A quantitative analysis of tracer diffusion was made of the adsorption of an isolated ethane molecule on Pt(111). In particular, deviations between the tracer diffusion of ethane in the simulations and the assumptions of the nearest-neighbor adsorbate-hopping model were examined at temperatures for which the kinetic energy of the molecule approached and exceeded the diffusion-barrier energy. The method of analysis could be implemented experimentally, using techniques such as scanning-tunnelling microscopy. It was shown that the adsorbate-hopping model could not accurately describe tracer diffusion at any of the temperatures probed. This was because ethane exhibited very long flights, with flight times that were not negligible compared to the time required for the molecule to escape from a binding site. A formula was proposed for the diffusion coefficient that included the influence of non nearest-neighbor jumps with non-negligible flight times. In the limit of low temperatures, this expression reduced to a hopping model while, at high temperatures, the model predicted that the diffusivity became analogous to that for a two-dimensional gas. It was shown that the model quantitatively described the tracer diffusion of ethane on Pt(111) in molecular-dynamics simulations over a wide temperature range, spanning both localized and non-localized adsorption.

Tracer-Diffusion Coefficients for Both Localized and Non-Localized Adsorption: Theory and Molecular-Dynamics Simulation. Raut, J.S., Fichthorn, K.A.: The Journal of Chemical Physics, 1995, 103[19], 8694-704