Inverse photoemission from metal surfaces.

Ultraviolet inverse photoemission spectroscopy (IPES) is a technique for exploring unoccupied electronic states, particularly in the energy range between the Fermi level and vacuum level, a range inaccessible in ordinary photoemission. Theories of inverse photoemission and its instrumentation requirements are outlined. IPES measurements on clean metal surfaces has revealed an abundance of new Shockley surface states and the Rydberg series of image states converging on the vacuum level. Empty surface states of d-like character have also been seen. The systematics of the occurrence of surface states (including image states) associated with s,p bulk gaps is well described by a simple adaptation of multiple-reflection theory. This model is propounded and its implications discussed with regard to effective masses, surface corrugation, and determination of the surface barrier potential.