Photochemistry of CH₃I(H₂O) _n Complexes: From CH₃IH₂O 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 (IO), 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.