Surface diffusion in the low-friction limit: Occurrence of long jumps

We present a molecular dynamics (MD) study of a Brownian particle in a two-dimensional periodic potential. For a separable potential? the study of the diffusion constant along the symmetry directions reduces to two one-dimensional problems. In this case, our MD study agrees with the existing analytical results on the temperature and the friction (eta) dependence of the diffusion constant (D). For a nonseparable and anisotropic potential such as the adsorption potential on a bcc(110) surface, the present study predicts an alternative D similar to 1/eta(0.5) dependence in the low friction regime as opposed to the D similar to 1/eta dependence found in previous studies of one-dimensional or separable potentials. We find that the dependence of D on eta in the low friction regime is directly related to the occurrence of long jumps. The probability for the long jumps depends not only sensitively on the value of the friction but also on the geometry of the surface. On the bcc(110) surface, the path connecting adjoining adsorption sites does not coincide with the direction of easy crossing at the saddle point. Consequently, the probability of deactivation is enhanced, leading to the reduction of long jumps and the different dependence of D on eta.