Production and isolation of OH radicals in water ice

Transcription

1 Mon. Not. R. Astron. Soc. 415, (2011) doi: /j x Production and isolation of OH radicals in water ice Emilie-Laure Zins, Prasad Ramesh Joshi and Lahouari Krim Laboratoire de Dynamique, Interactions et Réactivité, Université Pierre et Marie Curie, CNRS, UMR 7075, case Courrier 49, Bat F 74, 4 Place Jussieu, Paris, Cedex 05, France Accepted 2011 April 15. Received 2011 April 14; in original form 2011 March 23 ABSTRACT OH radicals may play an important role in reactions that take place on interstellar icy grain surfaces, due to their high reactivity. Unfortunately, laboratory experiments aiming at simulating such reactions are hindered by the high reactivity of these species. Indeed, a method to isolate and further study the reactivity of OH radicals is still missing. This paper presents a study we carried out to determine the best conditions to isolate OH radicals in water ice. OH radicals were produced using a microwave discharge from either pure gaseous water or a gaseous mixture containing diluted water. Species thus formed were further condensed on a cold mirror at temperatures ranging from 3.5 to 30 K. The samples were then probed with a Fourier transform infrared spectrometer. The dilution of water into a rare gas (He, Ne) was found to be the best way to form an ice containing a high proportion of OH radicals, especially at low temperatures. These conditions, namely low temperatures and low concentrations, allow for the cooling of species formed during the discharge and prevent radical recombination. Key words: astrochemistry ISM: clouds. 1 INTRODUCTION Chemical reactions involving OH radicals are of primary importance in different fields, especially in atmospheric chemistry and in astrochemistry. Indeed, many studies have proved that the OH radical, even present at low concentration in the troposphere, reacts efficiently with many trace species, including poisonous carbon monoxide, the greenhouse gas methane, sulphur dioxide and the nitrogen oxides, and other hazardous compounds (Comes 1994; Sennikov, Ignatov & Schrems 2005). OH radicals have been experimentally observed in interstellar space (Robinson & McGee 1967; Baud & Wouterloot 1980; Storey, Watson & Townes 1981). These radicals can be generated by the photodissociation of water (Bergeron et al. 2008; Watanabe & Kouchi 2008). The formation of OH radicals can also be due to the hydrogenation of O atoms on interstellar grains (Miyauchi et al. 2008). Furthermore, it has been proposed that these radicals might be involved in the formation of important molecules in interstellar clouds, such as H 2 O, H 2 O 2 and CO 2 (Bergeron et al. 2008; Miyauchi et al. 2008; Watanabe & Kouchi 2008). To gain some more insight into these processes, and prior to any reactivity study, it is of primary importance to propose an efficient way to prepare and isolate these radicals at sufficiently high concentrations. The latter point is challenging. Indeed, as highly reactive species, radicals are most likely to react in order to form more stable species. It is thus necessary to cool down the radicals experimen- zins@spmol.jussieu.fr tally generated. Such an experimental procedure will allow further investigation on the reactivity of OH radicals and the results thus obtained may be used to further propose models for the interstellar chemistry (van Dishoeck & Black 1986; Ruffle & Herbst 2000; Herbst 2001; Chang, Cuppen & Herbst 2007; Garrod 2008). In the context of applications to the chemistry in molecular clouds, the generation and the study of water ice including OH radicals would give more insight into the chemistry at very low temperature and low pressure. Indeed, it is now well established that the chemistry on the surface of interstellar ice plays a key role in the chemical evolution of interstellar clouds (Herbst 2001; Vidali et al. 2006; Bergeron et al. 2008). This is the reason why this paper is focused on the formation of OH radicals in the water ice generated by a microwave discharge in pure or diluted water. In the case of dilute samples, rare gases (RGs with RG = He or Ne) were used to cool down the radicals generated during the discharge. The species thus produced were then condensed on the surfaces of mirrors at different temperatures relevant in the context of interstellar clouds, namely 3.5, 10, 20 and 30 K, and at low pressure (10 7 mbar). The samples used were further probed by Fourier transform infrared (FT-IR) spectroscopy. 2 EXPERIMENTAL SETUP 2.1 General conditions of the experimental setup The system was evacuated at a base pressure of about mbar. IR spectra of the samples were recorded in the transmission-reflection mode between 5000 and 500 cm 1 using C 2011 The Authors

