Core level photoionization spectroscopy of supported metal clusters: Cu55 on silica

Inner-shell photoabsorption, photoelectron and Auger spectra of mass-selected Cu55 clusters are presented which have been deposited onto the natural silica surface layer of a doped Si-wafer. The core level spectra of the Cu55-cluster reveal distinct differences with respect to the bulk. Moreover, a non-fcc like structure of deposited Cu55 clusters can be concluded from the L3 photoabsorption spectrum. Upon airation of the deposited clusters an oxidation becomes very prominent in the absorption spectra. Both the formation of Cu2+ and Cu1+ ions are evident. Under continuous X-ray irradiation the Cu2+ ions are photoreduced to Cu1+ as seen by the changes in the photoabsorption spectrum as function of time. The 2p3/2 XPS line of the Cu2+ ions of oxidised Cu55 shows a positive binding energy shift while almost no shift is observed for the Cu1+ ions. The Auger spectrum of oxidised Cu55 is shifted to smaller kinetic energy compared to pristine Cu55 and bulk.