Evolution of the electronic and geometric structure of size-selected Pt and Pd clusters on Ag(110) observed by photoemission

Mass-selected Ptₙ and Pdₙ (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 antibondinglike 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 Ptₙ (n=1--7) and Pdₙ(n=1--17) clusters on the Ag(110) surface, lying along the [1{}10] direction.