Scattering and trapping dynamics of gas-surface interactions:: Theory and experiments for the Xe-graphite system

We report on molecular beam experiments and molecular dynamics simulations of xenon scattering with incident energies E = 0.06- 5.65 eV from graphite. The corrugation felt by an atom interacting with the surface is found to be influenced by both surface temperature, T-s, and E. Angular distributions are significantly broadened when T-s is increased, clearly indicating corrugation induced by thermal motion of the surface also at the highest E employed. Direct scattering dominates for high E, while trapping becomes important for kinetic energies below 1 eV. The coupling between atom translation and surface modes in the normal direction is very effective, while trapped atoms only slowly accommodate their momentum parallel to the surface plane. The very different coupling normal and parallel to the surface plane makes transient (incomplete) trapping-desorption unusually pronounced for the Xe/graphite system, and atoms may travel up to 50 nm on the surface before desorption takes place. The nonlocal and soft character of the Xe-graphite interaction compared to interactions with close packed metal surfaces explains the observed high trapping probabilities and the lack of structural corrugation effects at high kinetic energies. Experimental results and simulations are in good agreement for a wide range of initial conditions, and we conclude that the model contains the most essential features of the scattering system. (C) 1998 American Institute of Physics. [S0021-9606(98)70447-7].