Surface distribution of Cu adatoms deduced from work function measurements

We report on work-function changes due to Cu impurities on 1-ML Xe adsorbed on highly oriented pyrolytic graphite, as determined by normal-emission ultraviolet photoelectron spectroscopy. The dipole moment and polarizability of a single Cu impurity are deduced from the measurements within the Topping model. The results indicate that Cu atoms adsorb as isolated impurities, a picture which is supported by density-functional calculations. This finding confirms an earlier assumption in the scenario for the appearance of a Fano resonance in valence-band photoemission spectra.