© E. Dartois et al. 2020

1. Introduction

Cosmic rays pervade dense clouds and protostellar discs, the resident sites of interstellar ice mantles. They are a source of radiochemistry for these solids, in addition to photolysis from induced secondary vacuum ultraviolet (VUV) photons. They are also at the origin of a sputtering process releasing ice grain mantle species and products in the gas phase. This must therefore be considered as a desorption mechanism to be properly quantified to examine to what extent this sputtering in the electronic regime influences the astrochemical balance in dense regions of the interstellar medium. There is a special interest in the desorption of large molecules embedded in interstellar ice mantles, observed in the gas phase in abundances, often above what classical modelling can predict from pure gas phase chemistry, and even for some including grain chemistry (e.g. Vasyunin et al. 2017; Ruaud et al. 2016; Bacmann et al. 2012; Garrod et al. 2008). At the low temperature of interstellar dust grains, non-thermal chemical, or physical desorption mechanisms can effectively overcome the sublimation barrier.

The first organic molecule considered as “complex” with significant relative abundances in interstellar ices is methanol. Besides much discussion on its formation pathway, the composition of the ice matrix in which methanol is embedded is not always constrained, and it could vary from source to source (e.g. Bottinelli et al. 2010). The major elements constituting the ice matrix phases are water, carbon dioxide, and carbon monoxide. A carbon monoxide matrix is favoured by several authors on the basis of laboratory experiments showing that the formation of methanol is possible via the hydrogenation of carbon monoxide (Krim et al. 2018; Chuang et al. 2018; Fuchs et al. 2009; Watanabe & Kouchi 2002). Some experiments address alternative formation routes or branching ratios leading to other pathways/species (Qasim et al. 2018; Minissale et al. 2016; Chuang et al. 2016). Methanol ice is generally observed in lines of sight with large column densities. However, no clear correlation emerges between the carbon monoxide content of a cloud and methanol formation, with some lines of sight harbouring low column density upper limits, and others with apparently similar conditions with firm detections (Whittet et al. 2011; Bottinelli et al. 2010). Despite what is commonly assumed in current chemical models, there is no spectroscopic consensus that justifies a strict association of carbon monoxide and methanol formation, with several formation pathways being probably at work. In several lines of sight, the spectroscopic profiles of the CO₂ bending mode that are observed show sub-structures associated with the degeneracy breaking of this mode. It has been experimentally and theoretically demonstrated that such a profile can be assigned to the formation of a methanol/carbon dioxide complex and imply a relatively lower water ice content for this ice phase that would inhibit the complex formation, suggesting a segregation of the ices along these lines of sight (e.g. Klotz et al. 2004; Dartois et al. 1999; Ehrenfreund et al. 1999). The occurrence of high-energy cosmic rays penetrating into these dense regions brings into question their impact on the evolution of these ice mantles and their ability to sputter methanol and more complex organic molecules (COM), returning them to the gas phase. As ice mantles projected column densities are dominated by water ice, we recently performed experiments focusing on methanol in a water-ice-dominated matrix in Dartois et al. (2019).

In this article, we experimentally investigate the sputtering by cosmic rays (swift ions in the electronic regime) of methanol when it is embedded in a carbon dioxide ice matrix. In Sect. 2, the experiments are briefly described. Combined infrared and mass spectrometer analyses of the products ejected during swift heavy ion interaction with thin ice films deposited at low temperatures are presented in Sect. 3. A simple model analysis framework is used to draw figures. The comparison of the methanol sputtering in a carbon dioxide ice matrix with water ice matrix is also discussed. We conclude with the astrophysical implications of methanol sputtering rate.

2. Experiments

Swift ion-irradiation experiments were performed at the heavy-ion accelerator Grand Accélérateur National d’Ions Lourds (GANIL, Caen, France). Heavy-ion projectiles were delivered on the IRRSUD beam line¹. The Irradiation de GLaces d’Intérêt AStrophysique (IGLIAS) facility, a vacuum chamber (10⁻⁹ mbar under our experimental conditions) that holds an IR-transmitting substrate that can be cryocooled down to about 10 K, was coupled to the beam line. The ice films were produced by placing the cold window substrate in front of a dosing needle that was connected to the deposition line. Ice films were condensed at 10 K on the window from the vapour phase and were kept at this temperature during the irradiations. Methanol, purchased from Sigma Aldrich, was of spectrophotometric grade. The ¹³C labelled methanol, with a 99% ¹³C purity was purchased from Eurisotop. The carbon dioxide was from AirLiquide with 99.995% purity. They were used as received. Details of the experimental setup are given in Augé et al. (2018). The stopping power of the projectiles (⁵⁸Ni⁹⁺ at 0.57 MeV u⁻¹) in the electronic regime (calculated with the SRIM package, Ziegler et al. 2010) is close to 3.7 keV nm⁻¹ for a pure CO₂ ice film, adopting an ice density of 1.1 g cm⁻³ (Satorre et al. 2008). For pure methanol, the stopping power is slightly higher, about 5.1 keV nm⁻¹ for a pure ice density of 1.013 g cm⁻³ (Maté et al. 2009).

