Steps and the surface dissociation of metal dimers: Direct observations on Ir(111)

Lattice steps on metal surfaces are known to enhance the dissociation rate of diatomic gases and also of large metal clusters. To establish the effectiveness of steps in promoting dissociation of metal dimers, we use the field ion microscope to compare the behavior of individual iridium dimers ${\mathrm{Ir}}_2$ on the close-packed (111) plane of iridium with that at lattice steps bounding this plane. On the perfect (111) plane, dissociation takes place at ∼250 K; at sites at lattice steps, either of type A or B, dissociation occurs under similar conditions but at a rate three times faster than at interior sites.