Chemical and physical effects induced by heavy cosmic ray analogues on frozen methanol and water ice mixtures

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1 doi: /mnras/stu1208 Chemical and physical effects induced by heavy cosmic ray analogues on frozen methanol and water ice mixtures A. L. F. de Barros, 1 E. F. da Silveira, 2 H. Rothard, 3 T. Langlinay 3 and P. Boduch 3 1 Departamento de Física, Centro Federal de Educação Tecnológica - CEFET-RJ, Av. Maracanã 229, Rio de Janeiro, RJ, Brazil 2 Departamento de Física, Pontifícia Universidade Católica do Rio de Janeiro, Rua Marquês de São Vicente 225, Rio de Janeiro, RJ, Brazil. 3 Centre de Recherche sur les Ions, les Matériaux et la Photonique (CEA/CNRS/ ENSICAEN/Université de Caen-Basse Normandie), CIMAP-CIRIL-Ganil, Boulevard Henri Becquerel, BP 5133, F Caen Cedex 05, France Accepted 2014 June 17. Received 2014 June 15; in original form 2014 April 19 1 INTRODUCTION Frozen methanol water is a particularly important astrophysical ice mixture. Methanol has been observed in the coma of comets as gasphase species, probably produced by direct sublimation from the nucleus. Its abundance has been estimated to be around a few per cent with respect to the dominant water ice (Mumma et al. 1993)and varies between comets, some of which appear to be CH 3 OH rich. Methanol, the simplest organic alcohol, is an important precursor of more complex pre-biotic species, and its molecules are expected to occur in several astrophysical environments. The formation of many interstellar molecules is reasonably well explained by gas-phase reactions; for methanol, however, as gas-phase reactions cannot justify abarros@pq.cnpq.br ABSTRACT The chemical and physical effects induced by fast heavy-ion irradiation on a frozen mixture of methanol (CH 3 OH) and water (H 2 O) at 15 K are analysed. The laboratory experiment described here simulates the energy transfer processes that occur when cosmic rays bombard this particular ice mixture and helps to elucidate the understanding of the radiolysis of ices occurring in interstellar medium grains, at the surfaces of comets, and on icy Solar system bodies. Infrared spectroscopy (FTIR) was used before and during irradiation with a 40 MeV 58 Ni 11+ ion beam to determine the variation of the main absorption bands of methanol, water and products. In particular, the radiolysis of CH 3 OH:H 2 O (1:1) mixture leads to the formation of H 2 CO, CH 4,CO,CO 2, HCO and HCOOCH 3. Their formation and dissociation cross-sections are determined. H 2 CO, CH 4 and HCOOCH 3 molecules have relatively high destruction crosssections of around cm 2. Furthermore, atomic carbon, oxygen and hydrogen budgets are determined and used to verify the stoichiometry of the most abundant molecular species formed. Temperature effects are compared with irradiation effects, and the spectra of samples warmed-up to different temperatures are compared with those of the irradiated CH 3 OH:H 2 O mixtures. As an astrophysical application, the CH 3 OH:H 2 O dissociation cross-sections due to other ion beam projectiles and energies are predicted assuming validity of the S e 3/2 power law; calculation of the integrated dissociation rates confirms the importance of nickel and some other heavy-ion constituents of cosmic rays in astrochemistry. Key words: astrochemistry methods: laboratory circumstellar matter. the relatively high abundances detected, solid-phase reactions must be invoked (Willacy et al. 1998). The origin of methanol in interstellar objects is still the subject of controversy (Watanabe, Shiraki & Kouchi 2003; Garrod et al. 2006; Fuchs et al. 2009). Further, Dartois et al. (1999) and Lacy et al. (1998) showed that CH 3 OH is very abundant in the line of sight towards some massive protostars. Many laboratory spectra of methanol, as a single material or in a mixture with other important astrophysical species, have been studied and compared with observational spectra; see, for instance, Tielens & Allamandola (1987), Grima etal. (1991), Schutte, Tielens & Sandford (1991), Allamandola, Sandford & Valero (1988), Allamandola et al. (1992), Sandford & Allamandola (1993) and Palumbo, Castorina & Strazzulla (1999). Water ice is the most abundant molecule in grain mantles as well as in the astrophysical ices present in clouds; it has been observed in both the gas phase and in the solid phase covering dust grains. Observations have revealed that solid H 2 OandCO C 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society

2 2734 A. L. F. de Barros et al. Table 1. Spectroscopic properties of CH 3 OH and H 2 O virgin ices: position of the peak (cm 1 ), FWHM (cm 1 ) and wavelength (λ). Assignments and A-values ( cm molecule 1 ) were obtained from the literature. Vibration Position FWHM λ (µm) Mode A-values Ref CH 3 OH O H stretching ν [1,2] C H stretching (asym) ν 2,ν [1,2] CH 2 stretching (asym) [3] CH 2 asymmetric stretching ν [3] Combination /Overtone ν 4 + ν 5 / [3] ν 4 + ν 10 CH stretching ν 4 + ν 5 /ν 4 [2] C H stretching (sym) ν [1,2] CH asymmetric ν 4 [3] C H def & O-H bending ν 4,ν 5, ν [1,2,4] CH 2 (rocking + scissoring) ν [3,4] OH bending ν 6 [3,4] CH 2,CH 3 rocking ν 7, ν [1,5] (CH 2, CO) scissoring ν [3] C O stretching ν 2, ν [1,5,6] O H stretching ν 1 20 [1] H O H bending ν [1] OH-libration ν L 2.6 [2,3] References: [1] de Barros et al. (2011a), [2] Palumbo et al. (1999), [3] Bennett et al. (1997), [4] Merlin et al. (2012), [5] Sandford & Allamandola (1993), [6] Gibb et al. (2004). All the A-values given by Bennett et al. (1997) were predicted using models. Located at the same place as the water band. For the different ice mixtures, the symmetric CH stretching band shows a shoulder around 2858 cm 1, which is not present in the spectrum of pure methanol. Bands at 1371, 1356, 1162, 1144 and 1120 cm 1 (not