Hydrogen adsorption on Ru(0 0 1) studied by scanning tunneling microscopy

The adsorption of hydrogen on Ru(0 0 1) was studied by scanning tunneling microscopy at temperatures around 50 K. Hydrogen was found to adsorb dissociatively forming different ordered structures as a function of coverage. In order of increasing coverage θ in monolayers (ML) these were (√3 × √3)R30° at θ = 0.3 ML; (2 × 1) at θ = 0.50 ML, (2 × 2)-3H at θ = 0.75, and (1 × 1) at θ = 1.00. Some of these structures were observed to coexist at intermediate coverage values. Close to saturation of 1 ML, H-vacancies (unoccupied three fold fcc-hollow Ru sites) were observed either as single entities or forming transient aggregations. These vacancies diffuse and aggregate to form active sites for the dissociative adsorption of hydrogen.