Unoccupied surface states on metal surfaces as revealed by inverse photoemission

Abstract

We have discovered the first unoccupied electronic surface states on Ag(110) using k-resolved inverse photoemission. We call these surface states intrinsic, as they are split-off from bulk bands into projected bandgaps owing to the jump of the electrostatic potential at the surface. They reveal a material property and may be occupied or unoccupied. In contrast, the image potential at a metal surface gives rise to a series of bound electronic states with energies near the vacuum level. Their wavefunction maxima are located several ångströms away from the surface z = 0. On Ag(100), both image and intrinsic surface states exist at different k in the surface Brillouin zone. Both states can be quenched with oxygen or water at 110 K. However, Xe due to its lower interaction potential causes only a shift of the image-state emission, which follows the workfunction change of the substrate, while the intrinsic surface-state emission is again quenched by only 1 monolayer of Xe at 50 K. Finally, we find an increased effective mass (m ∗ = 1.5m) for the image-state band on Ag(100), which we attribute to the surface corrugation the electron is following in its x, y movement. This allows the identification of the binding energies of the different n image states, also on the Cu(110), Au(110) and Ag(111) surfaces.