displayed in this table) were also observed in the spectra analysed here and are due to precursor species. Table 2. Peak positions (cm 1 ) for the intense bands corresponding to the products formed by the CH 3 OH + H 2 O mixture under heavy-ion irradiation. The last column contains the A-values (10 17 cm molecule 1 ) used in the calculations in this study. Molecule Position FWHM λ Mode A-values (cm 1 ) (cm 1 ) (µm) H 2 CO ν a CH ν b,c CO ν d CO ν d,e ν d,e HCO ν f ν f HCOOCH ν f,g,h References: a Öberg et al. (2009), b de Barros et al. (2011b), c Mejía et al. (2013), d Gerakines et al. (1995), e Jamieson, Mebel & Kaiser (2006), f Bennett et al. (1997) g Gerakines et al. (1996), h Modica & Palumbo (2010). are the most abundant ice species in the interstellar medium (ISM) and are observed towards numerous protostars as well as towards background sources (e.g. Chiar et al. 1998). In the solid phase, water is the dominant species in many astrophysical environments, and its abundance can be of the order of 10 4 relative to H 2 gas (the most abundant molecule in molecular clouds; Öberg, van Dishoeck & Linnartz 2008). The abundance of condensed water may be due to H 2 O Figure 1. Comparison of the infrared spectra of the methanol + water ice mixture at 15 K before (solid line) and after (dotted line) irradiation with a 40 MeV 58 Ni 11+ ion beam at a fluence of ions cm 2. A detailed description of this figure is given in Section 3. processes other than direct condensation of the gaseous phase. The results of laboratory experiments showed that H 2 O can be formed on cold surfaces of silicate grains by hydrogenation of O, O 2 or O 3 (Ioppolo et al. 2008). In the gas phase, the formation of H 2 O

3 Heavy cosmic ray analogues on methanol and water ice mixtures 2735 Figure 2. Dependence of column density on beam fluence for eight CH 3 OH bands (open symbols) and three H 2 O bands (closed symbols). The solid lines are included to facilitate interpretation of the graph, and the H 2 O data are multiplied by 1.5 for clarity. The bands chosen are those for which A-values are available in the literature (see Table 1). is only effective above 300 K (Charnley & Latter 1997); hence, its abundance in cold clouds of ISM is reduced to approximately 10 7 relative to H 2 (Bergin, Langer & Goldsmith 1995). Water can be found in low abundance in regions where UV radiation fields are intense, suggesting that photodesorption may compete with thermal desorption in the sputtering of ice surfaces (Melnick &Bergin2005). The partition of water in the gas and solid phases is of extreme importance since it has a great impact on the chemical reactions responsible for the formation of more complex molecules (van Broekhuizen et al. 2006). Recent Infrared Space Observatory (ISO) observations indicate that H 2 OandCH 3 OH ices dominate the grain surface close to massive protostars, where volatile ices have already sublimed (Gerakines, Moore & Hudson 1999; Boogert 1999). The effects induced by UV irradiation of a mixture of CH 3 OH and H 2 Oand irradiation of pure CH 3 OH were studied by Allamandola et al. (1988), Gerakines, Schutte & Ehrenfreund (1996). The results of spectral identification of new species synthesized after ion irradiation of CH 3 OH and CH 3 OH:H 2 O ice targets have already been presented by Baratta et al. (1994), Hudson & Moore (1995), Strazzulla, Castorina & Palumbo (1995), Moore, Ferrante & Nuth (1996), Palumbo et al. (1999). de Barros et al. (2011a) studied the radiolysis of 15 K pure methanol ice by the following heavyion beams: 16 MeV 16 O 5+, 220 MeV 16 O 7+, 606 MeV 70 Zn 26+ and 774 MeV 86 Kr 31+. We compare their results with our findings using a40mev 58 Ni 11+ beam. The newly formed molecules identified by infrared spectroscopy of the methanol water mixture ice are formaldehyde (H 2 CO), methane (CH 4 ), carbon monoxide (CO), carbon dioxide (CO 2 ), methyl formate (CH 3 OCHO) and the formyl radical (HCO); these are the same molecular species found in the pure methanol ice experiment performed by de Barros et al. (2011a). It is worth mentioning that the O 2 molecule band was observed for the first time in radiolysis of CH 3 OH:H 2 O ice and is attributed to the high electronic stopping power of the heavy-ion components of cosmic rays. In this paper, we investigate irradiation of CH 3 OH:H 2 O ice at 15 K by 40 MeV 58 Ni 11+ ions. The effects of annealing and ion irradiation on the physical and spectral properties of the mixture Figure 3. Experimental infrared spectra obtained for the ice mixture at 15 K before (solid line) and after irradiation for two fluences: and ions cm 2 (dashed and dotted lines, respectively). Two spectral regions are shown: (a) cm 1 and b) cm 1. are also studied. In Section 2, the experimental set-up is described briefly. The results are shown in Section 3, where the formation of simple and complex new species is discussed. In Section 4, the dependences of the column densities of the precursor and newly formed molecular species on ion beam fluence are analysed, and the destruction cross-sections of CH 3 OH:H 2 Oandformation cross-sections of the daughter species are determined. The issue of the carbon, oxygen and hydrogen budgets is addressed in Section 5, where we determine whether the variation in column density of the destroyed precursors can account for the increase in column density of their products. In Section 6, the effects of the CH 3 OH:H 2 O concentration ratio and of annealing on the FTIR spectrum is analysed for virgin ices. Astrophysical implications are discussed in Section 7. The results of this and previous works are combined in an attempt to develop a formal description of the interaction between cosmic rays and ice over the extremely large energy range of cosmic rays. Section 8 contains the summary and conclusions.

