefficient (the two contributions are indistinguishable experimentally) is obtained from , after subtraction of the other terms appearing in Eq. ( 3). First, we used the HITRAN2016 database [START_REF] Gordon | The HITRAN2016 Molecular Spectroscopic Database[END_REF] to simulate the local CO 2 and H 2 O monomer contributions with Voigt profiles truncated at ±25 cm -1 excluding the pedestal of the absorption lines [START_REF] Ptashnik | Water vapor self-continuum absorption in near-infrared windows derived from laboratory measurements[END_REF]. As CO 2broadening coefficients for H 2 O are not given in HITRAN, the air-broadening coefficients were multiplied by a factor of 1.67 following the value reported in [START_REF] Brown | CO 2 broadened water in the pure rotation and ν 2 fundamental regions[END_REF] for the water vapor rotational band spectral region, and fitted for the most intense lines. Note that for the spectra around 2853 cm -1 , the H 2 O local monomer absorption is mainly due to HDO for which we take an isotopic abundance 20 % lower than the one given in HITRAN2016 as determined in our previous water samples (see for instance [START_REF] Mikhailenko | A spectroscopic database for water vapor between 5850 and 8340cm -1[END_REF]). The CO 2 cylinder used for this study contained methane impurities. In the 2853 cm -1 spectra, two CH 4 lines of intensity greater than 10 -22 cm/molecule allowed determining a CH 4 molar fraction of about 590 ppb. The methane lines were included in the monomer resonance line simulations for all spectra, although their contribution is mostly negligible except near 2853 cm -1 .

Table 2 summarizes the source of the water and CO 2 self-continuum cross sections used to subtract their corresponding contributions from the recorded spectra.

Spectral range (cm -1 ) Water self-continuum CO 2 self-continuum 2853

Fleurbaey in press [START_REF] Mondelain | The self-and foreign-absorption continua of water vapor by cavity ring-down spectroscopy near 2.35 μm[END_REF] This work

4244-4256

Mondelain 2015 [START_REF] Campargue | Accurate laboratory determination of the near infrared water vapor self-continuum: A test of the MT_CKD model[END_REF] Tonkov 1996 [START_REF] Tonkov | Measurements and empirical modelling of pure CO 2 absorption in the 2.3 µm region at room temperature: far wings, allowed and collision-induced bands[END_REF]

4302

Ventrillard 2015 [START_REF] Ventrillard | Accurate measurements and temperature dependence of the water vapor self-continuum absorption in the 2.1 μm atmospheric window[END_REF] Tonkov 1996 [START_REF] Tonkov | Measurements and empirical modelling of pure CO 2 absorption in the 2.3 µm region at room temperature: far wings, allowed and collision-induced bands[END_REF]

4421-4440

Richard 2017 [START_REF] Richard | Water vapor selfcontinuum absorption measurements in the 4.0 and 2.1 μm transparency windows[END_REF] Tonkov 1996 [START_REF] Tonkov | Measurements and empirical modelling of pure CO 2 absorption in the 2.3 µm region at room temperature: far wings, allowed and collision-induced bands[END_REF]

4518-4535

Campargue 2016 [START_REF] Mikhailenko | A spectroscopic database for water vapor between 5850 and 8340cm -1[END_REF] Tonkov 1996 [START_REF] Tonkov | Measurements and empirical modelling of pure CO 2 absorption in the 2.3 µm region at room temperature: far wings, allowed and collision-induced bands[END_REF]

5730-5800

Vasilchenko 2019 [START_REF] Vasilchenko | The water vapour self-and foreign-continua in the 1.6 μm and 2.3 μm windows by CRDS at room temperature[END_REF] Mondelain 2017 [START_REF] Mondelain | The CO 2 absorption continuum by high pressure CRDS in the 1.74 µm window[END_REF]

