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

Abstract

The interaction of methyl iodine with the surface of amorphous, cubic, and hexagonal ices has been investigated. The CH 3 I desorption process has also been evaluated. We have highlighted the difference in CH 3 I behavior depending on the trapping on the ice surface. The broadband UV photochemistry of CH 3 I trapped at the surface of ices has been studied. Those results have been compared with UV broadband photochemistry of CH 3 I bare monomer complexed with water and trapped in argon cryogenic matrices. It appears that if CH 3 I interacts with water molecules or water ice by hydrogen bonding, then CH 3 I 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 CH 3 I interacts by picnogen-type bonding (I•••O), then CH 3 I fragments, because the electronic relaxation seems to take place mainly at the intramolecular levels of CH 3 I. Finally, we demonstrated that water ice does not catalyze the photofragmentation of CH 3 I. 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− CH 3 I interaction and the role of these molecular interactions on CH 3 I photochemistry in the atmosphere.