Surface and bulk photoemission: Some old problems and new findings

Angle-resolved photoemission has been applied to a variety of materials for electronic structure determination. It is generally regarded as a routine technique. Yet the measured lineshapes are often much more complicated than simple Lorentzians riding on a smooth background as is usually assumed. This study is a reexamination of the photoemission properties of Ag and Cu. These are simple metals for which the standard models are expected to work well. Yet the measured peaks are significantly asymmetric, and the ''background'' shows interesting variations as a function of photon energy. These ''nonideal'' behaviors have been known for almost twenty years, but there has been no satisfactory explanation. This work shows that the underlying physics is related to a del . A term in the optical transition matrix element, which has been largely ignored in the past. This term has a significant contribution near a surface due to a rapid change in dielectric response. An analysis incorporating this surface term explains the observed lineshape features. A test employing an overlayer to modify the surface boundary condition provides a direct proof of the surface origin of these effects. It is thus important to consider the electromagnetic response of a surface for a complete understanding of the photoemission process.