5800-6665

Vasilchenko 2019 [START_REF] Vasilchenko | The water vapour self-and foreign-continua in the 1.6 μm and 2.3 μm windows by CRDS at room temperature[END_REF] This work The water vapor self-continuum contributions were obtained from the cross-sections reported in our previous papers [START_REF] Richard | Water vapor selfcontinuum absorption measurements in the 4.0 and 2.1 μm transparency windows[END_REF][START_REF] Ventrillard | Accurate measurements and temperature dependence of the water vapor self-continuum absorption in the 2.1 μm atmospheric window[END_REF][START_REF] Vasilchenko | The water vapour self-and foreign-continua in the 1.6 μm and 2.3 μm windows by CRDS at room temperature[END_REF][START_REF] Campargue | Accurate laboratory determination of the near infrared water vapor self-continuum: A test of the MT_CKD model[END_REF][START_REF] Mondelain | The self-and foreign-absorption continua of water vapor by cavity ring-down spectroscopy near 2.35 μm[END_REF], except at 2853 cm -1 where this value was newly determined and found to be 5.85(42)×10 -24 cm 2 molec -1 atm -1 [START_REF] Fleurbaey | Measurements of the water vapor continuum absorption by OFCEAS at 3.50 µm and 2.32 µm[END_REF].

In the case of the CO 2 self-continuum, the contributions were calculated from the measurements of [START_REF] Tonkov | Measurements and empirical modelling of pure CO 2 absorption in the 2.3 µm region at room temperature: far wings, allowed and collision-induced bands[END_REF] in the 2.2 µm window, and of [START_REF] Mondelain | The CO 2 absorption continuum by high pressure CRDS in the 1.74 µm window[END_REF] in the 1.6 µm window below 5800 cm -1 . In all other spectral regions, the CO 2 self-continuum cross-sections values were obtained from dedicated measurements as follows. The high finesse cell was filled with pure CO 2 gas at a pressure value equal to the total pressure used in the binary absorption experiments. The contribution from the monomer lines was subtracted as described above, and the remaining absorption was divided by to retrieve the self-continuum coefficient . The obtained values were smoothed using polynomial fits as a function of wavenumber, in order to calculate the self-continuum contribution to the H 2 O-CO 2 measurements. For the spectral point at 2853 cm -1 , the total absorption due to CO 2 (local lines and self-continuum) was retrieved from these pure CO 2 spectra and subtracted from the water+CO 2 mixture spectra recorded for the same CO 2 partial pressure, so that the self-continuum coefficient was not itself determined.

Smoothed values of the self-continuum absorption coefficients of water vapor and carbon dioxide used in this work are provided in a Supplementary Material together with their estimated uncertainty and their source. Figure 3 displays the different contributions to the absorption coefficient recorded around 4525 cm -1 for a total pressure of 700 Torr and a water vapor molar fraction of 10 000 ppm. In these conditions, the local line contribution of CO 2 , due to the 00021-00001 band of the 16 O 13 C 18 O isotopologue in natural abundance, dominates. In other spectral regions like near 4250 cm -1 , the local line contribution of water vapor is the largest subtracted absorption. At all the wavenumbers studied here, the absorption coefficient reduced by the two self-continua contributions and the resonance line contribution is found to be proportional to , as illustrated in Figure 4 (lower panel). This leads to a very good consistency of the binary coefficients retrieved for the different densities of CO 2 and H 2 O in the 2.3 µm window (Figure 4, upper panel), as well as in the other spectral regions, as shown in Figure 5 for the 1.6 µm window.

The retrieved binary coefficients are given in the Supplementary material along with their type-B uncertainty (see next section). We also provide smoothed values of the binary coefficient for ease of use, with a combined (type-A and type-B) uncertainty. These smoothed values are used in Figure 6 along with other calculated contributions to the absorption, to illustrate the various contributions to the absorption in part of the 1.6 µm window. 2). The (H 2 O+CO 2 ) binary continuum (hatched blue) corresponds to the smoothed values reported in the Supplementary Material. The water (yellow) and CO 2 (green) local line contributions are HITRAN simulations, with collisional broadening multiplied by 1.67 for CO 2 -broadened water.

