Resolving atomic motions on a crowded surface
Using a combined high speed-variable temperature STM we have resolved the diffusion of adsorbed atoms on a surface completely covered by co-adsorbed molecules. This process, diffusion on a crowded surface, is important in heterogeneous catalysis because it randomizes the adsorption layer and is thus the basis for the reaction kinetics. O atoms on a completely CO-covered Ru(0001) surface were observed to move across the surface almost as fast as on the bare ruthenium surface. This effect cannot be explained by one of the diffusion mechanisms known from 3D solids, namely vacancy diffusion, interstitial diffusion, direct exchange, ring exchange, etc. A new lattice diffusion mechanism is proposed in which fast density fluctuations in the adsorption layer frequently open low-energy pathways for the travelling atom. The mechanism may play a general role in heterogenous catalyis.