2 3108 E.-L. Zins, P. R. Joshi and L. Krim a Bruker 120 FT-IR spectrometer. A Ge/KBr beam splitter was used along with a N 2 -cooled narrow-band HgCdTe photoconductor. Mid-infrared (mid-ir) spectra were collected. The resolution was 0.5 cm 1. Bare mirror backgrounds, recorded prior to sample deposition, were used as references in processing the sample spectra. 2.2 Preparation of the samples in water ice Some of the experimental methods and setup used for this study have been previously described (Krim, Manceron & Alikhani 1999; Danset, Alikhani & Manceron 2005; Dozova et al. 2006; Pirim et al. 2010; Pirim & Krim 2011). Here we briefly mention the main points concerning the synthesis of the samples, and we describe the new aspects of the experiments. A gas was submitted to a microwave discharge (microwavedriven atomic source, SPECS PCR-ECS). The microwave discharge source is based on the principle of electron cyclotron resonance where low radio frequency is coupled to the plasma. The sample submitted to the microwave discharge was either pure water or a water/rg (RG = He or Ne) mixture. In the case of water/rg mixtures, different concentrations of water were used. Helium and neon were furnished by L Air Liquide S.A. with a purity of per cent. Natural demineralized water was degassed in a vacuum line prior to the experiments. The purity of the samples was confirmed spectroscopically. The samples were obtained by simple condensation of the species generated by the discharge on to one of the six flat, highly polished, Rh-plated copper mirrors maintained at low temperature using a pulse tube, closed-cycle cryogenerator (Cryomech PT405, USA). The temperatures chosen for this study were the following: 3.5, 10, 20 and 30 K. All these temperatures are relevant from an astrochemical point of view. Most of the experimental simulations for astrochemical purposes are carried out at 10 K. Slightly higher temperatures such as 20 and 30 K can also be found in some interstellar clouds. Complementarily, we also investigated the formation of OH radicals at 3.5 K, because it seemed logical to suppose that low temperatures will favour the observation of OH radicals by preventing side reaction. In the case of experiments for which pure water was submitted to the microwave discharge, samples obtained after condensation at 3.5 or 10 K were investigated. Concerning diluted water samples, two different procedures were used depending on the nature of the RG. For RG = He, the discharged mixture was condensed at the surface of the mirror at 3.5, 10, 20 or 30 K. On the other hand, for RG = Ne, experiments were performed to generate water ice, and in this context, it was not possible to work at very low temperatures. Indeed, to prevent the formation of a neon matrix, we chose to work at 20 K. This was also the only option we had to be able to directly compare the IR spectra obtained by different approaches (pure water, 10 per cent of water into He and 10 per cent of water into Ne). For the sake of comparison, samples obtained by simple condensation of water with no microwave discharge were also investigated. 3 RESULTS AND DISCUSSION The time of deposition was chosen depending on the nature of the sample. Indeed, preliminary studies have shown that, when pure water is subjected to the discharge, saturated spectra are obtained even after 5 min of deposition. On the other hand, when diluted water is used instead of pure water, the best spectra are obtained after 30 min of deposition. 