Estimated uncertainties on the experimental binary coefficients

The uncertainties on the derived binary coefficients have been calculated

with the error propagation approach using Eq. ( 1), assuming uncorrelated variables. Uncertainty on total pressure and temperature are estimated to 0.15% and 0.1K, respectively. A baseline uncertainty of 5×10 -10 cm -1 is adopted for the 1.6 µm window and for the measurements centered at 4525 cm -1 .

At 2853, 4250, 4302 and 4430 cm -1 , the baseline uncertainties are taken as equal to 5×10 -9 , 2×10 -9 , 1×10 -8 and 2×10 -9 cm -1 , respectively. Baseline stability was estimated from argon or nitrogen spectra.

As mentioned above, uncertainties on water vapor and carbon dioxide binary coefficients are reported as Supplementary Materials. The uncertainty on the water vapor partial pressure is the main contributor to the final error budget due to the local monomer contribution and to the selfcontinuum contribution (see Figs. 4 and6). As explained in part 2.3, the water vapor partial pressures retrieved from the dew/frost point temperature measured with the chilled-mirror hygrometer are in some cases slightly different from the ones derived from the recorded spectra. These latter are adopted to extract the binary coefficients ( from the spectra recorded between 5730 and 5950 cm -1 and the CRDS measurements in the 2.3 µm window, the values from the hygrometer being used in all other cases. A mean uncertainty of 2% is estimated for P H2O . In the case of P CO2 , the estimated uncertainties are much smaller and correspond to the uncertainty on the total pressure (0.15%).

Semi-empirical calculations and comparison with available experimental data

Few measurements of H 2 O+CO 2 continua are available in the literature [START_REF] Baranov | On the significant enhancement of the continuum-collision induced absorption in H 2 O+CO 2 mixtures[END_REF][START_REF] Tran | Measurements and modeling of absorption by CO 2 +H 2 O mixtures in the spectral region beyond the CO 2 ν 3 -band head[END_REF][START_REF] Tran | The CO 2 -broadened H 2 O continuum in the 100-1500 cm -1 region: Measurements, predictions and empirical model[END_REF], all by FTS. Table 2 summarizes the main experimental characteristics showing that only the work of [START_REF] Baranov | On the significant enhancement of the continuum-collision induced absorption in H 2 O+CO 2 mixtures[END_REF] provides data to compare with. More precisely, in this latter study, a value of 4(±1)×10 -5 cm -1 amagat -2 was reported around 2850 cm -1 for the binary coefficient with no observed temperature dependence. This value is 1.5 times higher than what we measured here (2.66(±0.32)×10 -5 cm -1 amagat -2 ). These two measurements are thus barely compatible considering their 1σ-error bars. From a theoretical point of view, recent progress has been done for the calculations of the far wings and collision induced absorption (CIA) with the classical molecular dynamic simulations (CMDS) which provide, as explained in [START_REF] Hartmann | Effect of humidity on the absorption continua of CO 2 and N 2 near 4 μm: Calculations, comparisons with measurements, and consequences for atmospheric spectra[END_REF], "the center-of-mass positions and orientations of many (rigid) linear molecules interacting through an input anisotropic potential". The auto-correlation functions (ACF) of the allowed-and induced-dipoles and of their cross-terms are then computed. The absorption spectrum is finally obtained after applying a Fourier-Laplace transform to these ACF. This approach has been applied to systems like CO Another way to get (independently) the and coefficients consists in adopting the widespread χ-factor empirical approach. Here, the Lorentzian/Voigt line-shape is corrected by applying χ-factors in the wings of the profile depending on the distance to the line center. The same profile is applied to all the transitions of the absorbing molecule which are provided by a spectroscopic database such as HITRAN. For the H 2 O absorbing molecule in CO 2 , the χ-factors have been obtained either by using quasi-static predictions [START_REF] Ma | A far wing line shape theory and its application to the foreign broadened water continuum absorption[END_REF] or by fitting these factors on experimental data sets as done in [START_REF] Tran | The CO 2 -broadened H 2 O continuum in the 100-1500 cm -1 region: Measurements, predictions and empirical model[END_REF] with FTS data in the 100-1500 cm -1 spectral range. The same procedure was applied to CO 2 as absorbing molecule and H 2 O as collider in the 2400-2600 cm -1 spectral range [START_REF] Tran | Measurements and modeling of absorption by CO 2 +H 2 O mixtures in the spectral region beyond the CO 2 ν 3 -band head[END_REF]. Figure 7 shows the different data sets mentioned just above as well as the sum of the binary coefficients calculated in the present work using the χ-factor approach developed in [START_REF] Tran | Measurements and modeling of absorption by CO 2 +H 2 O mixtures in the spectral region beyond the CO 2 ν 3 -band head[END_REF][START_REF] Tran | The CO 2 -broadened H 2 O continuum in the 100-1500 cm -1 region: Measurements, predictions and empirical model[END_REF] and the data obtained in this work. As can be seen, the χ-factor approach well reproduces our experimental binary coefficients on the low-and high-frequency edges of the 1.6 µm window and a relatively good agreement is observed for the 2853 cm -1 data point. Larger differences are observed in the 2.3 µm window where the calculated values underestimate by factors between 2.6 and 3.9 our experimental data. This is not so surprising considering that the χ-factor approach has limited extrapolation capabilities [START_REF] Hartmann | Chapter V -The far wings (beyond the impact approximation)[END_REF]. Interestingly, unexpected peak absorption is observed around 6000 cm -1 that we interpret as a signature of the collision induced absorption (CIA) corresponding to the ν 3 (CO 2 ) + ν 1 (H 2 O) simultaneous transition. Note that the possible detection of this combination band was anticipated in Ref. [START_REF] Pavlyuchko | Variational Solution of Anharmonic Vibrational Problems for Polyatomics and Molecular Pairs[END_REF]. Further investigations dedicated to this absorption peak will be done in a future work. In all cases, this additional absorption is not taken into account in the χ-factor approach.

