Evolution of metallic screening in small metal clusters probed by PCI-Auger spectroscopy.

Excitation-energy dependent Auger spectra of small copper clusters supported by a thin silica layer have been measured as function of cluster size. The Auger kinetic energy of the clusters clearly changes with the excess energy of the emitted photoelectron while not for the bulk. The kinetic energy shift is attributed to post-collision interaction (PCI) and exhibits a reduced metallic screening ability of small Cu-clusters. The spectroscopic data reveal an evolution from a long-range Coulomb-like interaction to a short-range "screened" electrostatic interaction within the sub-nm range. The data show that core electron spectroscopy such as PCI-Auger measurements can be used as a general tool to follow the metallic character of supported clusters.