Unoccupied states on Pd(110) and the surface potential barrier

Inverse photoemission spectra on Pd(110) reveal bulk band-structure features, and surface-state features in the projected gaps centered at points Y¯ and X¯. The energy dispersion of the bulk band-structure features is in agreement with direct-transition predictions. Surface-state energies and their dispersion are reproduced with a nearly-free-electron model and a surface-barrier model which places the image plane 2.3 a.u. beyond the outermost atomic layer. The experimentally determined energies of surface states are as follows: at point X¯, 6.6 eV above $E_F$, designated $S_0^{+}$; at point Y¯, 3.4 and 6.6 eV above $E_F$, designated $S_0^{+}$ and $S_1^{\mathrm{-}}$, respectively.