Ref

Conclusion

From spectra of water vapor in CO 2 mixtures recorded with highly sensitive and stable CRDS and OFCEAS techniques, we derived the binary coefficients at room temperature in different spectral regions corresponding both to H 2 O and CO 2 spectral windows. To our knowledge, this constitutes the first experimental dataset of the H 2 O+CO 2 continuum in those spectral regions.

Comparison to values obtained from the only available model based on the χ-factor approach is presented. For that, binary coefficients are extrapolated in the spectral intervals studied in this work using a model where the χ-factors were previously fitted to FTS data recorded in the 100-1500 cm -1 spectral range for H 2 O absorbing molecules in CO 2 [START_REF] Tran | The CO 2 -broadened H 2 O continuum in the 100-1500 cm -1 region: Measurements, predictions and empirical model[END_REF] and in the 2400-2600 cm -1 spectral range [START_REF] Tran | Measurements and modeling of absorption by CO 2 +H 2 O mixtures in the spectral region beyond the CO 2 ν 3 -band head[END_REF] for the CO 2 absorbing molecules in H 2 O. This comparison shows the ability of the model to quite well reproduce our experimental values on the edges of the 1.6 µm window and at 2853 cm -1 but a discrepancy is observed in the 2.3 µm window. It is important to notice that a peak of additional absorption (not predicted by the far-wing model) is observed around 6000 cm -1 . This peak is interpreted as due to a CIA band corresponding to a cooperative excitation of CO 2 and H 2 O molecules. Additional work is planned to confirm this hypothesis and, if applicable, to model the collision induced absorption band of the H 2 O and CO 2 molecules which has to be added to the farwings contribution. The impact of this kind of CIA bands in other spectral regions remains to be evaluated. Finally, a study of the temperature dependence of the different contributions of the H 2 O+CO 2 continuum is also of first importance for planetary applications and will be the subject of new dedicated measurements.

Figure 1 . 1 .

 11 Figure 1. Overview of the H 2 O and CO 2 absorption lines provided by the HITRAN2016 database [22] between 2700 cm -1 and 6800 cm -1 . The vertical light blue solid lines and red rectangles indicate where measurements were performed using OFCEAS and CRDS, respectively. The line intensities of water vapor have been divided by a factor 100 to roughly scale to the relative contributions of H 2 O and CO 2 in the used gas mixtures.

