SURFACE AND BULK PHOTOEMISSION - SOME OLD PROBLEMS AND NEW FINDINGS

Angle-resolved photoemission has been applied to a variety of material s for electronic structure determination. It is generally regarded as a routine technique. Yet the measured lineshapes are often much more c omplicated than simple Lorentzians riding on a smooth background as is usually assumed. This study is a reexamination of the photoemission p roperties of Ag and Cu. These are simple metals for which the standard models are expected to work well. Yet the measured peaks are signific antly 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 exp lanation. 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 analys is incorporating this surface term explains the observed lineshape fea tures. A test employing an overlayer to modify the surface boundary co ndition provides a direct proof of the surface origin of these effects . It is thus important to consider the electromagnetic response of a s urface for a complete understanding of the photoemission process.