UNOCCUPIED ELECTRONIC STATES OF THE CS CU(100) AND CS/CU(111) ADSORPTION SYSTEMS/

We have investigated the unoccupied electronic structure of submonolay er amounts of cesium on the Cu(100) and Cu(111) surfaces by inverse ph otoemission (IPE) and work-function measurements. At very low Cs cover ages on both substrates, IPE spectra obtained at normal incidence indi cate 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 Fe rmi level (E-F). As the Cs coverage is increased, this feature moves t o lower energy. On the Cu(100) surface, the energy of the Cs feature i nitially 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 obs erved a similar effect for the Cs/Cu(111) system. Off-normal spectra o btained 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 s ubstrates, we observed a correlation between modulations in the intens ity of the Cs feature and degeneracy between this feature and the proj ected bulk bands of the substrate. Energy-dependent IPE spectra sugges t that the adsorbate state on both Cu surfaces has predominantly d-lik e orbital character when the state is in the projected bulk band gap o f the substrate. The orbital character of the Cs feature changes to sp -like when the adsorbate state is degenerate with the Cu sp-derived bu lk 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 mo mentum-dependent hybridization of the adsorbate-substrate bond.