Figure 2 . 1 .

 21 Figure 2. Spectra of argon and of a (H 2 O+CO 2 ) mixture recorded at a total pressure of 700 Torr recorded by CRDS near 4525 cm -1 . The water vapor molar fraction in the mixture was 10 000 ppmv. The argon spectra (lower ones) recorded at the beginning (I) and at the end (4) of the 4 step-procedure give the zero-absorption baseline which is subtracted from the (H 2 O+CO 2 ) spectra (2) and (3). At the scale of the figure, the spectra (2) and (3), recorded successively are indistinguishable illustrating the stability of the setup. Note the water vapor traces (at the 100 ppm level) are present in the argon spectra.

Figure 3 .

 3 Figure 3. The different contributions to the total absorption coefficient measured by CRDS around 4525 cm -1 for a mixture of 10 000 ppmv of H 2 O in CO 2 at a total pressure of 700 Torr. Refer to the text for the different notations.

Figure 4 .

 4 Figure 4. Absorption coefficients due to the contribution (lower panel) and retrieved binary coefficients versus wavenumber (upper panel) for different water vapor and CO 2 densities (corresponding to different colors).

Figure 5 .

 5 Figure 5. Overview of the binary coefficients measured in the 1.6 µm window for various total pressures and water vapor molar fractions.

Figure 6 .

 6 Figure 6. Overview of the various contributions to absorption in the low-frequency part of the 1.6 µm window, for a pressure of 700 Torr and a water mole fraction of 15 000 ppm. The self-continua contributions (water, red and CO 2 , black) are calculated using the coefficients listed in the Supplementary Material (see sources in Table

The H 2 O

 2 and CO 2 local monomer contribution uncertainties are determined by taking into account the error bars on the line intensities, self-broadening, air-broadening coefficients and partial pressures. The uncertainties on the line parameters were taken from the HITRAN database [22]. In both cases (i.e. for H 2 O and CO 2 ), four simulations are performed, each of them with either, the intensities, γ self , γ air coefficients or P i values increased by their error bar. The resulting deviations from the HITRAN simulation, δ i (ν), lead to the uncertainty on α WML : δα WML (ν) = . The uncertainties detailed above are of type-B. Statistical (type-A) uncertainties are taken as the standard deviation of the experimental data fit residuals. Final residuals given in Supplementary material correspond to the sum in quadrature of type A and B uncertainties.

Figure 7 .

 7 Figure 7. Overview of the experimental and calculated binary coefficients obtained for H 2 O+CO 2 mixture. The calculated coefficients are obtained with the χ-factor approach adopted in[START_REF] Tran | The CO 2 -broadened H 2 O continuum in the 100-1500 cm -1 region: Measurements, predictions and empirical model[END_REF] for B CO2-H2O and[START_REF] Tran | The CO 2 -broadened H 2 O continuum in the 100-1500 cm -1 region: Measurements, predictions and empirical model[END_REF] for B H2O-CO2 .The black solid line corresponds to the sum of these two calculated binary coefficients, while the dashed line presents B H2O-CO2 . The smoothed experimental values are plotted as a red solid line with a shaded area corresponding to the 1 uncertainty.

-1 ) Technique Laser source P tot (Torr) X H2O (ppm) T (K) 2852.7-2853.4

		OFCEAS	ICL	450	5000 to 15000	303
	4243-4255	CRDS	DFB LD	500	3000 to 10000	298
	4301.3-4302	OFCEAS	DFB LD	700	5000 to 15000	306
	4421.5-4440	CRDS	DFB LD	700	5000 to 15000	298
	4518-4535	CRDS	DFB LD	500/700	5000 to 15000	298
	5720-6045	CRDS	DFB LD	750	5000 to 15000	296
	6390-6460	CRDS	DFB LD	750	10000	297.5
	6570-6665	CRDS	DFB LD	750	3000 to 7000	298

Table 1 .