3.1 Determination of a peak characteristic of the formation of OH radicals First of all, it was necessary to identify peaks that may be characteristic of the formation of OH radicals. To this end, gases containing various amounts of water were subjected to the microwave discharge, in order to prepare samples that may contain OH radicals at various concentrations (Fig. 1). The exact experimental conditions in which these samples were prepared will be discussed later in detail, but we have chosen to present these results first, because they allow the attribution of some peaks to hydrated OH radicals and aggregates. In the case of a sample obtained with the smallest amount of discharged water (Fig. 1a), isolated OH radicals are observed (3560 cm 1 ) as well as a small amount of water interacting with OH radicals (ν 1 and ν 3 modes at 3726 and 3628 cm 1, respectively). All these attributions are in full agreement with earlier studies (Acquista, Schoen & Lide 1968; Tinti 1968; Cheng, Lee & Ogilvie 1988; Suzer & Andrews 1988; Langford, McKinley & Quickende 2000; Cooper et al. 2003; Costa Cabral 2005). When the concentration of water in the discharged mixture increases, peaks due to water interacting with OH radicals (ν 1 and ν 3 modes at 3726 and 3628 cm 1, respectively) are more intense (Fig. 1b). Additionally, HO 2 is observed (3424 cm 1 ). With a still higher amount of water subjected to the microwave discharge, the peak at 3726 cm 1 almost disappears, whereas the peak at 3689 cm 1 dramatically increases. A bump is still observed at 3628 cm 1, and a water ice is formed, as shown by the wide band centred at 3400 cm 1 (Fig. 1c). The comparison of the latter IR spectrum (Fig. 1c) with the one obtained with no discharge (Fig. 1d) strongly suggests that, under our experimental conditions, namely diluted samples, water is totally dissociated by the microwave discharge. Indeed, the signature of the water ice formed by the direct condensation of water is characterized by a wide band with no structure, centred at 3200 cm 1 (Fig. 1d). On the other hand, the water ice observed from a diluted H 2 O/RG mixture is characterized by a strongly different IR spectrum. Indeed, structures are observed in the wide band, blueshifted by 200 cm 1 (Fig. 1c). This change in the IR spectrum reflects differences in the structure of the water ice obtained either by the direct condensation of non-dissociated water or by recombination reactions. Thus, we can consider that the amount of OH radicals condensed on the surface increases with the amount of water injected in the plasma. As described earlier, the comparison of spectra obtained with discharge mixtures containing various amounts of water is shown in Figs 1(a) (c). The changes in the relative intensities of the different peaks, in combination with the observations of peaks characteristic of OH radicals at 3576 and 3560 cm 1 (Fig. 1b), show that OH radicals are indeed formed when the gas containing water is subjected to the microwave discharge. The simultaneous observation of these peaks with the ones at 3726 and 3628 cm 1 (Fig. 1b) strongly suggests that the latter are due to interactions between one OH radical and one water molecule. The further increase in the peak at 3689 cm 1 when the amount of OH radicals formed is still increasing (Fig. 1c) leads to the conclusion that the latter peak is due to interactions between OH radicals and water ice. Thus, the peak at 3689 cm 1, in combination with the observation of water ice characterized by a wide and irregular band centred at 3400 cm 1, can be used to probe OH radicals trapped in water ice.

3 OH radicals in water ice 3109 Figure 1. Influence of the amount of water subjected to the microwave discharge. (a) shows the spectrum obtained with the smallest amount of discharged water; (b) and (c) show spectra obtained with an increasing amount of discharged water; and (d) shows water ice formed by simple condensation at 3.5 K with no discharge. The wavelength of each peak is specified in the table. The following two points are worth mentioning. (i) In addition to the above-mentioned species, CO 2 is observed in samples containing OH radicals (peak at 2345 cm 1, not shown here), whereas this species is absent in the case of a water ice obtained by simple condensation of gaseous water. Earlier studies have demonstrated that this peak is due to a reaction between CO, present as impurity traces in the chamber, and OH