Quantum mechanical atom scattering from adsorbates at high and low coverage

A multiple scattering theory was developed for the quantum mechanical scattering of atoms from an adsorbate on a smooth surface. For low coverage random adsorption, the specular intensity is shown to fall nearly exponentially with coverage. A linear decrease, however, is consistent with island formation. In the former case, the total cross section for an isolated adatom can be obtained from measured specular intensity and the differential scattering cross section from elastic incoherent scattering. Numerical calculations were done using realistic probe-adatom potentials but with a hard wall potential representing the substrate. The long range attractive probe-adatom interaction was found to make an important contribution to the scattering cross section. The energy dependence of the total cross section shows quantum mechanical interference oscillations sensitive to both the attractive and repulsive potential. Excellent agreement was found with the measured total cross section for He scattering from physisorbed Xe using the gas phase He-Xe potential. The scattering of H and He atoms from periodic Xe and Kr monolayers on graphite was calculated. Generally, atom-surface interaction potentials are not well known. In this case, however, a very good approximation can be obtained by summing over the rather well known probe-atom-adatom pair interactions