 1 Summary

of the experimental conditions of the different recordings. DFB LD and ICL mean distributed feedback laser diode and interband cascade laser, respectively.

Table 2 .

 2 Summary of the sources of the self-continuum coefficients used in this work. Note that near 2853 cm self-continuum and local line absorption were subtracted together from the mixture spectra, so that the value of the self-continuum was not derived in this work.

	-1 ,

Table 2 .

 2 Summary of the experimental works on H 2 O+CO 2 continuum available in the literature.

  2 -CO 2 in[START_REF] Hartmann | Molecular dynamics simulations for CO 2 spectra. II. The far infrared collision-induced absorption band[END_REF][START_REF] Hartmann | Molecular dynamics simulations for CO 2 spectra. III. Permanent and collisioninduced tensors contributions to light absorption and scattering[END_REF], CO 2 -H 2 O and N 2 -H 2 O in[START_REF] Hartmann | Effect of humidity on the absorption continua of CO 2 and N 2 near 4 μm: Calculations, comparisons with measurements, and consequences for atmospheric spectra[END_REF] where the first molecule is the absorbing molecule and the second one the collider. Good agreements with measured data have been obtained, without any scaling factor, for the ν 3 band for pure CO 2 , the high frequency wing of this band for CO 2 in H 2 O and the CIA of the N 2 fundamental band. Up to now, CMDS has only been applied to linear absorbing molecules and thus not to the water molecule as absorber so that the coefficient has not yet been calculated.

Acknowledgements

This work was performed in the frame of the ANR project COMPLEAT (ANR-19-CE31-0010-01). Authors want to thank J-M. Hartmann (LMD) for fruitful discussions and H. Tran (LMD) and M. Turbet (Univ. of Geneva) for providing them extended χ-factor data.


A Marciniak

A Bonnamy

S Zamith

L Noguès

O Coeur-Joly

D Murat

P Moretto-Capelle

J.-M L'hermite

G Mulas

C Joblin

Studying the interaction between VUV photons and PAHs in relevant astrophysical conditions

Keywords: PAHs, VUV photoprocessing in PDRs, cryogenic ion trap

PIRENEA and PIRENEA 2 are experimental setups dedicated to the study of fundamental molecular processes involving species of astrochemical interest. The coupling of a VUV source to PIRENEA has allowed us to study the fragmentation pathways and stability of polycyclic aromatic hydrocarbons (PAHs) containing aliphatic bonds under conditions relevant for astrophysical photodissociation regions. PIRE-NEA 2 will open the possibility to extend these studies to larger systems such as PAH clusters, and more generally to study gas-nanograin-photon interactions.

Introduction

Studying the interaction of vacuum ultraviolet (VUV) photons with polycyclic aromatic hydrocarbon (PAH) molecules is of high interest to understand the physics and chemistry of photodissociation regions (PDRs) associated with star forming regions (see A.G.G.M. Tielens in this volume). This interaction is responsible for the well-known aromatic infrared band (AIB) emission but it can also induce other mechanisms such as fragmentation (see e.g., Ref [START_REF] Montillaud | Evolution of polycyclic aromatic hydrocarbons in photodissociation regions -Hydrogenation and charge states[END_REF]).

