EVOLUTION OF THE ELECTRONIC AND GEOMETRIC STRUCTURE OF SIZE-SELECTED PT AND PD CLUSTERS ON AG(110) OBSERVED BY PHOTOEMISSION

Mass-selected Pt(n) and Pd(n) (n = 1-15) clusters, generated by ion bombardment, are deposited at room temperature in submonolayer quantities on an Ag(110) single-crystal surface and characterized by electron spectroscopy (x-ray-photoemission spectroscopy and ultraviolet photoemission spectroscopy). The monodispersed clusters indicate individual discrete electronic structure features of the Pt 5d and Pd 4d emission. For the monomers virtual bound-state formation as in dilute (3%) PtAg and PdAg alloys is observed. With increasing cluster size, first, the increasing splitting between the bondinglike and anti-bondinglike d states reflects the increase in cohesive energy due to molecular interaction and second, the shift of the center of gravity of the d emission towards the Fermi energy indicates the trend to metal formation. The Pt 4f and Pd 3d core levels show practically no shift with increasing cluster size indicating a two-dimensional cluster structure. Total-energy calculations, performed within this study using the embedded-atom method, suggest a chain structure for small Pt(n) (n = 1-7) and Pd(n)(n = 1-17) clusters on the Ag(110) surface, lying along the [110BAR] direction.