4 2736 A. L. F. de Barros et al. Table 3. Peak position of small peaks due to new products formed by the CH 3 OH:H 2 O mixture under heavy-ion irradiation seen particularly at fluences above ions cm 2. The last column shows the A- values (in cm molecule 1 ) existing in the literature. The evolution of the absorbance of the bands is shown in Figs 5 and 6. Molecule Position λ Mode A-value Ref. (cm 1 ) (µm) CH 2 OH ν [1,2] CH ν 2 + ν [3] 13 CO ν [5,7] CO ν 1 +ν [5,6] (ν 2 +ν 3 ) [5] 13 CO ν [5] C 3 O ν 1 19 [6,7] H 2 CO ν [2,5] ν [8] HOCH 2 CHO ν [9] HOCH 2 CH 2 OH ν [8] ν [8] ν [8] ν [8] ν [8] O [10] [10] [1] Gerakines et al. (1996), [2] de Barros et al. (2011a), [3] Mejía et al. (2013), [4] Yamada & Person (1964), [5] Gerakines et al. (1995), [6] Moazzen-Ahmadi & Zerbetto (1995), [7] Jamieson et al. (2006), [8] Bennett et al. (1997), [9] Bennett & Kaiser (1997), [10] Vandenbussche et al. (1999). Figure 4. Column density evolution N(F) for (top) the CH 3 OH (1129 cm 1 ) and H 2 O (1655 cm 1 ) precursor molecules and for (bottom) the most abundant daughter species. Dashed curves are predictions from equations (4) (6): the extracted cross-sections (Section 4) are shown in Table 5 for daughters molecules (H 2 CO, CO, HCO, CH 4,CO 2 and HCOOCH 3 ). 2 EXPERIMENTAL The experiments were performed using 40 MeV 58 Ni 11+ ions (i.e. v MeV/u ions projectiles) in the IRRSUD energy beam-line at the GANIL (Grand Accélérateur National d Ions Lourds) heavy-ion accelerator in Caen, France. A detailed description of the experi- Figure 5. Experimental infrared spectra of the irradiated mixture at 15 K, for different fluences. Note the peaks of new species that appear at higher fluences ( to ) in the regions: (a) cm 1, (b) cm 1 and (c) cm 1. mental apparatus can be found in Seperuelo Duarte et al. (2009) and de Barros et al. (2011a). The target, a CH 3 OH:H 2 O (1:1) mixture deposited on a CsI substrate, was placed at the centre of a high vacuum chamber (10 8 mbar) and the substrate was kept at 15 K by a closed cycle helium cryostat during the experiment. The pure

5 Heavy cosmic ray analogues on methanol and water ice mixtures 2737 Figure 6. Segments of the infrared spectra relative to the irradiated ice mixture at 15 K for high fluencies (i.e to ). The IR peaks observed near 1550 (a) and 2139 (b) cm 1 are attributed to O 2. methanol sample (Aldrich, 99.9 per cent) was purified with several freeze thaw cycles to remove any volatile gas; the same procedure was used for the pure water which was of similar purity. The gas inlet tubes were also cleaned previously. The beam impinged perpendicularly on the sample at a constant beam flux of ions cm 2 s. The final fluence was up to ions cm 2. A sweeping device ensured homogeneous irradiation of the sample surface. The ice modifications were probed by a Nicolet Magna 550 Fourier Transform Infrared Spectrometer operating in transmission mode at right angles to the ice sample to obtain the infrared transmission spectra. FTIR spectrum analysis was performed using the Lambert Beer equation (Gerakines et al. 1996). The ice thickness was determined based on densities of g cm 3 for methanol (Bennett et al. 1997) and 0.94 g cm 3 for water (Hama & Watanabe 2013). From peak area measurements performed before irradiation and using the appropriate A-values reported in Table 1, the initial ice thickness of each constituent was determined: 2.93 µmforch 3 OH and 1.79 µm forh 2 O. These values should be compared to the range of 40 MeV 58 Ni projectiles in the 1:1 methanol water homogeneous mixture. Calculation for this projectile ice system, using the SRIM code (Ziegler, Ziegler & Biersack 2010), yields the stopping power of kev/µm (i.e ev/10 15 atoms cm 2 ) and a range equals to 28 µm; as a consequence, the beam should traverse completely the ice target and no implantation effects are expected. The residual pressure of 10 8 mbar is constituted basically by atmospheric gases which condense (particularly H 2 O) on the target surface during the irradiation. If, on one hand, sputtering occurs continuously in this thin layer, on the other hand it protects the methanol water mixture from the same process. Experiments performed with MeV ion bombardment on CH 4 ice in the same experimental set-up have shown (references in Table 4.5 of Mejía et al. 2013a) that 10 water monolayers per hour is a typical target grow rate due to residual gas condensation. After 10 h (time interval of the irradiation), the water thickness increased approximately by 1 per cent (i.e molecules cm µm). Based on this result, the corresponding corrections were neglected in our cross-section analysis. The absorbed dose per molecule at the end of irradiation (F = ions cm 2 ) is estimated to be 88 ev molecule 1. Segments of typical FTIR spectra before and after irradiation obtained from our mixture sample (Fig. 1) are in good agreement with previous studies (Palumbo et al. 1999) using 3 kev helium ions. 3 LABORATORY RESULTS Fig. 2 shows the different absorption bands due to molecular vibrations of the methanol mixture at 15 K. Peak position, vibration mode and integrated absorbance A of the main methanol features are reported in Table 1. In the current work, the formation of new species is studied; the profiles (shape, width and peak position) of the different methanol and water absorption bands before and after irradiation are examined. 3.1 CH 3 OH and H 2 O ice analysis IR bands Fourteen CH 3 OH and three H 2 O optical absorptions bands, listed in Table 1, are identified. Methanol are observed in the infrared spectrum in particular by the transitions 1026, 1053, 1129, 1426, 1446, Table 4. Spectroscopic information for precursors: peak positions, A-values and final column density, N 1,. The destruction (σ d ) and formation (σ f ) cross-sections (with 5 per cent error bar) were obtained by fitting the precursor CH 3 OH and H 2 O data with equation (2). The destruction and formation G-values are defined as 100 σ d,f /S e,wheres e = ev/10 15 atoms cm 2 for the ice mixture. Precursors Position A-values N 1, σ d σ f σ f /σ d G d G f (cm 1 ) (cm molecule 1 ) (molecules cm 2 ) (cm 2 ) (cm 2 ) ratio (molecules/100 ev) (molecules/100 ev) CH 3 OH H 2 O de Barros et al. (2011a); Sandford & Allamandola (1993)