PIRENEA, from the acronym of "Piège à Ions pour la Recherche et l'Etude de Nouvelles Espèces Astrochimiques" [START_REF] Joblin | A piece of interstellar medium in the laboratory: the PIRENEA experiment[END_REF], is a dedicated laboratory astrophysics setup which was developed to meet the following functionalities: (i) isolation of molecular species of astrophysical interest for minutes in a cold (≤ 30 K) and collision-free (≤ 10 -11 mbar) environment, (ii) photoprocessing with photons in the visible to VUV range, and (iii) ultra high resolution non-destructive Fourier transform mass spectrometry (FT-MS) to record fragmentation products. The functioning principle of PIRENEA is shown and described in Figure 1. Here, we illustrate the ability of PIRENEA to study the VUV photoprocessing of aliphatic PAHs [START_REF] Marciniak | Photodissociation of aliphatic pah derivatives under relevant astrophysical conditions[END_REF]. We put these results in perspective of future studies with the second generation setup, PIRENEA 2. marciniak@irsamc.ups-tlse.fr PAHs containing aliphatic C-H bonds are considered as good candidates for the 3.4 µm AIB observed in PDRs [START_REF] Joblin | Spatial Variation of the 3.29 and 3.40 Micron Emission Bands within Reflection Nebulae and the Photochemical Evolution of Methylated Polycyclic Aromatic Hydrocarbons[END_REF]. We have recently studied the chemical evolution of aliphatic PAHs under VUV irradiation [START_REF] Marciniak | Photodissociation of aliphatic pah derivatives under relevant astrophysical conditions[END_REF] by coupling a 10.5 eV photon source to PIRENEA (Figure 1). This source has a low photon flux (∼ 10 13 photon s -1 cm -2 ), which means that only one-photon processes are considered in this experiment. Besides, ions or their fragments have a sufficient time (∼ 10 2 s) between two photon absorption to relax most of their internal energy by radiative cooling [START_REF] Joblin | Photoprocessing of astro-PAHs[END_REF]. Thus, this setup allows the study of fragmentation cascades (see Figure 2) under conditions that are relevant for PDRs [START_REF] Joblin | Photoprocessing of astro-PAHs[END_REF].

We studied the fragmentation kinetics of the coronene cation (C 24 H + 12 , Cor + ) and its alkylated derivatives, namely methylcoronene (C 25 H + 14 , MeCor + ), and ethylcoronene (C 26 H + 16 , EtCor + ). The kinetic curves of the parents and main fragmentation channels of these three PAHs are shown in Figure 3(a-c). We observe that the fragmentation of the bare Cor + is much less efficient than its methylated or ethylated derivatives. The kinetic curves are analyzed with a simple population/depopulation model whose solution fits the data (full line and R 2 in Figure 3(a-c)). From this model, we derive a fragmentation map with fragmentation rates (k frag ) and branching ratios for the different fragmentation channels. We show a simplified version of these maps in Figure 3(d-f) for Cor + , MeCor + and EtCor + , respectively. They illustrate well the difference of fragmentation pathways between a bare PAH and its alkylated derivatives. Interestingly, we observe that the first H-loss step in MeCor + is mimicked by a CH 3 -loss in EtCor + . It shows that similar photoproducts are involved in the fragmentation cascades of these alkylated PAHs. Figure 4 summarizes the VUV photostability of the studied PAHs and their fragments. Our results provide experimental support for the isomerization of Cor + upon VUV irradiation, which is presumably due to the migration of an H atom [START_REF] Trinquier | Pah chemistry at ev internal energies. 1. h-shifted isomers[END_REF] (e.g see Figure 3(d), see also discussion by A.G.G.M. Tielens in this volume). In addition, Cor + is found to be very stable compared to MeCor + or EtCor + which have higher fragmentation rates and exhibit carbon loss channels. However, the VUV fragmentation cascade of alkylated PAHs leads to the formation of a fragment bearing a peripheral pentagonal ring (C 23 H + 11 ), which is as stable as Cor + or [Cor-2H] + . All of the latter species have a higher radiative cooling fraction (see Figure 4) and thus a higher probability of survival in PDRs. This work supports a scenario in which the carriers of the 3.4 µm AIB are efficiently photo-dissociated and this leads to the production of small hydrocarbons in PDRs [START_REF] Pety | Are PAHs precursors of small hydrocarbons in photodissociation regions? The Horsehead case[END_REF][START_REF] Pilleri | Mixed aliphatic and aromatic composition of evaporating very small grains in NGC 7023 revealed by the 3.4/3.3 µm ratio[END_REF]. This process results in the formation of PAHs containing a pentagonal ring, which can be a step in the formation of fullerenes [START_REF] Berné | Formation of buckminsterfullerene (C 60 ) in interstellar space[END_REF]. Finally, our study provides evidence for isomerization processes which could result in the formation of aliphatic C-H bonds in PAHs and provide an additional contribution to the 3.4 µm AIB [START_REF] Jolibois | Hydrogen dissociation of naphthalene cations: a theoretical study[END_REF].

