Photodissociation dynamics of CH₃I adsorbed on MgO(100): Theory and experiment

Theoretical and experimental results are compared for the 257 nm photolysis of methyl iodide adsorbed on an MgO(100) crystal. Molecular-dynamics calculations treat CH₃I as a pseudodiatomic molecule and describe the geometry and the vibrational and librational frequencies of ground state CH₃I on the surface of a solid at 125 K. The simulations modeled the photodissociation dynamics of the adsorbed species. The photoexcitation of CH₃I at 257 nm is to the ³Q₀ state which is, in turn, coupled to the ¹Q₁ state. The electronic surface coupling allows for two dissociation pathways, producing either ground- or excited-state iodine atoms in concert with ground-state methyl radicals. The I*/I branching ratio and the veloc-ity and angular distributions of both photofragments are predicted by the theory. A comparison is made between these predictions and experimental observation of the I*/I branching ratio, the velocity distribution of the methyl fragment, and the internal state distribution of the methyl. A substantial lowering of the I*/I ratio as compared to data from the gas-phase photodissociation studies is both predicted by theory and seen experimentally. Theoretical simulations attribute this change to efficient trapping of the I* photofragments by the surface. Further comparisons between the theoretical predictions and the experimental data support a model where the molecule is aligned perpendicular to the surface and the escape of iodine atoms from the surface following the photodissociation of adsorbed methyl iodide involves collisions with the surface.