Unoccupied electronic states of the Cs/Cu(100) and Cs/Cu(111) adsorption systems

We have investigated the unoccupied electronic structure of submonolayer amounts of cesium on the Cu(100) and Cu(111) surfaces by inverse photoemission (IPE) and work-function measurements. At very low Cs coverages on both substrates, IPE spectra obtained at normal incidence indicate that an adsorbate-induced feature develops in the projected bulk band gap of the substrate at an energy of similar to 3 eV above the Fermi level (E-F). As the Cs coverage is increased, this feature moves to lower energy. On the Cu(100) surface, the energy of the Cs feature initially follows the decrease of the work function, and levels off at similar to 0.3 eV above E-F as the Cs coverage approaches 1 ML. We observed a similar effect for the Cs/Cu(111) system. Off-normal spectra obtained for 0.08 ML of Ca on Cu(111) indicate that, at low coverages, the Cs state does not disperse with k(parallel to). However, on both substrates, we observed a correlation between modulations in the intensity of the Cs feature and degeneracy between this feature and the projected bulk bands of the substrate. Energy-dependent IPE spectra suggest that the adsorbate state on both Cu surfaces has predominantly d-like orbital character when the state is in the projected bulk band gap of the substrate. The orbital character of the Cs feature changes to sp-like when the adsorbate state is degenerate with the Cu sp-derived bulk bands. This effect was observed whether the degeneracy was achieved by varying the Cs coverage at a fixed k(parallel to), or by varying k(parallel to) at a fixed coverage. We interpret this as evidence of momentum-dependent hybridization of the adsorbate-substrate bond.