Towards a study of gas-nanograin-photon interactions

Upcoming observations from the James Webb Space Telescope (JWST) will allow us to study, with unprecedented sensitivity and precision, the chemical evolution of matter in PDRs, including that of AIB carriers. We expect to better characterize PAH precursors in PDRs, the so-called evaporating very small grains [START_REF] Pilleri | Evaporating very small grains as tracers of the UV radiation field in photo-dissociation regions[END_REF]. We also expect to detect nano-sized species, including molecular clusters, and their interaction with gas phase molecules through adsorption and desorption processes. In this context, PIRENEA 2 has been designed to generate and study a large panel of species (see Figure 5, [START_REF] Bonnamy | Pirenea 2: A new setup to study cosmic dust in the laboratory[END_REF]). The setup combines two versatile nanograin sources, namely (i) a laser vaporization source to produce species of interest for stardust chemistry [START_REF] Bérard | Impact of metals on (star)dust chemistry: A laboratory astrophysics approach[END_REF] and (ii) a source of molecular clusters to produce homogenous or heterogenous weakly bounded clusters (e.g. PAH + n or PAH + n (H 2 O) + m ) [START_REF] Zamith | Thermal evaporation of pyrene clusters[END_REF][START_REF] Zamith | Water attachment onto size-selected cationic pyrene clusters[END_REF]. PIRENEA 2 includes a number of features that are or will be soon commissioned: (i) a post-source mass selection, (ii) a cryogenic electrostatic trap to perform adsorption of small molecules (e.g. CO or H 2 O) or tagging with rare gas atoms (e.g. He or Ne) and (iii) a transfer and storage in the cryogenic ICR cell (similar to PIRENEA but with increased sensitivity). This will allow us to study photoinduced mechanisms at different photon energies. We plan to perform vibrational spectroscopy, including the characterization of C-C and C-H modes in relation to AIBs, and explore, by UV-VUV photoprocessing, the fragmentation and reactivity of a variety of nanograins containing carbon.

Conclusion and outlook

PIRENEA provides the necessary environment to study the photophysics of PAHs under VUV irradiation in physical conditions that are relevant to PDRs. The extracted fragmentation rates and maps are useful for astrochemical models. In the near future, PIRENEA 2 will allow us to study the properties of isolated nanograins of interest for the interpretation of JWST observations and for astrochemistry in general. From a more fundamental point of view, we wish to study the role of intermolecular interactions in the energy redistribution of photoinduced processes.

Aknowledgments. This research has received funding from the European Research Council (ERC-2013-SyG, Grant agreement N o 610256 NANOCOSMOS).

Fig. 1 .Fig. 2 .

 12 Fig. 1. Scheme of PIRENEA. PAH + species are produced from a sample target by laser desorption and ionization at 266 nm. They are stored in the cryogenic ion cyclotron resonance (ICR) cell where they are irradiated by the VUV laser. The photoproducts are then measured by non-destructive Fourier transform mass spectrometry.

Fig. 3 .

 3 Fig. 3. (a-c) VUV photoprocessing kinetic data (experimental points and fitting curves from the kinetic model) of the parents and main fragmentation channels of Cor + , MeCor + and EtCor + and(d-f) their associated simplified fragmentation maps. We display the fragmentation rates (k, in ×10 -3 s -1 ) and branching ratios extracted from the fit performed with the kinetic model.

Fig. 4 .

 4 Fig.4. Fragmentation rates as a function of theoretical (TD-DFT) photoabsorption rates for Cor + , MeCor + , EtCor + , and their photofragments. The dotted line indicates species that dissociate for each absorbed VUV photon, while species far below have high radiative cooling rates. Adapted from Ref.[START_REF] Marciniak | Photodissociation of aliphatic pah derivatives under relevant astrophysical conditions[END_REF].

[Fig. 5 .

 5 Fig.5. Scheme of PIRENEA 2 with its most important features and illustrations of the possible gas-nanograin-photon interactions which are planned to be studied.