The yield-dependency on the stopping power for pure carbon dioxide ice predicts a total sputtering yield for intact plus radiolysed molecules of 5.4${}_{-2.4}^{+4.3}\times {10}^4$ sputtered CO₂/ion (using the data shown in Rothard et al. 2017). The ion flux, set between 10⁹ and 2 × 10⁹ ions cm⁻² s⁻¹, was monitored online using the current measured on the beam entrance slits that define the aperture. The relation between the current at different slit apertures and the flux was calibrated before the experiments. We used a Faraday cup that was inserted in front of the sample chamber to do this. The deposited ice-film thicknesses allowed the ion beam to pass through the film with an almost constant energy loss per unit path length. A Bruker Fourier Transform InfraRed (FTIR) spectrometer (Vertex 70v) with a spectral resolution of 1 cm⁻¹ was used to monitor the infrared (IR) film transmittance. The evolution of the IR spectra was recorded as a function of the ion fluence. The irradiation was performed at normal incidence, whereas the IR transmittance spectra were recorded simultaneously at 12° of incidence (a correction factor of ≈0.978 was therefore applied to determine the normal column densities). A sweeping device allows for uniform and homogeneous ion irradiation over the target surface. Mass spectrometry measurements were performed simultaneously using a microvision2 mks quadrupole mass-spectrometer (QMS). The QMS signals were noise-filtered to smooth out high-frequency temporal fluctuations using a Lee filter algorithm with a typical box size of five to seven points. The mass-to-charge (m/z) ranges that we scanned were varied for the different experiments in order to optimise the integration time. The m/z channels we used to follow a given species were selected in order to avoid overlap with channels that are dominated either by contamination or to avoid confusion with a byproduct of the radiolysis. Labelled ¹³C-methanol was chosen so that the clean m/z = 33 can be used to follow unambiguously ¹³CH₃OH⁺ with the QMS. Methanol (¹²C) was also used in the first experiments, in order to follow also the ice mixture infrared spectral behaviour expected in space. Carbon dioxide can be monitored using m/z = 44 (${\text{CO}}_2^{+}$). When only selected optimum m/z are used, a correction factor needs to be applied to estimate the abundance of a species so that the mass-fragmentation pattern following electron impact ionisation can be taken into account. The experimental fragmentation pattern within our QMS for a species X was monitored during the injection of the gas mixture, that is, when the ice film was deposited. A self-calibration of the QMS for the mass-fragmentation pattern of species X was then obtained at m/z as follows:

$$\begin{array}{c}\hfill f(X,m/z)=I(m/z){\displaystyle \sum _{m/z=X_{\mathrm{fragments}}}}I(m/z),\end{array}$$(1)

where only the expected m/z from possible (major) fragments (X_fragments) were included. For ¹³CH₃OH, the main m/z are 30(H¹³CO⁺), 31(${\text{H}}_2^{13}{\text{CO}}^{+}$), 32(¹³CH₃O⁺), and 33(¹³CH₃OH⁺). For CO₂, the main m/z are 12(C⁺), 16(O⁺), 22(${\text{CO}}_2^{2+}$), 28(CO⁺), and 44(${\text{CO}}_2^{+}$). Therefore, the sum of the chosen m/z was used and divided by their relative fragmentation-pattern percentage (obtained for this QMS, as explained above) to retrieve the total number of species X. The spectra were also corrected for the total electron-impact ionisation cross section σ^impact(X) at 70 eV (energy of the QMS electron ionisation source) for each molecule: CO₂ (3.521 Ų, NIST database), CH₃OH (4.8 Ų, Vinodkumar et al. 2011), also used for ¹³CH₃OH, and for one main radiolysis product used in the analysis, CO (2.516 Ų, NIST database), to compensate for the higher ionisation efficiency of larger species (which carry more electrons).