radicals. Reaction (1) can be proposed to explain this result: CO + OH CO 2 + H (1) Thus, the OH radicals formed under our experimental conditions can further react with other species present in the system (Zins, Joshi & Krim 2011; see also Frost, Sharkey & Smith 1993; Lester et al. 2000; Watanabe & Kouchi 2008). (ii) The difference between the structure of water ice generated by simple condensation of water, on the one hand, and that in the case of samples subjected to the microwave discharge, on the other hand, is likely to be due to reaction (2): OH + OH H 2 O + O (2) 3.2 Influence of the medium in which the microwave discharge is carried out Discharge in pure water Fig. 2(a) shows the spectrum obtained when pure water is condensed at 3.5 K. The formation of a water ice is characterized by the wide band centred at 3200 cm 1. The absence of any peak centred at 3600 cm 1 indicates that the water ice formed at this temperature is compact, with no dangling OH. When water molecules are subjected to a microwave discharge prior to their condensation, the spectrum obtained is fundamentally different (Fig. 2b). Indeed, the shape of the band corresponding to This is a new route to form water ice, which will be discussed later. As a conclusion of this preliminary discussion, the results presented above show that, when gaseous water is subjected to a microwave discharge, OH radicals are formed. These radicals can be trapped in water ice as (OH)(H 2 O) n aggregates, which leads to the observation of a peak at 3689 cm 1. Besides, a certain amount of OH radicals undergo reactions, mainly leading to the formation of H 2 OorCO 2. Figure 2. Spectra obtained at 3.5 K when pure water is: (a) condensed without any discharge; (b) subjected to the discharge and deposited during 1 min; (c) subjected to the discharge and deposited during 1.5 min; and (d) subjected to the discharge and deposited during 2 min.

4 3110 E.-L. Zins, P. R. Joshi and L. Krim the water ice changed, which indicates that the ice formed by simple condensation of water has a different structure from that of the ice obtained from discharged water, as already noted in Section 3.1. Moreover, the peak due to OH radicals trapped in the water ice, at 3689 cm 1, is observed. Thus, the presence of these two signals (the wide band centred at 3400 cm 1 and the peak at 3689 cm 1 ) proves that OH radicals are indeed formed and trapped in the water ice. Additionally, if this water ice has a different structure from the one obtained by simple condensation of gaseous water, this may be due to the fact that the water ice observed after the microwave discharge is due to the recombination between two OH radicals. Indeed, due to their high reactivity, they might react to form water molecules and atomic oxygen: H 2 O OH + H (3) 2OH H 2 O( ) + O (4) where reaction (3) is initiated by the microwave discharge, and H 2 O( ) is a water ice characterized by a specific structure. Thus, reaction (3) takes place in the gas phase, whereas reaction (4) is likelytooccuronthesurface. These hypotheses about the reaction pathways are strongly supported by the formation of HO 2. Indeed, a successive reaction may occur from atomic oxygen formed during reaction (4): OH + O HO 2 (5) Figs 2(b), (c) and (d) show the spectra obtained after 1, 1.5 and 2 min of the deposition of the pure discharged water, respectively. The peak attributed to OH radicals interacting with H 2 Obulkice is observed in each spectrum as well as peaks due to water ice. Besides, the relative abundances of water and hydrated OH radicals are particularly interesting with respect to our search for the best experimental conditions to observe OH radicals in water ice. Indeed, it appears that when the deposition time is increasing, both the bands due to hydrated OH radicals and water ice are increasing. Thus, it appears that when the amount of OH radicals is increasing, reaction (4) leading to the formation of water ice is favoured. As a conclusion, in order to further increase the amount of OH radicals trapped into the water ice, one way would be to try to reduce the recombination of OH radicals. To this end, the pure gaseous water subjected to the microwave discharge was replaced by a mixture of water