Empty electronic states of graphite and the growth of Au and Pd clusters

The empty electronic states of Au and Pd clusters grown on the basal plane of cleaved, highly oriented pyrolytic graphite have been probed with inverse photoemission. Momentum-resolved inverse photoelectron spectroscopy of the clean surface prior to metal deposition shows an interlayer band and several nondispersing features at $E_F$+3.5eV, $E_F$+9.5 eV, and $E_F$+13.1 eV, which have been attributed to the empty states of graphite. Fixed photon-energy features at 14.7, 18.8, and 20.3 eV are associated with graphite plasmon luminescence, and resonant enhancement of empty-state features is observed. Thereafter, the coverage-dependent inverse photoemission spectra for cluster growth show that hybridization between the initially-empty s and the filled metal d states increases with cluster size as the d character evolves from the $d^{10}$ configuration of the isolated atom to a more bandlike configuration. This hybridization is significant after the deposition of 2 Å of Pd or 4 Å of Au. At higher coverage, we show the convergence of the empty states to those representative of the bulk metals. These results indicate that the electronic structure near the Fermi level is significantly affected by metal cluster size in the early stage of metal deposition. This dependence on the cluster sizes is estimated by using a simple growth model.