Anisotropy of the physisorption interaction betweenH2and metal surfaces

The orientational dependence of molecule-surface interaction has been studied both experimentally and theoretically. Rotational substrate splittings of corrugation-mediated selective adsorption resonances observed for ${\mathrm{H}}_2$ molecular beams scattered from a Cu(100) surface are compared with predictions from the standard theory of physisorption. We find that the measured anisotropy is dominated by the van der Waals interaction and that the repulsive contribution is much smaller than anticipated. We show that an extension of the theory that includes interaction of the metal electron states with the antibonding resonance of ${\mathrm{H}}_2$ strongly reduces the repulsive anisotropy and brings theory into agreement with experiment.