diluted into a RG. The idea was that such a procedure will not hinder the formation of OH radicals, but it will reduce further collisions between these radicals, thus reducing the amount of water formed. Further collisions between OH radicals and inert gas particles may also cool down the former Discharge in a water/ne mixture As already mentioned, these experiments were carried out at 20 K to prevent the formation of a Ne matrix. As in the case of experiments in pure water, the samples obtained from 10 per cent of water into Ne are characterized by the presence of CO 2, and the structure of the water ice formed is strongly different from the one obtained by simple condensation of water. Nevertheless, the presence of OH radicals was not clearly observed (Fig. 3). Indeed, the band attributed to hydrated OH radicals is partially hindered by the wide band due to water ice. This result strongly suggests that the presence of a RG during the microwave discharge is not sufficient to hinder the recombination of OH radicals. However, the presence of a RG might lead to the relaxation of the internal energy of the radical formed, by means of inelastic collisions. On the other hand, the Figure 3. Spectra obtained when a microwave discharge is carried out in a water/ne = 10/100 mixture at 20 K. relatively high temperature of this experiment may favour reactions between OH radicals, leading to the formation of a water ice. To take into account this point, and to be able to generate a water ice with no RG matrix from a dilute gaseous water mixture at temperatures as low as 3.5 K, water/he mixtures were prepared and subjected to the microwave discharge Discharge in a water/he mixture A water/he = 10/100 mixture was subjected to the microwave discharge and the products thus formed were then condensed at 3.5 K. Graph (a) of Fig. 4(A) shows the spectrum obtained after 15 min of deposition. As previously, the most abundant species is water. Nevertheless, hydrated OH radicals are also observed and characterized by an intense peak at 3689 cm 1 [see graph (a) of Fig. 4(A)]. These results confirm the fact that the dilution of water into He leads to the stabilization of the generated radicals. This cooling of the radicals is more efficient at low temperatures. The longer the deposition to the discharged mixture, the higher the intensity of peaks due to both water ice and hydrated OH radicals [see graph (b) of Fig. 4(A)]. Fig. 4(B) further shows that HO 2 (1101 cm 1 ) is formed when OH radicals are produced and trapped in water ice. Moreover, H 2 O 2 (1069 and 1252 cm 1 ) was not observed, even if the formation of this species is reported to be from OH radicals in the literature under different experimental conditions (see, for instance, Pan et al. 2004; Zheng, Jewitt & Kaiser 2006, Cuppen et al. 2010). Indeed, reaction (6) has been proposed: OH + OH H 2 O 2 (6) However, our results suggest that when mixtures containing diluted water into a RG are subjected to the discharge, reaction (2), leading to the formation of water, takes place instead of reaction (6). Thus, reactions (2) and (6) are probably competitive routes, and the dilution of water prior to the discharge is probably a necessary prerequisite to observe reaction (2) instead of reaction (6). This may explain why the formation of water from OH radicals has not been reported previously. Furthermore, ozone (O 3 ) (1032 cm 1 ) was not formed during these experiments with dilute water mixtures. More precisely, we noted during different experiments (results not shown here) that a high concentration of water in the mixture subjected to the discharge

