Palladium clusters on graphite: A Bremsstrahlung Isochromat Spectroscopy study

We report a combined Bremsstrahlung Isochromat Spectroscopy (BIS) and XPS investigation of Pd clusters deposited on polycrystalline graphite. The density of the empty states close to the Fermi level is modified by subsequent Pd depositions until the continuous film is formed. With reducing cluster size, the d portion of the conduction band broadens and shifts to higher energy while its valence counterpart shows the opposite shift. We interpret the changes of both filled and empty states as the modification of the hybridized graphite π^* and Pd 4d electronic states during the formation of the metallic cluster. Charging effects are estimated and found to be negligible. The BIS results agree with X-ray absorption data where cluster particles are uncharged. Our analysis is consistent with the opening of a gap, across the Fermi level, in the d-density of states with decreasing cluster size.