6 2738 A. L. F. de Barros et al. shownintable1. For clarity, in Fig. 2, the water data are multiplied by a factor of 1.5. Figure 7. Evolution of column density (N(F)) forthe most abundantdaughter species produced during irradiation of CH 3 OH:H 2 O. Dashed lines are fittings based on equations (4) (6) for the evolution of the cross-sections. Figure 8. Normalized CH 3 OH band area as a function of dose for a pure CH 3 OH sample irradiated with 606 MeV 70 Zn 26+ (de Barros et al. 2011a) ions and for CH 3 OH:H 2 O mixture (this study). 1460, 1479, 2827, 2858, 2905, 2920, 2937, 2961 and 3250 cm 1 (Table 1); five other bands (1371, 1356, 1162, 1144 and 1120 cm 1 ) are observed in the current spectra behaving like precursor species. To determine the column densities shown in Fig. 2, six of the most important methanol bands were chosen because they were also observed in a previous study (de Barros et al. 2011a) and their A-values are well known in the literature. For the precursor H 2 O peak, three peaks are observed in the spectrum, the band at 1655 cm 1 associated with the H O H bending mode of water molecules; the strongly modified profile of the 3250 cm 1 band due to the O H stretching and a stronger water peak at 765 cm 1 due to the ν L mode. Fig. 2 shows the dependence of the column density (molecules cm 2 ) of the eight most intense methanol bands and the three water bands as a function of fluence. The absorbance of the observed bands were calculated from the integrated intensities 3.2 Formation of new species New molecular species are produced in the ice mixture as a result of recombination or fragmentation of radicals or molecular fragments formed by radiolysis. Figs 3 and 4 shows the main IR bands of the precursors, CH 3 OH and H 2 O, as well as those of all the new species produced by bombardment with the 40 MeV 58 Ni 11+ beam. Spectrometric parameters of the main products are displayed in Table 2. The spectral region of interest was divided into two spectrum ranges for clarity: (a) cm 1 and (b) cm 1. The solid line corresponds to the virgin mixture and the two dashed/dotted lines to irradiation at fluences of 1 and ions cm 2. Small contamination peaks due to CO 2 (ν cm 1 ) and CO (ν cm 1 ), already reported by Ehrenfreund et al. (1996), were also observed before irradiation in the virgin spectrum (Fig. 3b). Fig. 4 shows an overview of columns density evolution for the main species. It appears from the results that the bands are affected differently by ion irradiation and hence give conflicting information about the amount of methanol and water in the irradiated mixture. Table 3 shows the band position, assignments, characterization and A-values reported in the literature for weak vibrational mode transitions of species formed during irradiation of the mixture (1:1) (CH 3 OH:H 2 O) Small daughter molecules: CH 4,CO,H 2 CO and CO 2 In the present experiment, variation in abundance of CH 4 molecules was determined by their relatively isolated ν 4 band at 1304 cm 1 (D Hendecourt et al. 1986). The A-value used was cm molecule 1 (Gerakines et al. 1996; debarrosetal.2011b; Mejía et al. 2013). The final column density, determined after irradiation, was N f = molecules cm 2. For CO absorption, only a broader component around cm 1 was observed (Tielens et al. 1991; Ehrenfreund et al. 1997; Boogert, Hogerheijde & Blake 2002; Pontoppidan et al. 2003). As this particular feature does not correspond to any other species, identification is clear. The column density of carbon monoxide was traced through the CO stretch using an A- value of cm molecule 1 (Gerakines et al. 1995) and was found to increase with fluence, reaching a final column density of N f = molecules cm 2. This CO band can also be observed for higher fluences as shown in Fig. 8(b) close to O 2 molecule. The H 2 CO band appears as a relatively isolated peak at 1246 cm 1 (ν 2 ). This band was used for the cross-section calculations with an A-value of cm molecule 1 (Öberg et al. 2009; de Barros et al. 2011a). The increased column density after irradiation was N f = molecules cm 2, as can be seen in Fig. 3(a). It can also be observed in Fig. 4 that H 2 CO is one of the most abundant daughter species. CO 2 can be observed via two bands: the strong CO-stretching at 2346 cm 1 and the band at 662 cm 1 (Gerakines et al. 1995; Jamieson et al. 2006), as can be seen in Figs 3(a) and 5(a). To calculated the cross-sections, the absorbance of the 662 cm 1 band was used. The column density of carbon dioxide after irradiation using an A-value of cm molecule 1 (Gerakines et al. 1995), was N f = molecules cm 2. It can be seen in

7 Heavy cosmic ray analogues on methanol and water ice mixtures 2739 Table 5. Final column density (N k,f ), destruction (σ d ) and formation (σ f ) cross-section, obtained by fitting with equations (4) (6), and destruction G-values (radiochemical yields) of the daughter species formed in the CH 3 OH:H 2 O mixture. Species Position N k,f σ d σ f σ f /σ d G d (cm 1 ) (molecules cm 2 ) (cm 2 ) (cm 2 ) ratio (molecules/100 ev) H 2 CO (9.7 ± 0.6) (2.0 ± 0.8) CO (3.0 ± 0.8) (1.4 ± 0.8) HCO (9.5 ± 3.5) (0.52 ± 0.05) HCOOCH (9 ± 2) (1.6 ± 0.3) CH (3.2 ± 0.2) (1.8 ± 0.2) CO (7.1 ± 0.8) (0.6 ± 0.04) Cross-sections are multiplied by two as these species need two precursor molecules. Table 6. Summary of carbon, oxygen and hydrogen column density variations for precursor and most abundant daughter molecules. The bands used for the calculations are given in the second column. N = N f N 0 is the column density variation (expressed in molecules cm 2 ) observed after irradiation (fluence of ions cm 2 ). The average destruction and formation yields in 10 5 molecules destroyed or produced per projectile are also given. Species Wavenumber N N N Yield Yield Yield (cm 1 ) Carbon Oxygen Hydrogen Carbon Oxygen Hydrogen Precursor CH 3 OH H 2 O Total Formed species H 2 CO (0.8 per cent) 0.25 (0.8 per cent) 0.5 (5.3 per cent) CH (0.5 per cent) (6.1 per cent) CO (0.2 per cent) 0.14 (0.4 per cent) 0 CO (0.1 per cent) 0.03 (0.1 per cent) 0 HCO (0.3 per cent) 0.08 (0.2 per cent) 0.08 (0.8 per cent) HCOOCH (2.5 per cent) 0.75 (2.2 per cent) 1.5 (16 per cent) Total (4.4 per cent) 1.25 (3.7 per cent) 2.68 (28.6 per cent) For molecules with more than one carbon, oxygen and hydrogen atom, the corresponding atomic column density is obtained by multiplying the molecular column density by the number of carbon, oxygen or hydrogen atoms. Fig. 3(b) that the column density behaviours of CO (2136 cm 1 ) and CO 2 (2346 cm 1 ) are very similar Daughter radicals: HCO We identified the formyl radical (HCO) by means of two bands: (i) the ν 1 mode at 1096 cm 1, the OH-rocking band, and (ii) the ν 3 mode at 2543 cm 1, the CH-stretching band, which allows us to determine its final column density as be N f = or N f = molecules cm 2 using the A-values of 1.37 or cm molecule 1 (Bennett et al. 1997), respectively CHO-bearing complex daughter molecule: HCOOCH 3 We observed a band at 1160 cm 1 corresponding to a C 2 H 4 O 2 isomers previously characterized as CH 2 -wagging + OH-rocking at ν 8 (Gerakines et al. 1996; Bennett et al. 1997) and identified through its fundamental mode (CH 3 -rocking; Modica & Palumbo 2010) as methyl formate (HCOOCH 3 ). The same band was observed by de Barros et al. (2011a) and Modica & Palumbo (2010) for pure methanol and by Modica & Palumbo (2010)foraCO:CH 3 OH mixture after ion irradiation. This band is a key element that proves that methyl formate is formed in the solid phase after irradiation. Using an A-value of cm molecule 1 (Gerakines et al. 1996; Bennett et al. 1997; Modica & Palumbo 2010), the column density for this band before irradiation was found to be N f = molecules cm New species formed at high fluences At fluences above ions cm 2, new daughter bands (Table 3) are observed. These are discussed in this subsection. They were not used to determine the cross-sections. Among the complex organic molecules detected so far, special attention has been paid to the formation of the C 2 H 4 O 2 isomers methyl formate (HCOOCH 3 ), acetic acid (CH 3 COOH) and glycolaldehyde (HOCH 2 CHO) molecules that have the same stoichiometry, but differ in the way atoms are connected. The glycolaldehyde start to be observed at fluence (Fig. 7c) with A-value from Bennett & Kaiser (1997). The hydroxymethyl radical (CH 2 OH) at 1193 cm 1, carbon monoxide ( 13 CO) at 2091 cm 1 and formaldehyde (H 2 CO) at 1494 cm 1 did not appear at the beginning of irradiation of the CH 3 OH:H 2 O mixture; however, they could be observed in the pure methanol measurement for four ion beams (de Barros et al. 2011a).

8 2740 A. L. F. de Barros et al. Figure 9. (a) Total ( i,a) hydrogen, carbon and oxygen atomic column densities (N A (F))and molecular column densities of daughters species as a function of beam fluence. The total values are the sum of the molecular densities according to stoichiometry and are labelled as hydrogen (squares), carbons (stars) and oxygen (triangles). (b) Atomic column density variation (N 0 N(F)) for hydrogen, carbon and oxygen atoms existing on the precursor molecules (CH 3 OH and H 2 O) as a function of beam fluence. These values are to be compare to the total atomic N(F),shownin(a)and repeated in (b). In the CH 2 OH low-temperature experiments performed by Milligan et al. (1973), Gerakines et al. (1996)and Bennett et al. (1997), a strong CO-stretching band was found around ν 4 at 1195 cm 1, in agreement with their calculations. The ν 1 13 CO band was observed by Jamieson et al. (2006) at 2091 cm 1. The evolution of the absorption of several IR bands is shown in Figs 5(b) and (c); the corresponding A-values are given in Table 3. Unexpected peaks, such as those due to O 2 at 1550 cm 1 and 2139 cm 1, were also observed in Figs 6(a) and (b), respectively. 4 DETERMINATION OF CROSS-SECTION 4.1 Destruction and formation cross-sections of the precursors The destruction rates of the precursor molecules in the sample (CH 3 OH and H 2 O) exhibit decreasing exponential behaviour, a result of the molecular dissociations induced by the fast projectile. This is shown in Fig. 4 for the 1129 and 1655 cm 1 bands. This behaviour is usual in beam collision experiments, in which the target molecules are destroyed by induced chemical reactions and by sputtering. As discussed in previously studies, the column density evolution with fluence for each precursor species i can be approximately described by the differential equation system: dn i df = σ f,ij N j + L i σ d,i N i Y i. (1) j i The quantities L i and Y i are layering and sputtering yields, respectively. N i (F)andN j (F) are functions describing the column densities of molecular species i and j as a function of the beam fluence F; σ f and σ d are their formation and destruction cross-sections, respectively (de Barros et al. 2014). For the studied mixture, the precursor molecules are CH 3 OH (i = 1) and H 2 O(i = 2). It should be noted that for the current experiment a thin surface layer of water ice grew continually on the sample target, preventing sputtering of the precursor and the daughter molecules, simplifying therefore the analysis of their column density evolution. Assuming no layering (L 1 = 0) for CH 3 OH and, because of a thin water layer on the target surface no sputtering (Y 1 = 0) even for lower fluences, the approximate solution for equation (1) is N 1 (F ) = (N 1,0 N 1, ) exp( σ ap d,1 F ) + N 1,. (2) The quantities N 1,0 and N 1, are the initial and final column densities and σ ap d,1 is the apparent destruction cross-section of CH 3OH. In this equation, N 1, is the asymptotic value of the column density of methanol considering the molecular reconstruction process and σ d,1 = j σ d,1j. In fact, these results tend to be exact if the daughter molecules j = i are much more abundant than the others products formed. Then, N 1, /N 1,0 σ f,1i /(σ f,1i + σ d,1i )andσ ap d,1 (σ f,1i + σ d,1i ). Fitting the data with equation (2) gives the CH 3 OH destruction cross-sections (Table 4). As can be seen in Fig. 4, the experimental variation in column density confirms the beam used confirms the decreasing exponential behaviour, validating the use of equation (2) over this fluence range. The following parameters were determined: N 1,0 = N CH3 OH(0) = and N 1, = molecules cm 2 ; σ ap d,1 = (8.3 ± 0.35) cm 2 for the band 1129 cm 1. Equation (2) was also used for H 2 O, but this time taking into account the fact that there was a small but continuous water flow towards the cold target during irradiation. The molecules deposited during this layering (L 2 ) did not participate in chemical reactions with CH 3 OH but created a superficial thin film. The values obtained for the water column density evolution are: N 2,0 = N H2 O(0) = , N 2, = molecules cm 2 and σ ap d,2 = (7.9 ± 0.56) cm 2 for the 1665 cm 1 band. The formation cross-sections obtained for these bands are shown in Table 4. Fig. 8 compares the decrease in the CH 3 OH (1026 cm 1 )band areas as a function of dose for two different measurements: pure CH 3 OH ice bombarded by 606 MeV 70 Zn 26+ and CH 3 OH:H 2 O ice mixture bombarded by 40 MeV 58 Ni 11+ (this work). For clarity, both sets of data are normalized to unity at zero fluence. Since the stopping powers are different for each case, it is not surprising that the observed slopes (which depend on cross-sections) are also different (see Table 7). In contrast to the values for pure methanol, which tend to fall to zero, the values for the mixture data level off saturate at 0.4. A possible explanation is that, above a certain beam fluence through the mixture sample ( ), the concentration of free OH and H radicals in the water ice matrix is so high that

