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 EF, designated S₀⁺; at point Y{}, 3.4 and 6.6 eV above EF, designated S₀⁺ and S₁^-, respectively.