DYNAMICS OF MOLECULAR-SURFACE DIFFUSION - ORIGINS AND CONSEQUENCES OF LONG JUMPS

The mechanics of molecular surface diffusion have been studied in a theoretical model CO/Ni(111). Using molecular dynamics, diffusion rates have been calculated over a wide range of temperatures and interpreted using methods typically applied to experimental measurements. This interpretation is based on transition state theory and a model of uncorrelated hops between near neighbors. An Arrhenius plot of diffusion constants from simulations is linear from 175 to 1000 K. However, the underlying dynamics do not conform to the model of uncorrelated hops. Instead, molecules that have been excited to a transition state tend to fly past several sites before settling onto a new one. These multiple site flights ("long jumps") make the Arrhenius prefactor larger than the transition state theory prediction by more than an order of magnitude. Transition state recrossings have a small effect on the diffusion rate. Long jumps are typical of a "low friction" regime in which energy exchange is slow between lateral translation and other modes. Completely freezing the surface motion has a relatively small effect on flight lengths, and coupling of adsorbate lateral translations to other adsorbate modes is as important as coupling to the surface. The dependence of these results on details of the model is discussed and the frictional forces in this model are compared to other theoretical and experimental estimates of these forces.