Thermal and non-thermal processes of simple molecules on model interstellar ices
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Thin film growth and desorption behaviour of simple molecules have been studied by means of surface science techniques, such as mass spectrometry and reflectionabsorption infrared spectroscopy (RAIRS), in order to understand the physiochemical processes and intermolecular interactions in model interstellar ices. The systems of interest comprise a silica surface, representing the bare grains in the interstellar medium, and films of water (H2O), methanol (CH3OH), diethyl ether ((CH3CH2)2O) and benzene (C6H6). While H2O and CH3OH are key components of the icy mantles, (CH3CH2)2O and C6H6 are found in lower abundances being two products, among many, of the rich chemistry occurring in these environments. Temperature programmed desorption and IR signatures of pure solid H2O, CH3OH, and (CH3CH2)2O adsorbed on amorphous silica were compared as a function of surface coverage and temperature. H2O and (CH3 CH2)2O display opposite behaviours, consistent with two-dimensional island formation and wetting of the amorphous silica surface respectively. CH3OH, being intermediate between the two species, exhibited aspects of both behaviours. Temperature programmed RAIRS has revealed evidence for thermal activation of di↵usion of H2O over the amorphous silica surface between 40 K and 60 K, and of CH3OH between 20 K and 40 K, while no conclusive evidence was found for such with (CH3CH2)2O. Experiments have been performed to study the thermal desorption and the IR features of C6H6 on CH3OH and (CH3CH2)2O solids in comparison to those on a solid H2O substrate at 110 K. The results give a clear picture of the C6H6 film growth from low to high coverages. Ab initio quantum chemical calculations highlight the key interactions between the two species for each system, C6H6/H2O, C6H6/CH3OH and C6H6/(CH3CH2)2O, in support of the interpretation of the data. Building on this basis, 250 eV electron irradiation of C6H6 on thick ices of H2O, or CH3OH, or (CH3CH2)2O was investigated to demonstrate the crucial role of hydrogen-bonding in propagating electronic excitation to the solid-vacuum interface where C6H6 desorption can occur. Competitive electron-induced chemistry in the form of molecular hydrogen (H2) formation was also observed. The electron beam used in the these experiments is inelastically scattered by the molecules in the solid ices forming a similar flux of electrons to that associated to cosmic rays. Conclusions related to the impact of these observations on the early phase of icy interstellar grain chemistry are discussed.