DYNAMICS OF ADATOMS ON SOLID-SURFACES

A microscopic theory for the diffusive and vibrational motion of adsorbed atoms on a solid surface is presented. We evaluate and analyze the dynamical structure factor and the velocity autocorrelation function. the quasielastic scattering peak in the dynamical structure factor is found to consist of mixed contributions from diffusive and vibrational motion. The vibrational peak indicates a shift and broadening from anharmonic effects and frictional damping. As the temperature is lowered the diffusion constant crosses over from a continuous Brownian motion behavior to a thermally activated Arrhenius form. When the frictional coefficient is varied at low temperatures, the diffusion constant crosses over smoothly from the multisite jumping regime at low friction, via the transition-state regime, to the high-friction overdamped regime. Applied to Na on the Cu(001) surface, our theoretical predictions on the quasielastic-peak width and the temperature dependence of the shift of the vibrational mode agree quantitatively with recent experimental measurements. For adatoms on anisotropic bcc (110) surfaces, we also study the diffusion anisotropy as well as the coupling of the two frustrated translational modes.