Novel Electronic Properties of Solids Revealed by Auger-Photoelectron Coincidence Spectroscopy (APECS)

The basic principles and practical considerations of Auger-Photoelectron Coincidence Spectroscopy (APECS) are discussed. In this technique, core photoelectrons and their associated Auger elecrtons are detected in time coincidence. For true coincidence counts, both electrons originate from the same photoexcitation event enabling one to prove local electronic structure and excitation phenomena with unprecedented discrimination. Originally developed by Haak, Sawatzky and Thomas [1, 2] using a conventional x-ray source, we have adapted APECS to a synchrotron radiation source [3], making it a practical probe of electronic structure. We discuss the results of APECS experiments performed on TaC(111) and Cu(100). In the former study, the self convolution of the density of states from a single atomic layer was determined and a novel off site Auger decay mode revealed. In the latter case, coincidence spectroscopy narrowed the photoemission line width so that the lifetime of the two hole final state could be measured. The implications of these results and other applications of APECS to probe the electronic properties of solids are discussed.