9 Heavy cosmic ray analogues on methanol and water ice mixtures 2741 Figure 10. IR spectra, in the cm 1 range of virgin CH 3 OH:H 2 O(1:x) mixtures, where x = 10, 5, 4, 2, 1 and 0. The variation in the mixture concentration was taken into account in the analysis of the 1026, 1129, 1460, 2827 and 2937 cm 1 methanol bands. dissociations such as CH 3 OH CH 3 + OH and CH 3 OH CH 2 OH + H are inhibited. Consequently, the organic fragments produced receive the OH or H radicals back and re-form CH 3 OH. For pure CH 3 OH ice, the production of H 2,O 2 and H 2 O reduces the concentration of free OH and H in the ice, as a result of which re-formation is low and the saturation level of the column density is close to zero. 4.2 Formation and destruction cross-sections of daughter species Establishing correct parametrization for cross-section expressions for daughter molecules in the case of a mixture of precursors is more involved. All the column densities of the daughter species used to calculate the cross-sections here are shown in Fig. 7. The procedures used to analyse the formation and destruction crosssections of precursors were separated into two parts: (a) Daughter species produced directly by the dissociation of just one precursor If sputtering is negligible (Y k = 0) and if production of the daughter species k occurs directly from a given precursor i without interference of the evolving environment, equation (1) can be written as dn k df = σ f,kin i σ d,k N k, (3) where σ d,k = j σ d,kj. Assuming σ f,ki = σ d,ik and j = i, the solution of equation (3) can be written approximately as N k (F ) N k ( ) = 1 exp[ (σ f,ki + σ d,ki )F ], (4) where σ f,ki N k ( ) = N i,0. (5) σ f,ki + σ d,ki Note that, for (σ f,ki + σ d,ki )F 1 there is a linear dependence N k (F ) σ f,ki F. (6) N i,0 Equations (4) and (5) were used to fit the CH 3 OH (i = 1) and H 2 O (i = 2) daughter column densities. For the CH 3 OH group in particular, this means that N 1,0 and σ d,1 are the values previously obtained by fitting the precursor data with equation (2) and that pairs of σ f,k1 and σ d,k1 are extracted independently using the H 2 CO, CO, CH 4 and HCO data. (b) Daughter species produced directly by two or more precursor molecules If at least n molecules of the same species i are needed to form the observed new species, equation (4) or equation (6) may also be used, but N i,0 /n is considered the precursor column density. This is equivalent to considering synthesis involving n precursor molecules as a more efficient process than dissociation of a single molecule; the formation cross-sections of daughters molecules are then increased by a factor n with respect to daughter molecules that need only one precursor σ = n.σ. Therefore, n = 2 was considered for the production of CO 2 and HCOOCH 3 from two neighbouring CH 3 OH molecules. The results are displayed in Table 5. 5 COMPUTING THE ATOMIC BUDGET The carbon, oxygen and hydrogen budgets are addressed by testing whether the column density of the mixture destroyed can account

10 2742 A. L. F. de Barros et al. Figure 11. Experimental infrared spectra for the CH 3 OH:H 2 O mixture when it is warming up from 16 to 95 (a, b, f) and cooled down (c, d) and for pure CH 3 OH when it is warming up from 15 to 105 K (f) and cooled down (e) FTIR spectra are shown in the cm 1 region for the mixture and in the cm 1 region for pure methanol. The inserts in (a) and (c) are the profiles of the methanol C O stretching, which change significantly depending on the annealing. for the column densities of the products. After irradiation it was found that molecules cm 2 of CH 3 OH were destroyed per square centimetre; for H 2 O this value is of the same order of magnitude: molecules cm 2.Table6 shows the total number of carbon, oxygen and hydrogen atoms per unit area for all products after irradiation. It is found that ion average carbon atoms cm 2, oxygen atoms cm 2 and hydrogen atoms cm 2 per impact participate in chemical reactions, corresponding to approximately (73 ± 5) per cent of the atoms from the destroyed CH 3 OH:H 2 O mixture. Fig. 9(a) shows, in solid lines, the evolution of the six most abundant daughter molecules column densities, N k (F). The label k corresponds to H 2 CO, CH 4,CO 2, CO, HCO and HCOOCH 3.The dashed lines represent atomic column densities N A (F ) = k n A,k N k (F ), (7) where n A, k is the number of atoms of the element A (A holds for carbon, oxygen or hydrogen) in daughter molecule k. These data are compared with the atomic column density losses calculated for the CH 3 OH and H 2 O precursor molecules, N A = N 0A,A N A (F), where N 0A,A is the initial value of N A (F). These three functions are also plotted in Fig. 9(b). The discrepancies between N A (F) and N A allow the amount of unobserved material to be estimated and indicate possible inaccurate A-values. 6 ANALYSIS OF VIRGIN CH 3 OH:H 2 MIXTURES 6.1 Different mixture concentrations Infrared spectroscopy of non-irradiated CH 3 OH:H 2 O mixtures was performed for several concentrations (1:x), where x = 10, 5, 4, 2, 1 and 0. Fig. 10 shows the resulting FTIR spectra for the cm 1 range. These data may be useful for estimating the relative methanol/water abundance in interstellar icy mantles. The C O stretching mode is often used to estimate methanol abundance from laboratory spectra because it is not superimposed on bands of other species. The comparison of the C O stretching band profile obtained from spectra of pure virgin methanol and of CH 3 OH:H 2 O mixtures is shown in Fig. 10 for different concentrations. Analysis of the virgin ice bands reveals that, as the amount of water increases in the mixture, the band profile changes: its peak position is shifted to lower wavenumbers (from 1034 to 1020 cm 1 ) and its profile is asymmetric. This shift is probably due to methanol molecules embedded in a water matrix. As the amount of water increases in CH 3 OH:H 2 O virgin mixtures, the peak shape becomes slightly narrower. The methanol band located at 1460 cm 1 is due to both the CHdeformation and OH-bending modes (see Fig. 1). The pure methanol (Fig.10f) band is composed of three equally intense peaks at about 1480, 1463 and 1450 cm 1, and a shoulder at about 1430 cm 1.The same structure is observed in the 1:1 (Fig. 10e) and 1:2 (Fig. 10d)