The abundance ratios of species X and Y were thus evaluated from

$$\begin{array}{c}\hfill \frac{[X]}{[Y]}=\frac{{\sum }_{m/z=X_{\mathrm{fragments}}}I(m/z)}{{\sum }_{m/z=X_{\mathrm{fragments}}}f(X,m/z)}\frac{{\sum }_{m/z=Y_{\mathrm{fragments}}}f(Y,m/z)}{{\sum }_{m/z=Y_{\mathrm{fragments}}}I(m/z)}\frac{{\sigma }^{\mathrm{impact}}(Y)}{{\sigma }^{\mathrm{impact}}(X)},\end{array}$$(2)

with the fragments chosen so that they have a significant signal-to-noise ratio and do not overlap with other dominant species and/or potential residual gas m/z contribution. In the mixture experiments, in order to establish the relative abundance of species, the QMS signals are presented corrected for the relative mass-fragmentation pattern for a given mass attributed to a dominant species, normalised by its electron-impact ionisation cross section, as discussed above. The signals are then scaled by a global factor. During the experiments, when the ion beam was stopped, we recorded and followed the evolution of the QMS and chamber background signal. The QMS spectra we present are background subtracted. A summary of the ice-film parameters, such as ice mixture and irradiation temperatures, is given in Table 1. A summary of the integrated band strengths from the literature used in the IR spectra analysis to determine ice-film column densities is listed in Table B.1.

3. Results

The outcome of the irradiation experiments performed on methanol or ¹³C-methanol isotopically labelled ices mixed in various proportions with carbon dioxide are reported below. The usefulness of the ¹³C-methanol isotopologue in the yield determination is emphasised. The sputtering yield is analysed with a model including the evolution of the ice composition upon ion irradiation.

3.1. CH₃OH:CO₂ ice mixtures

Mixtures of methanol and carbon dioxide ice with two different proportions CH₃OH/CO₂ (≈0.07 and 0.3) were irradiated. The recorded IR and QMS evolution in function of the ion fluence are shown in Fig. 1. Using the C–O stretching mode of methanol and the CO₂ antisymmetric stretching mode, we report the column density of these molecules in the ice. The abundance ratio of gaseous methanol to carbon dioxide measured with the QMS during the ice deposition (blue diamonds in Fig. 1, lower panels) gives, within uncertainties, the same value as the ratio measured after deposition for the ice film in the IR (and providing therefore an independent measurement of the expected deposited ice-mixture ratio). The abundance ratios of gas- and solid-phase methanol to carbon dioxide evolve in parallel during irradiation. The measured abundance ratio in the gas phase is lower than measured in the bulk of the ice film, showing that the methanol sputtering yield is influenced both by sputtering and by higher radiolytic destruction efficiencies. In the figures, we also provide scaled ion flux variations to show the ion beam variability. Note that the ion beam was fluctuating significantly during the second experiment, and thus the magnitude of the species in the QMS signals. However, the abundance ratios calculated are largely unaffected, only marginal correlation exists with the fluctuations. The mass spectra in Fig. 1 show that carbon dioxide and methanol are desorbed as soon as the ion irradiation begins (middle panels) and decreases (if corrected with beam fluctuations, this is also the case for the second experiment). We note that, at the very beginning of the irradiation, for fluences lower than a few 10¹¹ cm⁻², a slight phase change can occur with the first ions impinging the freshly deposited ice film, involving a restructuring of the ice. During this early phase, a steady state is not reached in the chamber, the ice volume can change by up to about ten percent ; therefore, the sputtering efficiency can be slightly altered. For the m/z = 28 (gas-phase CO) and m/z = 32 (dominated by O₂), the intensities are lower at the beginning of the irradiation, then rise, and finally, decrease. This behaviour is a way to distinguish the direct products from the accumulated bulk ones. The observed gaseous CO arises partly due to direct sputtered CO₂ and methanol radiolytic product (value at the very beginning of the irradiations), and partly from the bulk of the ice at later times (bell-shaped curve behaviour). In addition, as it is more volatile, the desorbing CO can come from deeper layers of the ice film than carbon dioxide and methanol. This is shown by the amount of m/z = 28 that desorbs with respect to methanol, whereas the mean bulk ratio of these ices is lower (upper panel of Fig. 2) than what the QMS observes. The extent of the radiolysis of carbon dioxide and methanol with respect to the sputtering of intact molecules is discussed in a following section.

