Vacuum Ultraviolet Photolysis of Condensed Methyl Chloride in Interstellar Model Conditions and Trapping of Intermediates at Intergrain Interfaces

Photochemistry can generate chemical complexity in an interstellar space. This may occur due to photolysis and associated events that can happen in condensed molecular solids under the prevailing temperature and pressure conditions. In the present study, using reflection absorption infrared spectroscopy (RAIRS), three different condensed phases of methyl chloride ice, namely, amorphous, crystalline, and polycrystalline, were detected in ultrahigh vacuum (UHV) (p = 5 × 10–10 mbar) and cryogenic conditions (T = 10–90 K). Upon vacuum ultraviolet (VUV) photoirradiation, crystalline methyl chloride formed more photoproducts than amorphous and polycrystalline forms. This unusual finding is attributed to the rapid diffusion and reaction of photochemical intermediates in a crystalline matrix, whereas the intermediates are trapped at grain boundaries in the polycrystalline solid. Normally, the intermediate diffusion is high in the case of the amorphous phase as compared with the crystalline phase. During long-term irradiation, discontinuity in the polycrystalline phase is removed, resulting in enhancement in the formation of photoproducts in the matrix, which was observed by the intensified desorption of photoproducts in temperature-programmed desorption mass spectrometry (TPD-MS). Further, all major and minor neutral photoproducts were detected by highly surface-sensitive Cs+ ion-based secondary ion mass spectrometry (SIMS).