11 Heavy cosmic ray analogues on methanol and water ice mixtures 2743 mixtures, where the shoulder at 1430 cm 1 becomes less intense. In the 1:10 (Fig. 10d) mixture, the peak at 1480 cm 1 appears lower than the others. As the amount of water increases, the band becomes narrower: the FWHM decreases from 86 cm 1 in pure methanol to 68 cm 1 in a 1:10 mixture. The CH-stretching modes of the CH 3 OH bands occur at about 2827 cm 1 (symmetric) and 2961 cm 1 (asymmetric) according to the concentration. For virgin CH 3 OH:H 2 O mixtures, as the amount of water increases, the 2961 cm 1 asymmetric vibration becomes broader. The profile of the symmetric CH-stretching band does not change significantly in the different mixtures as the amount of water increases: the peak shifts slightly from 2826 to 2832 cm 1, while the FWHM decreases from 32 to 16 cm 1. For the different mixtures, the symmetric CH-stretching band shows a shoulder at 2850 cm 1 which is not visible in the pure methanol spectrum (Fig. 10f). As can be seen in Fig. 10, while the mixing ratio mainly governs the shape of the absorption bands, the temperature affects the location of the band peaks. It can be reasonably be supposed that this is true as long as there are no amorphous crystalline phase transitions. 6.2 Annealing effects Another very important effect is the changes in the IR peak shapes during warming up of both pure CH 3 OH and the CH 3 OH:H 2 O mixture (Figs 11a d). Fig. 11 shows the experimental infrared spectra for the CH 3 OH:H 2 O mixture when warming up from 16 to 95 K (a), (b), (f) and cooled down (c), (d) and for pure CH 3 OH when it is warmed up from 15 to 105 K (f) and cooled down (e). FTIR spectra are shown in the cm 1 region for the mixture and in the cm 1 region for pure methanol. The insets in (a) and (c) represent the profiles of the methanol C O stretching band whose shapes are temperature dependent. For pure methanol (Figs 11e and f) the 1026 cm 1 C O stretching band changes very little during warming up and cooling down but splits into 1016 and 1034 cm 1 peaks for mixtures ices. The OH-stretch band of CH 3 OH at 3250 cm 1 seems to have many modifications at pure methanol sample during the cooling down process (Fig. 11e). Another characteristic is that their absorbance depend on the initial mixture concentration and on temperature. The formation of a new peak at 1034 cm 1 has been already observed by Schutte et al. (1991) in laboratory spectra of a CH 3 OH:H 2 O mixture (1:20). They suggested that this peak is due to methanol embedded in annealed H 2 O ice. It can also be observed that the OH-stretching band in the mixture (Figs 11a and c) did not change during annealing. 7 ASTROPHYSICAL IMPLICATIONS Determining radiochemical yield (G-factor) and the change in crosssection behaviour with stopping power is important as it allows a better comparison of the radiation effects produced by electrons, photons and ions reported in the literature. The increase in destructive effect with stopping power is assumed to be linear in the definition of G. Nevertheless, examination of the values for G d (CH 3 OH) in Table 7 for different projectiles shows that they are dispersed over two orders of magnitude. This means that the energy transferred is not the only relevant quantity in molecular dissociation by fast ions. In general, there is no pure CH 3 OH ice in space and typical abundance of CH 3 OH in water ice is 1 per cent. Cross-sections for mixtures are different from those for pure ices and much more relevant for astrophysics. The higher the stopping power, the greater the chemical modifications in the ice are expected to be. Accordingly, the formation and destruction cross-sections for irradiation of methanol ice in our study (using a 40 MeV 58 Ni 11+ beam) were expected to be higher than cross-sections previously reported for 16 MeV 16 O 5+, 220 MeV 16 O 7+, 606 MeV 70 Zn 26+ and 774 MeV 86 Kr 31+ beams (de Barros et al. 2011a). The results for destruction and formation cross-sections are shown in Figs 12(a) and (b). Although the ice concentrations are not the same, it is assumed that the law (σ d,f = a d,f S n e,wheren 3/2) still holds approximately. It is worth noting that this power-law dependence was also reported by Godard et al. (2011) for aliphatic C H hydrogenated carbons, by Andrade et al. (2013) for HCOOH irradiated with 267 MeV 56 Fe 22+,byMejía et al. (2013) forch 4 irradiated with 6 MeV 16 O 2+, 220 MeV 16 O 7+, 267 MeV 56 Fe 22+ and 606 MeV 70 Zn 26+ and most recently by de Barros et al. (2014)forH 2 O:H 2 CO:CH 3 OH (100:2:0.8) ice mixture bombarded by 220 MeV 16 O 7+ ions. An interesting point regarding the new species that were formed at higher fluences is the appearance of unexpected peaks such as those due to O 2 (Figs 6a and b) or CH 4 (Fig. 5b). The Short Wavelength Spectrometer at the ISO was used to search for solid O 2 in cold dense clouds at 6.45 µm ( 1550 cm 1 ). O 2 bands were observed in an analysis of solid O 2 on the Galilean satellites (Vidal et al. 1997; Baragiola & Bahr 1998)haveanalysed Table 7. Comparison of radiochemical yields induced in CH 3 OH mixtures ices by different irradiation sources. G-values is defined as 100 σ d,f /S e. Projectile E/m S e ev/10 15 G d G f G f G f G f G f G f Ref. (MeV/u) atoms cm 2 (CH 3 OH) (H 2 CO) (CH 4 ) (CO) (CO 2 ) (HCO) (HCOOCH 3 ) 220 MeV 16 O MeV 16 O MeV 70 Zn < MeV 86 Kr MeV 58 Ni MeV protons MeV protons kev protons kev He keVHe ev photons UV de Barros et al. (2011b), 2 This work for CH 3 OH:H 2 O (1:1), 3 Gerakines et al. (1999), 4 Gerakines, Moore & Hudson (2001), 5 Brunetto et al. (2005), 6 Baratta, Leto & Palumbo (2002), 7 Baratta et al. (1994).

