Momentum-dependent orbital character of unoccupied Cs-induced levels on Cu(100) and Cu(111)

We present inverse photoemission spectra from the Cs/Cu(100) and Cs/Cu(111) systems, obtained with a photon-energy dispersive grating spectrograph. On both substrates, the intensity of a Cs-derived unoccupied electronic state increased with increasing excitation energy in regions of the surface Brillouin zone (SBZ) which contained a projected bulk band gap of the substrate. However, when the Ca feature was degenerate with the substrate sp-derived bulk band, the intensity of the Ca feature decreased with increasing excitation energy. We interpret this as evidence for a change in the orbital character of a Cs-induced unoccupied electronic state from d-like in the former case to sp-like in the latter, owing to hybridization in the adsorbate/substrate bond. This effect was observed whether sampling of the SBZ was achieved by varying the Cs coverage or by probing a different value of k(parallel to).