Potential-energy surface for H-2 dissociation over Pd(100)

The potential-energy surface (PES) of dissociative adsorption of H-2 on Pd(100) is investigated using density-functional theory and the full-potential linear augmented plane-wave method. Several dissociation pathways are identified which have a vanishing energy barrier. A pronounced dependence of the potential energy on ''cartwheel'' rotations of the molecular axis is found. The calculated PES shows no indication of the presence of a precursor state in front of the surface. Both results indicate that steering effects determine the observed decrease of the sticking coefficient at low energies of the H-2 molecules. We show that the topology of the PES is related to the dependence of the covalent H(s)-Pd(d) interactions on the orientation of the H-2 molecule.