Fig. 1. Methanol carbon dioxide ice-film mixture experiments. Left: CH3OH/CO2 (6.8%). Right: CH3OH/CO2 (30.5%). Upper panels: ice column-density measurements from IR spectra. The column densities are estimated using the integrated cross sections from Table B.1. The cyan and black dashed lines represent the fits to the destruction cross section of carbon dioxide and methanol, respectively. Middle panels: QMS-normalised signals used to follow the relative abundance of carbon dioxide (m/z = 44), methanol (m/z = 31; m/z = 32 cannot be used, as it is strongly overlapping with dioxygen), and carbon monoxide (m/z = 28, after subtracting the contribution of the mass fragmentation of CO2) during irradiation. The black line is the scaled monitoring signal of the ion flux, showing its stability. The m/z = 28 signal increases at the beginning, because it contains fragments from the radiolytic products of the ice mixture. Lower panels: CH3OH/CO2 abundance ratio deduced from the IR spectra of the ice film in function of fluence (red dots, uncertainty filled in in red). Comparison with the QMS-determined abundance ratio of the same desorbed molecules (dark blue line, from m/z = 31 for CH3OH and m/z = 44 for CO2). The blue and cyan diamond symbol indicates the gas ratio measurement with the QMS during the ice film deposition. The dashed blue and cyan line represents χ = βmeas × (fCH3OH/fCO2)bulk. See text for details.

Fig. 2. Same as Fig. 1 for 13C-methanol carbon dioxide ice-film mixtures. From left to right: CH3OH/CO2 (5.3%, 11.8%, 21.6%). 13C-methanol is followed using the radiolytic and fragmentation pattern free m/z = 33 QMS channel. It should be noted that the maximum fluence for the left panel experiment is only about half that of the others, as the initial methanol fraction is low and no longer detectable in the IR for fluences above about 4 × 1012 cm−2. See text for details.

3.2. ¹³CH₃OH:CO₂ ice mixtures

Mixtures of ¹³C-labelled methanol and carbon dioxide ice with varying proportions (¹³CH₃OH/CO₂ ≈ 0.05, 0.12, 0.22) were irradiated. The recorded IR and QMS evolution in function of the ion fluence are shown in Fig. 2. The benefit of using an isotopic marker is that it provides a clean channel for the QMS measurements at m/z = 33 to directly follow ¹³CH₃OH⁺ with neither disturbance from radiolysis products of methanol nor CO₂. Otherwise, the description is the same as for the non-labelled methanol experiments discussed above, and we confirm the behaviour observed.

3.3. Sputtering yield evaluation

A model of the evolution of column densities for different species with the projectile ion fluence as monitored by infrared spectroscopy for the ice (bulk), and a first-order model to estimate the abundance ratio measured in the gas phase by the QMS, were explained in detail in Dartois et al. (2019). Here, we recall this simple model describing the fraction of intact sputtered molecules χ in the gas phase. For instance, the abundance ratio measurements with the QMS can be expressed as

$$\begin{array}{cc}\hfill \chi ={\left(\frac{{\mathrm{CH}}_3\mathrm{OH}}{{\mathrm{CO}}_2}\right)}^{\mathrm{QMS}}& \hfill \approx \frac{Y_{\mathrm{eff}}-{\sigma }_{{\mathrm{CH}}_3\mathrm{OH}}{N}^{\mathrm{d}}}{Y_{\mathrm{eff}}-{\sigma }_{{\mathrm{CO}}_2}{N}^{\mathrm{d}}}{\left(\frac{f_{{\mathrm{CH}}_3\mathrm{OH}}}{f_{{\mathrm{CO}}_2}}\right)}^{\mathrm{bulk},}\hfill \\ \hfill & \hfill =\beta {\left(\frac{f_{{\mathrm{CH}}_3\mathrm{OH}}}{f_{{\mathrm{CO}}_2}}\right)}^{\mathrm{bulk}}\hfill \end{array}$$(3)

with f_CH3OH and f_CO2 being the fraction of methanol and carbon dioxide molecules in the sputtered ice layers, respectively (i.e. their fraction in the ice composition), whereas σ_CH3OH and σ_CO2 are their destruction cross sections. Adopting a cylindrical geometry for the sputtered volume (e.g. Dartois et al. 2018, Fig. 1), N^d corresponds to the column density of molecules in the ice film at the depth of desorption (i.e. the height of the cylinder). Y_eff is the semi-infinite thickness effective-sputtering total yield for the ice mixture under study. A total yield means that it includes the intact and radiolysed molecules sputtered from the volume considered. Unlike water ice, which is relatively resilient against destruction (e.g. Dartois et al. 2019, and references therein), the carbon dioxide destruction cross section becomes important in the evaluation of the yield, and it cannot be neglected. To this simple model, we add an additional radiolysis efficiency factor, called η_CO2, for carbon dioxide. In an ice matrix dominated by CO₂, the absolute intact carbon dioxide sputtering yield is therefore

$$\begin{array}{c}\hfill Y_{\mathrm{abs}}^{{\mathrm{CO}}_2}=Y_{\mathrm{eff}}^{{\mathrm{CO}}_2}{\eta }_{{\mathrm{CO}}_2,}\end{array}$$(4)