5 OH radicals in water ice 3111 Figure 4. A discharged water/he = 10/100 mixture was deposited at 3.5 K during (a) 15 min; and (b) 30 min. This figure also shows the IR spectra of the samples thus generated in the ranges (A) cm 1 and (B) cm 1. is needed to observe the formation of O 3. In-depth studies on this specific point are presently being carried out in our laboratory. 4 INFLUENCE OF THE TEMPERATURE 4.1 Study of a water/he mixture at different temperatures According to the results presented up to now, an efficient way to trap OH radicals in water ice is to condense species formed when a water/he = 10/100 mixture (10 per cent of water) is subjected to the microwave discharge. This procedure was repeated at higher temperatures, in order to determine whether the amount of radicals trapped in the ice is temperature-dependent or not. Fig. 5 clearly shows that the amount of water ice formed is increasing with the temperature, whereas the quantity of OH radicals is decreasing when the temperature is increasing. The spectra obtained prove that at higher temperatures, the OH radicals formed during the microwave discharge react to form water ice. This series of experiments also show that at 30 K almost all the OH radicals disappeared. Thus, the best temperature to observe a high proportion of OH radicals seems to be 3.5 K. Moreover, these temperature effects strongly suggest that the OH radicals formed under our experimental conditions are in their Figure 5. Spectra obtained when a discharged water/he =10/100 mixture is condensed at different temperatures: (a) 3.5 K; (b) 10 K; (c) 20 K; and (d) 30 K. ground state. Indeed, with no excess of energy, these OH radicals are sufficiently stable to be isolated in a water ice. The comparison between Figs 3 and 5(c) leads to a further remark concerning the nature of the RG. Indeed, these two spectra were obtained by condensing a discharged water/rg = 10/100 mixture at 20 K. The RG used for the dilution of water was Ne in the case of Fig. 3 and He in the case of Fig. 5(c). As a consequence, the comparison of these two spectra allows us to gain some more insight into the influence of the RG on the sample obtained. In the case of experiments in Ne, hydrated OH radicals are only observed as a bump in front of the water ice band. On the other hand, when a water/he mixture is subjected to the discharge, hydrated OH radicals are clearly observed. Thus, OH radicals obtained from a water/ne mixture are much more likely to react than OH radicals obtained from a water/he mixture. These results show that the cooling of the radicals is more efficient with He rather than with Ne. Thus, the lighter the gas used for the dilution, the softer the collisions between the RG and the radicals, and the smaller the kinetic energy gained by the radicals during the collisions. This may be explained as follows. Before the collision, the kinetic energy of the radicals formed during the discharge is given by KE lab = mv 2 /2. During the collision, the linear momentum of the system is not affected. To take into account this point, it may be easier to analyse the kinematics of the collision in the centre-of-mass frame. During the collision, the amount of energy that is converted into the internal energy of the partners is given by the relation KE rel = KE lab m RG /(m RG + m radical ). As a consequence, the lighter the RG, the smaller the increase in the internal energy of the radical during the collision. For a detailed discussion on this topic, see, for instance, Vekey (1996). 4.2 Influence of heating and annealing A sample containing OH radicals trapped in a water ice was further heated up to 10 K step by step (spectra shown in Figs 6a e). Under these conditions, no change was noted in the spectra obtained, which is proof that this heating does not lead to any reaction between the compounds spectroscopically observed. When the sample is further annealed up to 35 K, however, a gradual decrease in the peak attributed to (OH)(H 2 O) n with a concomitant increase in the band due to water ice is observed (spectra shown in Figs 6f j). Thus, when the sample is sufficiently heated, OH radicals can migrate and further react inside the water ice, leading to the formation of water ice.

