Photochemistry of CH3I···(H2O)n Complexes: From CH3I···H2O to CH3I in Interaction with Water Ices and Atmospheric Implications

The interaction of methyl iodine with the surface of amorphous, cubic, and hexagonal ices has been investigated. The CH₃I desorption process has also been evaluated. We have highlighted the difference in CH₃I behavior depending on the trapping on the ice surface. The broadband UV photochemistry of CH₃I trapped at the surface of ices has been studied. Those results have been compared with UV broadband photochemistry of CH₃I bare monomer complexed with water and trapped in argon cryogenic matrices. It appears that if CH₃I interacts with water molecules or water ice by hydrogen bonding, then CH₃I does not fragment under UV irradiation. Thus, energy transfer to the network of hydrogen bonds in the ice or matrix is effective. On the other hand, to a first approximation, if CH₃I interacts by picnogen-type bonding (I···O), then CH₃I fragments, because the electronic relaxation seems to take place mainly at the intramolecular levels of CH₃I. Finally, we demonstrated that water ice does not catalyze the photofragmentation of CH₃I. Rather, it modifies the electronic relaxation paths, some of which lead to the fragmentation of iodomethane. This fundamental work provides an understanding of the molecular processes involved in water/ice–CH₃I interaction and the role of these molecular interactions on CH₃I photochemistry in the atmosphere.