Dynamics of surface-aligned photochemistry (theory). 4. H atom reactions in the HBr(ad)/LiF(001) + hν system

This paper reports a classical trajectory study of surface-aligned photochemistry in HBr(ad)/(LiF(001) using the detailed interaction potential developed in an earlier study (Polanyi et al. J. Chem. Phys. 1991, 94, 978. Exchange reaction, H + H′Br → HBr + H′, and the minor pathway of abstraction reaction, H + H′Br → HH′ + Br, were incorporated in the trajectories by means of a London-Eyring-Polanyi-Sato function. The energy of the reactant H was, in general, E_xs = 2.6 eV. Marked changes in angular and energy distributions of the scattered H and H′ were found with increasing coverage in the range 0.2-1.0 ML (monolayer). This was related to altered dynamics, especially to the increasing importance of collisions with coadsorbate at higher coverage resulting in energy loss and in scattering nearer to the surface normal (termed "channeling"). It was predicted that at low coverages the products of exchange reaction induced by polarized light would show an angular anisotropy that mirrored the collision geometries: bent or collinear. Reaction probability was found to increase with coverage, saturating at ∼0.5 ML. This saturation was found to be due to a marked 2D "surface-aligned caging" effect (SAC). Reducing the surface temperature increased the reaction probability at low coverage due to increased alignment. By contrast, reducing the surface temperature at higher coverage (≥0.5 ML) decreased the reaction probability due to more pronounced SAC. Exchange reaction exhibited a higher threshold energy in the adsorbed state as compared with gas, due to the less than optimal surface-aligned collision geometry in the present system.