η_CO2 being the intact fraction of sputtered carbon dioxide molecules when compared to the sum of the intact and products species sputtered. The by far dominant radiolytic product of CO₂ is CO (see Figs. A.1 and A.2), therefore

$$\begin{array}{c}\hfill {\eta }_{{\mathrm{CO}}_2}\approx {\left(\frac{{\mathrm{CO}}_2}{{\mathrm{CO}}_2+\mathrm{CO}}\right)}^{\mathrm{QMS}}.\end{array}$$(5)

The CO value is evaluated at early stages of the ion irradiation, when CO has not yet accumulated in the ice. The η value is, threrefore, probably underestimated, and more intact CO₂ molecules are sublimating, since CO is more volatile, and some CO contribution can come from deeper layers than CO₂. This intact fraction can be related to the processes occurring in the solid phase by

$$\begin{array}{c}\hfill {\left(\frac{{\mathrm{CO}}_2}{{\mathrm{CO}}_2+\mathrm{CO}}\right)}^{\mathrm{QMS}}\approx {\left(\frac{Y_{\mathrm{eff}}^{{\mathrm{CO}}_2}-{\sigma }_{{\mathrm{CO}}_2}{N}^{\mathrm{d}}}{Y_{\mathrm{eff}}^{{\mathrm{CO}}_2}}\right)}^{\mathrm{bulk},}\end{array}$$(6)

where the superscript “bulk” indicates that these values are measured with the evolution of the ice film in the infrared. We experimentally measured σ_CO2. $Y_{\mathrm{eff}}^{{\mathrm{CO}}_2}$ can be evaluated from infrared measurements. Then we can infer

$$\begin{array}{c}\hfill N^{\mathrm{d}}\approx \frac{Y_{\mathrm{eff}}^{{\mathrm{CO}}_2}(1-{\eta }_{{\mathrm{CO}}_2})}{{\sigma }_{{\mathrm{CO}}_2}}·\end{array}$$(7)

The radiolytic destruction cross sections ${\sigma }_{{\mathrm{CO}}_2}^{\mathrm{destruction}}$ of carbon dioxide, and ${\sigma }_{{\mathrm{CH}}_3\mathrm{OH}}^{\mathrm{destruction}}$ of methanol, are obtained in each experiment by fitting the column density evolution as in function of fluence using IR measurements (the films used in this study are thick enough for the sputtering not to contribute significantly to the IR evolution). The fitted cross-section curves are shown overlaid as dashed lines in the upper-right panels of Figs. 1 and 2. In the case of a carbon dioxide dominated ice mantle, the effective sputtering yield should be close to the yield of carbon dioxide ice. The sputtering yield with heavy ions in the MeV/u range is calculated from previous measurements (Ni, Xe, Ti; Seperuelo Duarte et al. 2009; Mejía et al. 2015; Rothard et al. 2017). Assuming a quadratic dependency with the electronic stopping power, we calculate that $Y_{\mathrm{eff}}^{{\mathrm{CO}}_2}=5.4_{-2.4}^{+4.3}\times {10}^4$ CO₂/ion.

An approximate value for N_d can therefore be estimated from Eq. (7), and used in Eq. (3) to infer the value of the proportionality factor β_calc. This factor is reported in Table 2. The calculated values of β_calc are in fair agreement, taking into account the large uncertainties, with the value measured in our experiments. The χ = β_meas × (f_CH3OH/f_CO2)^bulk values are shown as dashed blue and cyan lines in the lower panels of Figs. 1 and 2. We used an isotopic marker for methanol in some ice mixtures, providing a clean channel (m/z = 33) for its QMS detection and measurements, ensuring a safer β_meas determination. The ice mixtures explored, with ¹²C or ¹³C isotopically enriched methanol, give consistent results for the sputtering yields, as shown in Table 2.

