Core level photoionization spectroscopy of supported metal clusters: Cu 55 on silica

Abstract

Inner-shell photoabsorption, photoelectron and Auger spectra of mass-selected Cu 55 clusters are presented which have been deposited onto the natural silica surface layer of a doped Si-wafer. The core level spectra of the Cu 55-cluster reveal distinct differences with respect to the bulk. Moreover, a non-fcc like structure of deposited Cu 55 clusters can be concluded from the L 3 photoabsorption spectrum. Upon airation of the deposited clusters an oxidation becomes very prominent in the absorption spectra. Both the formation of Cu 2+ and Cu 1+ ions are evident. Under continuous X-ray irradiation the Cu 2+ ions are photoreduced to Cu 1+ as seen by the changes in the photoabsorption spectrum as function of time. The 2 p 3/2 XPS line of the Cu 2+ ions of oxidised Cu 55 shows a positive binding energy shift while almost no shift is observed for the Cu 1+ ions. The Auger spectrum of oxidised Cu 55 is shifted to smaller kinetic energy compared to pristine Cu 55 and bulk.