THE ROLE OF PARALLEL MOMENTUM IN THE DISSOCIATIVE ADSORPTION OF H-2 AT HIGHLY CORRUGATED SURFACES

Molecular beam experiments of H-2 adsorption on Pt, Fe and Ni(100) show an inhibition of dissociation with increasing parallel momentum, which has been ascribed to surface corrugation. Here, we present the results of quantum dynamical calculations of the dissociation on a three-dimensional potential energy surface with strong corrugation across the unit cell. We show that there are two types of corrugation; energetic, where the barrier height varies across the unit cell, and geometric, where the barrier location above the surface is modulated. In the first case, dissociation is inhibited by momentum parallel to the surface, while in the second, off-normal incidence enhances dissociation. We demonstrate that a combination of the two is capable of giving approximate normal energy scaling in dissociation, thus explaining how the H-2/CU PES can appear ''flat'' in sticking experiments, while total energy calculations show it to be highly corrugated.