4. Discussion

4.1. Absolute sputtering rate

According to the beta value we determined, about one third of the methanol-to-carbon-dioxide ratio observed in our experiments in the ice (bulk) is ejected by sputtering. To obtain the absolute methanol sputtering yield, this value must be multiplied by η_CO2, the ejected intact CO₂ fraction, which is of the order of sixty percent. As a consequence, overall, when methanol is within a CO₂ rich ice mixture, about twenty percent of the methanol present in the ice is desorbed intact. Assuming that the fraction of dissociated molecules does not depend strongly on the stopping power, we calculated the astrophysical sputtering rate in the same way as discussed in Dartois et al. (2015), with an effective total yield proportional to the square of the electronic stopping power. We obtain a rate of 83.8 cm⁻² s⁻¹ for a pure CO₂ ice for a reference cosmic-ray-ionisation rate of ζ = 10⁻¹⁶ s⁻¹, therefore, the absolute methanol rate is about ≈16.8 × (f_CH3OH/f_CO2)^bulk cm⁻² s⁻¹(ζ/10⁻¹⁶ s⁻¹). For the pure water ice case, the equivalent sputtering rate would be 13.9 cm⁻² s⁻¹ for a reference ζ = 10⁻¹⁶ s⁻¹. According to Eqs. (6) and (7) in Dartois et al. (2019), we expect between about 30% and 40% of the methanol-to-water fraction to be desorbed by the sputtering process in the electronic regime. The fraction radiolysed in a methanol-CO₂ mixture is slightly higher, as can be seen from the values of the β_meas in Table 2, but the higher sputtering efficiency of the CO₂ matrix by a factor of about six largely dominates the net sputtering rate.

The relation between the expected secondary VUV photon flux and the cosmic-ray ionisation rate, under dense-cloud conditions where ices predominate and when the external UV field is fully attenuated, is discussed in, e.g., Shen et al. (2004), Prasad & Tarafdar (1983). Based on these works, we adopt a value of 7940 VUV photons.cm⁻² s⁻¹ for ζ = 10⁻¹⁶ s⁻¹. To match the cosmic-ray electronic sputtering efficiency, the absolute photodesorption yield should be of the order of ≈ 2.1 × 10⁻³ (f_CH3OH/f_CO2)^bulk/VUV photon for methanol/carbon dioxide mixture in the range explored in this study. Photodesorption measurements for methanol in the literature include pure methanol-ice desorption. Bertin et al. (2016), and Cruz-Diaz et al. (2016), deduced rates of the order of 10⁻⁵/VUV photon for the former, and an upper limit of about < 3 × 10⁻⁵/VUV photon for the latter. Bertin et al. (2016) also studied a methanol/CO mixture, and the rate they inferred falls to less than 10⁻⁶ molecules/photon, meaning lower than pure methanol, whereas pure CO photodesorbs more efficiently. One cannot infer directly from these measurements what would be the case for a methanol/CO₂ mixture (as explored in this article) but the lowering of the effective methanol-photodesorption rate for the methanol/CO mixture probably betrays the fact that mixtures open new photodissociation/recombination routes. Methanol/CO₂ mixtures may thus also give a lower photodesorption rate value than pure methanol.

4.2. Methanol position-FWHM diagram

Methanol was investigated with Spitzer space telescope observations in dense regions (Bottinelli et al. 2010). Botinelli and colleagues showed in their Fig. 12 that most of the observed methanol C–O stretching-mode position versus full width at half-maximum (FWHM) is in a mantle whose composition is dominated neither by a H₂O-rich nor by a CO-rich matrix. The methanol C–O stretching-mode position versus FWHM diagram for our experiments with ¹²C-methanol/carbon dioxide mixtures are reported in Fig. 3, for the first infrared measurements during irradiation. As the irradiation proceeds, the positions shift progressively towards higher FWHM and redder centroid positions. At this stage, the ice mixtures are heavily irradiated and no longer defined by the initial methanol/carbon dioxide mixtures. The ice mixtures used initially, and their evolution upon moderate irradiation fluences, are also compatible spectroscopically with the observed astronomical positions and FWHM values.

Fig. 3. Methanol C–O stretching-mode position FWHM diagram of the experiments performed in this work, compared to ISM measurements presented in Bottinelli et al. (2010; yellow squares). See text for details.

