Core Level Photelectron and X-Ray Absorption Spectroscopy of Free Argon Clusters: Size-Dependent Energy Shifts and Determination of Surface Atom Coordination

Core level photoelectron and x-ray absorption spectra for free argon clusters from $〈N〉\ensuremath{\approx}5$ to $〈N〉\ensuremath{\approx}4000$ are presented. Spectral features originating from surface and bulk sites of the clusters are identified. These are seen to shift with cluster size. From the development of the spectra from the isolated atom to the largest clusters, information about both the size-dependent cluster-specific electronic structure and that of the ``infinite'' solid is obtained. Using a simple model for the core level binding energy shifts, effective surface coordination numbers are derived. These range from 5.3 for $〈N〉\ensuremath{\approx}10$ to 8.5 for the solid.