Diffusion by chemical bond hopping of single O2 and H on Si(111)-7×7 surfaces

Using a variable temperature STM to trace in detail the path of single particle movement, it is possible to derive diffusion parameters of individual atoms and molecules on solid surfaces as well as to probe the mechanisms. Below ˜370 °C, O₂ molecules adsorb on Si(111)-7×7 surfaces at the top site of Si-adatoms as bright image spots. An O₂ molecule can hop between two adatom sites within the half unit cell it adsorbs via two rest-atom sites. Above this temperature, it can either hop out of the half cell, or can go through other reaction pathways. In contrast, for H atoms, the adsorption sites are rest-atom sites. An H atom darkens the rest-atom in filled state image, but the surrounding adatoms will appear brighter because of a reverse charge transfer. Above ˜280 °C, it can hop to a neighbor rest atom site within the half cell via an adatom site. The adatom in the short lived intermediate state appears darker because of the saturation of its dangling bond. Above ˜340 °C, it can hop out of the half cell via two adatom sites. Thus diffusion of H and O₂ on this surface is achieved by hopping of chemical bonds via intermediate states. We have also derived site and pathway-specific activation energies and frequency factors and the potential energy curves for the hopping of O₂ and H on Si(111)-7×7 surfaces.