5. Conclusions

We measured the sputtering yield of methanol embedded in a carbon dioxide ice matrix at 10 K irradiated by swift heavy ions in the electronic regime of energy deposition, simulating experimentally interstellar cosmic rays. We conclude that a large fraction of intact molecules are desorbed by cosmic rays with a sputtering yield close to that of the carbon dioxide ice matrix. A significant fraction of the sputtered CO₂ is also radiolytically processed, leading to the ejection of fragments (dominated by CO). The fraction of carbon dioxide molecule sputtered as products must be taken into account to obtain the absolute sputtering yield. The methanol-to-carbon-dioxide ratio observed in the gas phase by mass spectrometry is proportional to the bulk ice mantle composition, as measured with infrared spectroscopy. The proportionality factor is about one third in the experiments presented here, which reflects the fact that the destruction cross section of methanol is higher than that of the carbon dioxide one when exposed to cosmic rays. The overall efficiency of the initial methanol present in the ice mantle sputtered intact is about twenty percent. The higher efficiency of sputtering of the carbon dioxide ice matrix as compared to more bounded systems, such as a water-ice-dominated ice mantle, imply a very efficient release of complex organic molecules such as methanol.

Acknowledgments

This work was supported by the Programme National “Physique et Chimie du Milieu Interstellaire” (PCMI) of CNRS/INSU with INC/INP co-funded by CEA and CNES, by the P2IO LabEx program: “Evolution de la matière du milieu interstellaire aux exoplanètes avec le JWST” and the ANR IGLIAS, grant ANR-13-BS05-0004 of the French Agence Nationale de la Recherche. Experiments performed at GANIL. We thank T. Madi, T. Been, J.-M. Ramillon, F. Ropars and P. Voivenel for their invaluable technical assistance. We would like to acknowledge the anonymous referee for constructive comments that significantly improved the content of our article.

References

Appendix A: Irradiation at start

The average of the mass spectra recorded during the methanol and carbon dioxide mixture injection are shown in Fig. A.1, as well as QMS spectra recorded at the beginning of the ion irradiation. These diagrams show that the main radiolysis product of CO₂ is, as expected, CO (m/z = 28). The m/z = 32 QMS channel cannot be used to follow methanol, as it is also populated by dioxygen arising from the radiolysis of carbon dioxide. Instead, m/z = 31 is a good alternative before the use of labelled ¹³C-methanol to unambiguously separate these species in the QMS detection. The average of the mass spectra recorded during the labelled ¹³C-methanol and carbon dioxide mixture injection are shown in Fig. A.2, as well as QMS spectra recorded at the beginning of the ion irradiation. These diagrams show that the main radiolysis product of CO₂ is, as expected, CO (m/z = 28), and the main radiolysis product of ¹³C-methanol is ¹³CO (m/z = 29).

Fig. A.1. Average of mass spectra recorded during the 12C-methanol and carbon dioxide mixture injection to form the ice film (orange histogram), tracing the QMS mass-fragmentation patterns of pure methanol and carbon dioxide; average of a few spectra that were recorded at the beginning of the ice-film irradiation (blue histogram). Both mass spectra are scaled to m/z = 44 (${\text{CO}}_2^{+}$) for comparison. From top to bottom: mixtures with increasing proportions of methanol in relation to carbon dioxide. See text for details.

Fig. A.2. Average of mass spectra recorded during the labelled 13C-methanol and carbon dioxide mixture injection to form the ice film (orange histogram), tracing the QMS mass-fragmentation patterns of pure methanol and carbon dioxide; average of a few spectra that were recorded at the beginning of the ice-film irradiation (blue histogram). Both mass spectra are scaled to m/z = 44 (${\text{CO}}_2^{+}$) for comparison. From top to bottom: mixtures with increasing proportions of 13C-methanol in relation to carbon dioxide. See text for details.

Appendix B: Infrared spectra and band strengths

The infrared spectra, baseline corrected, used to evaluate the column densities of the bulk ice mixture evolution during irradiation are shown. Band strengths used in the analysis are given in Table B.1.

Fig. B.1. Methanol (13C-isotopologue) carbon dioxide ice-film mixtures experiment W1. Evolution of the infrared spectra as a function of fluence. The panel is split into two wavenumber ranges to maximise the optical depth scale.

Fig. B.2. Methanol (13C-isotopologue) carbon dioxide ice-film mixtures experiment W1 BIS. Evolution of the infrared spectra as a function of fluence. The panel is split into two wavenumber ranges to maximise the optical depth scale.

Fig. B.3. Methanol (13C-isotopologue) carbon dioxide ice-film mixtures experiment W1 TER. Evolution of the infrared spectra as a function of fluence. The panel is split into two wavenumber ranges to maximise the optical depth scale.

Fig. B.4. Methanol (12C-isotopologue) carbon dioxide ice-film mixtures experiment W2. Evolution of the infrared spectra as a function of fluence. The panel is split into two wavenumber ranges to maximise the optical depth scale.

