Wave packet studies of gas–surface inelastic scattering and desorption rates

Abstract

A previously formulated semiclassical wave packet method is used to investigate the importance of different surface phonon modes and the Debye surface temperature upon inelasticity in atomic gas–surface collisions. Desorption rates are calculated as a function of potential-well depth and the rate law for the process is examined. The incident beam is represented by a quantum mechanical wave packet whose momentum distribution is nearly square. This wave packet is coupled to a three-dimensional model lattice through a time-varying potential field obtained by solution of the classical motion equations for the lattice. Calculated final-state momentum and energy distributions are found to be strongly dependent upon the particular surface phonon mode into which the initial lattice energy is partitioned. In general, energy transfer occurs predominantly to and from those modes for which the lattice atom in the impact region have motion in the direction of the momentum vector of the incoming wave packet. The inelasticity of the collision is found to increase as the lattice force constants and the surface Debye temperature decrease. The peak spacings in the final-state momentum and energy distributions are found to correlate well with the surface phonon frequencies. Desorption is found to be well described by a first-order rate law for small potential-well depths. For larger well depths, the first-order decay plots begin to show an increasing amount of curvature. Desorption rate coefficients obtained from the slopes of the decay plots show an approximate exponential dependence upon the potential-well depth.