Auger photoelectron coincidence experiments from solids

Abstract

Auger photoelectron coincidence spectroscopy (APECS) is a technique in which Auger electrons are counted only when the photoelectron that caused the ionisation is also detected. This experimental technique was first demonstrated successfully from solid targets in 1978. There have been few experiments since then, but several groups have constructed apparatus in the late 1980s and a steady stream of new APECS data has been generated since. These data have begun to shed new light on Auger emission from transition materials. The strength of APECS as a tool for understanding is that one generally has good knowledge of the origins of the photoelectron spectra. The coincidence data allow one to identify the source of features in the Auger spectra. The Auger spectra of transition metals are particularly complex. It is possible to identify some 11 separate processes that may contribute intensity or modify the Auger lineshape. APECS is one of the few means of separating out the relative importance of these processes in particular Auger lines. There have been three successful APECS experiments from surfaces to date. Each of these is examined in detail. The relative merit of each approach to solving the APECS problem is considered. The principal difficulty in APECS is rejecting the electrons that are not in coincidence, and the implications of this are examined. The consequence is that practical systems have notoriously low count rates. It is shown that there is significant room for improvement in instrument design and some promise of systems that will have much better count rates. APECS has now been applied to approximately 15 different surfaces. The data from these experiments are reviewed. These data make clear the subtlety of the Auger process and the contribution APECS can make to improved understanding.