Interlayer diffusion of adatoms: A scanning-tunneling microscopy study

Low coverages of Co are deposited at room temperature on a Pt(111) surface with vacancy islands providing a high density of steps. Scanning-tunneling microscopy images with chemical contrast show that Co atoms do not descend Pt steps by diffusing over the step, but descend from the upper terrace to the lower by an exchange diffusion process with the Pt atoms at the step ed ge. The Co atoms descend a Pt step edge by this process neither at straight A nor at B steps, but rather at the corners or kinks of the vacancy island s.