Walk on a Flickering Path: Tracer Diffusion of Adsorbed O Atoms on a Ru(0001) Surface in the Limit of CO Saturation

Abstract

Previous work has shown that tracer diffusion of adsorbed O atoms on a CO-covered Ru(0001) surface is driven by fluctuations of the CO clusters. Here, a study is presented on the question of whether this diffusion mechanism is suppressed at CO saturation. Experiments were performed using high-speed scanning tunneling microscopy (STM). Hopping rates of O atoms were extracted from atomic trajectories, and an activation energy was determined from temperature-dependent data. The energetics of the embedded O atoms in the CO layer was evaluated by density functional theory (DFT). DFT was also used to determine the structures occurring during the O/CO exchange process. CO at saturation forms densely packed 2D clusters on the Ru(0001) surface, which is known from previous work. The present STM data show that the trajectories of the O atoms are restricted to the narrow gaps between the CO clusters but that the atoms still move by randomly hopping between neighboring hcp sites. The molecular structures of the clusters change as the O atoms move. Surprisingly, the mobility is higher than at lower CO coverages. DFT shows that, as an O atom jumps from an hcp site to a neighboring hcp site via an intermediate fcc site, several CO molecules at the edges of the clusters are displaced, leading to low-energy hopping pathways for the O atoms. The O atoms walk on “flickering paths” driven by fluctuations of the CO clusters. CO displacements cost more energy than at the lower coverages, but this effect is overcompensated by a reduced hopping barrier of the O atoms at the high CO coverage.