THEORY OF H-BONDING TO A PSEUDOMORPHIC MONOLAYER OF Ni ON A W(001) SURFACE.

Self-consistent Linearized Augmented Plane Wave total energy calculations have been performed for H adsorption on a thin film comprised of a Ni monolayer adsorbed pseudomorphically on a W(001) substrate. Because Ni atoms are considerably smaller than W atoms, the preferred binding site for H is found to be a 2-fold bridge rather than a 4-fold hollow site. The binding energy per H atom is more than 0.17eV greater for two H atoms per surface unit cell in the bridge configuration than for 1 H per cell in the hollow site geometry. The symmetric stretch vibration frequency in the bridge site at the center of the surface phonon Brillouin zone and its first overtone are predicted to be 67meV and 139meV, respectively. The barrier to penetration of the H's into the subsurface region is found to be greater than 1eV per H atom, based on the rather crude assumption that an entire monolayer of H's penetrates the surface simultaneously along surface normals through bridge sites.