COVERAGE-DEPENDENT SURFACE-DIFFUSION EXPECTED FROM A MULTIPLE-SITE HOPPING MODEL

Monte Carlo simulations have been employed to examine the coverage-dependent surface diffusion expected from a multiple-site hopping model. The Monte Carlo simulations modeled the prepare-and-probe laser-induced thermal desorption (LITD) experiment used to measure surface diffusion on macroscopic single-crystal surfaces. For multiple-site hopping, the simulated refilling curves for the diffusion experiment revealed a strong decrease in the surface diffusion coefficient versus coverage. This coverage-dependent decrease was more pronounced for larger jump lengths. As coverage increased from theta/theta-s = 0.01 to 1.00, the surface diffusion coefficient decreased by factors of 6 and 23 for jump lengths of r = 4 and r = 8 sites, respectively. The magnitude of the surface diffusion coefficient was also proportional to the square of the jump length at low coverage. Assuming a multiple-site hopping mechanism, the expected coverage dependence of the surface diffusion coefficient was calculated for various jump lengths. These results can be utilized to obtain a jump length for a system that displays a coverage-dependent surface diffusion coefficient. A jump length of sixteen sites was estimated for the coverage-dependent surface diffusion coefficient of neopentane on Ru(001).