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) sho w an inhibition of dissociation with increasing parallel momentum, whi ch has been ascribed to surface corrugation. Here, we present the resu lts of quantum dynamical calculations of the dissociation on a three-d imensional 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 fir st 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 approxi mate normal energy scaling in dissociation, thus explaining how the H- 2/CU PES can appear ''flat'' in sticking experiments, while total ener gy calculations show it to be highly corrugated.