Image-Potential States Observed by Inverse Photoemission

Abstract
Momentum-resolved photoemission spectra from Cu(100) exhibit structure at 4 eV above the Fermi energy in a bulk band gap. This emission is pinned to the vacuum level and shows a free-electron-like dispersion. It is polarized with electric vector normal to the surface. The emission intensity is independent of the sample temperature unlike that of a simultaneously observed bulk direct transition. These properties identify the observed phenomenon as transitions into image-potential surface states.