6 3112 E.-L. Zins, P. R. Joshi and L. Krim Figure 6. Heating and annealing effects on a sample obtained at 3.5 K from a discharged water/he = 10/100 mixture. (a) Spectrum obtained at 3.5 K. The sample was then heated up to: (b) 5 K; (c) 6 K; (d) 8 K; and (e) 10 K. Annealing was then carried out: (f) 15 K 10 K; (g) 20 K 10 K; (h) 25 K 10 K; (i) 30 K 10 K; and (j) 35 K 10 K. The right-hand panel shows the evolution of the intensities of peaks corresponding to water, hydrated species and OH radicals within the temperature. 5 SUMMARY Experiments on the formation and isolation of OH radicals in water ice were performed. To this end, a preliminary study has shown that a peak of 3689 cm 1, in combination with the observation of a structured water ice band centred at 3400 cm 1,andCO 2,ischaracteristic of hydrated OH radicals. Under our experimental conditions, CO 2 is generated from reactions between CO, present as an impurity in the system, and OH radicals (Zins, Joshi & Krim, 2011). Gaseous pure water or a water/rg (RG = He or Ne) mixture was subjected to a microwave discharge, and the species thus formed were then condensed on the surface of a mirror at different temperatures. The influence of the temperature and the dilution with a RG has been investigated. The presence of He favoured the observation of hydrated OH radicals. This result is attributed to two different effects of the RG: a dilution of the radicals, leading to less probable radical/radical collisions, and a cooling effect of the radicals due to collisions between radicals and the RG. With a heavier RG, namely Ne, this cooling is less efficient. Moreover, and as expected, the concentration of OH radicals observed is higher when the mixture is deposited at lower temperatures after the microwave discharge. Based on these results, it is possible to draw three main conclusions: (i) a microwave discharge into gaseous water leads to the formation of OH radicals; (ii) with no cooling of the system, these radicals react to form water ice; and (iii) one efficient way to cool down the radicals formed and to prevent their reaction is to perform the microwave discharge in a water/he = 10/100 mixture and to condensate the species thus formed at 3.5 K. This experimental procedure will be used to further study the reactivity of OH radicals in water ice. ACKNOWLEDGMENT This work was supported in part by the SMART Federation from the Université Pierre et Marie Curie, Paris VI and the CMI (Chimie des Milieux Interstellaires) grant. REFERENCES Acquista N., Schoen L. J., Lide J. D. R., 1968, J. Chem. Phys., 48, 1534 Baud B., Wouterloot G. A., 1980, A&A, 90, 297 Bergeron H., Rougeau N., Sidis V., Sizun M., Teillet-Billy D., Aguillon F., 2008, J. Phys. Chem. A, 112, Chang Q., Cuppen H. M., Herbst E., 2007, A&A, 469, 973 Cheng B. M., Lee Y. P., Ogilvie J. F., 1988, Chem. Phys. Lett., 151, 109 Comes F. J., 1994, Angew. Chem., Int. Ed., 33, 1816 Cooper P. D., Kjaergaard H. G., Vaughan V. S., McKinley A. J., Quickenden T. I., Schofield D. P., 2003, J. Am. Chem. Soc., 125, 6048 Costa Cabral B. J., 2005, Int. J. Quant. Chem., 103, 610 Cuppen H. M., Ioppolo S., Romanzin C., Linnartz H., 2010, Phys. Chem. Chem. Phys., 12, Danset D., Alikhani M. E., Manceron L., 2005, J. Phys. Chem. A, 109, 97 Dozova N., Krim L., Alikhani M. E., Lacome N., 2006, J. Phys. Chem. A, 110, Frost M. J., Sharkey P., Smith I. W. M., 1993, J. Phys. Chem., 97, Garrod R. T., 2008, A&A, 491, 239 Herbst E., 2001, Chem. Soc. Rev., 30, 168 Krim L., Manceron L., Alikhani M. E., 1999, J. Phys. Chem. A, 103, 2592 Langford V. S., McKinley A. J., Quickende T. I., 2000, J. Am. Chem. Soc., 122, Lester M. I., Pond B. V., Anderson D. T., Harding L. B., Wagner A. F., 2000, J. Chem. Phys., 113, 9889 Miyauchi N., Hidaka H., Chigai T., Nagaoka A., Watanabe N., Kouchi A., 2008, Chem. Phys. Lett., 456, 27 Pan X. N., Bass A. D., Jay Gerim J. P., Sanche L., 2004, Icarus, 172, 521 Pirim C., Krim L., 2011, Chem. Phys., 380, 67 Pirim C., Krim L., Laffon C., Parent P., Pauzat F., Pilmé J., Ellinger Y., 2010, J. Phys. Chem. A, 114, 3320 Robinson B. J., McGee R. X., 1967, ARA&A, 5, 183 Ruffle D. P., Herbst E., 2000, MNRAS, 319, 837 Sennikov P. G., Ignatov S. K., Schrems O., 2005, Chem. Phys. Chem., 6, 392 Storey J. W. V., Watson D. M., Townes C. H., 1981, ApJ, 244, L27 Suzer S., Andrews L., 1988, J. Chem. Phys., 88, 916 Tinti D. S., 1968, J. Chem. Phys., 48, 1459 van Dishoeck E. F., Black J. H., 1986, ApJS, 62, 109 Vekey K., 1996, J. Mass Spectrom., 31, 445 Vidali G., Roser J. E., Ling L., Congiu E., Manico G., Pirronello V., 2006, Faraday Discuss., 133, 125 Watanabe N., Kouchi A., 2008, Prog. Surf. Sci., 83, 439 Zheng W., Jewitt D., Kaiser R., 2006, ApJ, 639, 534 Zins E. L., Joshi P., Krim L., 2011, ApJ, submitted This paper has been typeset from a Microsoft Word file prepared by the author.