Theoretical modeling of photodissociation dynamics of CH3I on LiF(001)

Abstract

A new method is developed for the simulation of atoms and molecules interacting with ionic surfaces. This approach, based on a quasi-two-dimensional Ewald sum and a two-dimensional Fourier transformation, is capable of evaluating the long-range Coulomb interactions for a semi-infinite ionic solid. We have applied this method to investigate the photodissociation dynamics of CH3I on a LiF(001) surface. All the degrees of freedom of the adsorbed molecule are considered and the excited state dissociation potentials of CH3I are described by analytical functions derived from a recent ab initio calculation. The substrate (LiF) is represented by 6×6×3 movable atoms surrounded by static ions at their equilibrium positions. The adsorbate/substrate interaction is modeled as a sum of Coulomb and Lennard-Jones pairwise potentials. A phenomenological term is introduced to account for the adsorbate/adsorbate interaction. The equilibrium configurations of the system at a given temperature are obtained by a Monte Carlo method, which shows that there exist two stable configurations with the CH3I molecular axis perpendicular to the surface, either methyl up or down. The dissociation dynamics of the adsorbate is studied with a classical molecular dynamics method and the angular, kinetic energy, and rovibrational distributions of the fragments are calculated. When the molecule is adsorbed with the methyl up, the methyl fragment dissociates into the vacuum promptly with kinetic energy and internal state distributions similar to those in the gas phase. If the molecule is adsorbed with the methyl down, however, the methyl fragment could collide with iodine after rebounding from the surface, transferring a significant amount of kinetic energy to the iodine. A much broader and more energetic kinetic energy distribution of the iodine fragments is observed under such circumstances. The energy transfer is most effective when the parent molecule orients parallel to the surface normal and decreases as the angle deviates from this direction. We also observed a substantial increase in the rotational angular momentum of the methyl fragment and a cooler vibrational distribution for the umbrella mode as a result of the collision.