An infrared-spectroscopic study of methyl iodide and methylene iodide on Ag(111)

The adsorption of methylene iodide and methyl iodide is studied on Ag(111) using reflection-absorption infrared spectroscopy. Molecular methylene iodi de adsorbs in a random orientation at 80 K and converts into a species with the H-C-H plane adsorbed perpendicular to the surface on heating to 155 K, and also dissociates into iodomethyl species. These undergo further decomp osition eventually to form ethylene, although no methylene species are dete cted on the surface. Methyl iodide forms both multilayers and a metastable state on Ag(111) at 180 K, where the C-I axis of the metastable state is or iented nearly parallel to the surface. Infrared data suggest that the metas table state both desorbs almost simultaneously with the multilayer and conv erts to adsorbed methyl iodide with the C-I axis oriented nearly perpendicu lar to the surface. The latter species decomposes into adsorbed methyl frag ments and adsorbed iodine where the frequency of the methyl bending mode fo r CH3(ads) and for CH3I(ads) are identical at 1230 cm(-1).