Fig. B.5. Methanol (12C-isotopologue) carbon dioxide ice-film mixtures experiment W3. Evolution of the infrared spectra as a function of fluence. The panel is split into two wavenumber ranges to maximise the optical depth scale.

All Tables

All Figures

Fig. 1. Methanol carbon dioxide ice-film mixture experiments. Left: CH3OH/CO2 (6.8%). Right: CH3OH/CO2 (30.5%). Upper panels: ice column-density measurements from IR spectra. The column densities are estimated using the integrated cross sections from Table B.1. The cyan and black dashed lines represent the fits to the destruction cross section of carbon dioxide and methanol, respectively. Middle panels: QMS-normalised signals used to follow the relative abundance of carbon dioxide (m/z = 44), methanol (m/z = 31; m/z = 32 cannot be used, as it is strongly overlapping with dioxygen), and carbon monoxide (m/z = 28, after subtracting the contribution of the mass fragmentation of CO2) during irradiation. The black line is the scaled monitoring signal of the ion flux, showing its stability. The m/z = 28 signal increases at the beginning, because it contains fragments from the radiolytic products of the ice mixture. Lower panels: CH3OH/CO2 abundance ratio deduced from the IR spectra of the ice film in function of fluence (red dots, uncertainty filled in in red). Comparison with the QMS-determined abundance ratio of the same desorbed molecules (dark blue line, from m/z = 31 for CH3OH and m/z = 44 for CO2). The blue and cyan diamond symbol indicates the gas ratio measurement with the QMS during the ice film deposition. The dashed blue and cyan line represents χ = βmeas × (fCH3OH/fCO2)bulk. See text for details.

In the text

Fig. 2. Same as Fig. 1 for 13C-methanol carbon dioxide ice-film mixtures. From left to right: CH3OH/CO2 (5.3%, 11.8%, 21.6%). 13C-methanol is followed using the radiolytic and fragmentation pattern free m/z = 33 QMS channel. It should be noted that the maximum fluence for the left panel experiment is only about half that of the others, as the initial methanol fraction is low and no longer detectable in the IR for fluences above about 4 × 1012 cm−2. See text for details.

In the text

	Fig. 3. Methanol C–O stretching-mode position FWHM diagram of the experiments performed in this work, compared to ISM measurements presented in Bottinelli et al. (2010; yellow squares). See text for details.
In the text

Fig. A.1. Average of mass spectra recorded during the 12C-methanol and carbon dioxide mixture injection to form the ice film (orange histogram), tracing the QMS mass-fragmentation patterns of pure methanol and carbon dioxide; average of a few spectra that were recorded at the beginning of the ice-film irradiation (blue histogram). Both mass spectra are scaled to m/z = 44 (${\text{CO}}_2^{+}$) for comparison. From top to bottom: mixtures with increasing proportions of methanol in relation to carbon dioxide. See text for details.

In the text

Fig. A.2. Average of mass spectra recorded during the labelled 13C-methanol and carbon dioxide mixture injection to form the ice film (orange histogram), tracing the QMS mass-fragmentation patterns of pure methanol and carbon dioxide; average of a few spectra that were recorded at the beginning of the ice-film irradiation (blue histogram). Both mass spectra are scaled to m/z = 44 (${\text{CO}}_2^{+}$) for comparison. From top to bottom: mixtures with increasing proportions of 13C-methanol in relation to carbon dioxide. See text for details.

In the text

	Fig. B.1. Methanol (13C-isotopologue) carbon dioxide ice-film mixtures experiment W1. Evolution of the infrared spectra as a function of fluence. The panel is split into two wavenumber ranges to maximise the optical depth scale.
In the text

	Fig. B.2. Methanol (13C-isotopologue) carbon dioxide ice-film mixtures experiment W1 BIS. Evolution of the infrared spectra as a function of fluence. The panel is split into two wavenumber ranges to maximise the optical depth scale.
In the text

	Fig. B.3. Methanol (13C-isotopologue) carbon dioxide ice-film mixtures experiment W1 TER. Evolution of the infrared spectra as a function of fluence. The panel is split into two wavenumber ranges to maximise the optical depth scale.
In the text

	Fig. B.4. Methanol (12C-isotopologue) carbon dioxide ice-film mixtures experiment W2. Evolution of the infrared spectra as a function of fluence. The panel is split into two wavenumber ranges to maximise the optical depth scale.
In the text

	Fig. B.5. Methanol (12C-isotopologue) carbon dioxide ice-film mixtures experiment W3. Evolution of the infrared spectra as a function of fluence. The panel is split into two wavenumber ranges to maximise the optical depth scale.
In the text