Ordered hydrogen overlayers on metal surfaces

Abstract

The formation of ordered hydrogen overlayers on the close-packed surfaces of fcc and hcp metals is investigated. We consider hydrogen adsorption on seven metals (Ti, Co, Ni, Cu, Ru, Rh, and Pd) and we assume that the hydrogen atoms are adsorbed in the threefold hollow sites. We examine the question of site preference between the fcc and hcp sites and find substantial differences in binding energies between the two sites for all but one of these metals (Ni). The interaction energies between adatoms are then calculated. It is found that the multiple-adatom interactions can, to a high degree of accuracy, be written as the sum of the relevant pair interactions. Short-range interactions involving adatoms located at nearest-neighbor sites are found to be extremely repulsive indicating that nearest-neighbor exclusion is a prerequisite in the choice of the ordered overlayer. Long-range interactions (up to sixth neighbor) are found to be non-negligible and play an important role in determining which ordered overlayer is most likely to be formed. Information regarding the stability of different structures was obtained from considerations of the ground-state energies of a number of plausible ordered structures. The results for each metal are analyzed and compared to experiment, whenever possible. Excellent agreement with experiment is found for H/Ni(111) but a less satisfactory state of affairs is recorded for H/Pd(111).