12 2744 A. L. F. de Barros et al. Figure 12. Dependence on electronic stopping power of (a) destruction cross-section and (b) formation cross-section of H 2 CO, CH 4,CO,CO 2, HCO and CH 2 OHCHO daughter species. Fittings (solid lines) refer to the function σ d S e n with n = 3/2 for the current data. These results were compared with those obtained by de Barros et al. (2011b) in an experiment with pure CH 3 OH. them later on by heavy-ion irradiation in vapour-deposited water ice at 15 K. Additional constraints on O 2 abundance were obtained from analysis of the 4.67 µm( 2139 cm 1 ) on solid CO absorption profile observed from the ground (Vandenbussche et al. 1999). Both bands are observed in the current data in Table 3 and Figs 6(a) and (b). Two undetermined bands at 1545 and 1554 cm 1 were also observed (Fig. 6a). 8 REMARKS AND CONCLUSIONS The interaction of MeV 58 Ni 11+ ion beam with the CH 3 OH:H 2 O (1:1) ice mixture was studied. Fast nickel ions are important components of heavy galactic cosmic rays, and a methanol and water ice mixture is a natural constituent of grains and mantles in the astrophysical environment. Our measurements demonstrate that 40 MeV ion irradiation of this ice leads to dissociation into CH 4,HCOand CO, as well as synthesis of H 2 CO, CO 2, HCOOCH 3 and some heavier molecules. The most abundant daughter molecules are H 2 CO, HCOOCH 3 and CH 4. The radicals CH 2 OH (at higher fluence) and HCO appear to be very fragile under ion bombardment and are barely seen for high stopping power projectiles. At fluences above ions cm 2, new daughter bands are observed, such as those corresponding to a hydroxymethyl radical (CH 2 OH), carbon monoxide ( 13 CO), tri-carbon monoxide (C 3 O) and ethylene glycol (HOCH 2 CH 2 OH) in addition to those already observed at lower fluences. The budget calculation for carbon, oxygen and hydrogen atoms relative to the main daughter molecules was performed. This not only allowed the consistency of A-values to be monitored but also allowed the determination of the abundance of O 2 molecules (for which the band was observed for the first time in the radiolysis of this mixture) as well as that of the molecules which were not observed, such as H 2. The destruction cross-sections of the molecules in the ice mixture were determined, as well as the formation cross-sections of the molecular species produced. The dependence of the cross-section on stopping power for the mixture studied here when bombarded by Ni projectiles was found to follow an approximate power law, i.e. σ d Se n,wheren 3/2. Similar results have been reported and can be expected for other projectile ice systems, encouraging further investigation. Measurements for other mixtures are necessary to analyse whether this law can be generalized. Finally, with regard to the astrophysical implications, the current results contribute to the establishment of a data base of physical chemical-processes that will help elucidate the formation of complex organic molecules in the ISM. ACKNOWLEDGEMENTS This work was supported by the French-Brazilian exchange programme CAPES-COFECUB. We are grateful to T. Been, C. Grygiel, T. Madi, I. Monnet, A. Domaracka, X.Y. Lv, J.J. Ding and J. M. Ramillon for their invaluable support. The Brazilian agencies CNPq (INEspaço) and FAPERJ are also acknowledged. REFERENCES Allamandola L. J., Sandford S. A., Valero G. J., 1988, Icarus, 76, 225 Allamandola L. J., Sandford S. A., Tielens A. G. G. M., Herbst T. M., 1992, ApJ, 399, 134 Andrade D. P. P., de Barros A. L. F., Pilling S., Domaracka A., Rothard H., Boduch P., da Silveira E. F., 2013, MNRAS, 632 Baragiola R. A., Bahr D. A., 1998, J. Geophys. Res., 103, Baratta G. A., Castorina A. C., Leto G., Palumbo M. E., Spinella F., Strazzulla G., 1994, Planet. Space Sci., 42, 759 Baratta G. A., Leto G., Palumbo M. E., 2002, A&A, 384, 343 Bennett C. J., Kaiser R. I., 2007, ApJ, 661, 899 Bennett C. J., Chen S. H., Chang Agnes H. H., Kaiser R. I., 2007, ApJ, 660, 1588 Bergin E. A., Langer W. D., Goldsmith P. F., 1995, ApJ, 441, 222 Boogert A. C. A., 1999, PhD thesis, Kapteyn Astronomical Institute Boogert A. C. A., Hogerheijde M. R., Blake G. A., 2002, ApJ, 568, 761 Brunetto R., Baratta G. A., Domingoc M., Strazzulla G., 2005, Icarus, 175, 226 Charnley S. B., Latter W. B., 1997, MNRAS, 287, 538 Chiar J. E., Gerakines P. A., Whittet D. C. B., Pendleton Y. J., Tielens A. G. G. M., Adamson A. J., Boogert A. C. A., 1998, ApJ, 498, 716 D Hendecourt L. B., Allamandola L. J., Grim R. J. A., Greenberg J. M., 1986, A&A, 158, 119 Dartois E., Schutte W., Geballe T. R., Demyk K., Ehrenfreund P., D Hendecourt L., 1999, A&A, 342, L32 de Barros A. L. F., Domaracka A., Andrade D. P. P., Boduch P., Rothard H., da Silveira E. F., 2011a, MNRAS, 418, 1363 de Barros A. L. F., Bordalo V. Seperuelo Duarte E., Domaracka A., da Silveira E. F., Rothard H., Boduch P., 2011b, A&A, 531, A160