A MECHANISM FOR THE QUENCHING OF I-STAR IN PHOTODISSOCIATION OF METHYL-IODIDE ADSORBED ON A MGO SURFACE

We report a time-dependent wave packet study of non-adiabatic transitions in the photodissociation of methyl iodide adsorbed on a MgO surface. The dissociation dynamics is characterized using a two-degree-of-freedom model in which the CH3 group is replaced by an atom, and the CH3I is assumed to be perpendicular to the surface with CH3 either up or down. The surface is taken to be rigid. Two coupled, excited, potential surfaces, correlating to I and I*, were used. When CH3 is up, the dissociation dynamics is very similar to that in the gas phase. When CH3 is down, chattering of the CH3 between I and the surface results in an important second crossing of the intersection between the two excited surfaces, leading to partial quenching of the excited iodine atom. This result provides one possible mechanism for recent surface experiments in which the I* yield was found to be lower than in gas phase CH3I dissociation.