Extended fine structure analysis of surfaces

Abstract

Fine structure in the excitation probability of core states is observed to extend for hundreds of volts above the excitation thresholds. It results from the interference of an outgoing spherical wave of the ejected core electron with backscattered components from the neighbors of the absorbing atom. Apart from corrections due to the scattering phase shifts, the periodicities in the excitation probability versus the momentum of the exciting particle are the reciprocal inter-atomic spacings. This simple picture holds best at energies far above threshold, where multiple scattering of the outgoing electrons is less important. The excitation of the core state can either be by soft X-rays photons or by electrons. In the X-ray case, which is termed “surface extended X-ray absorption fine structure (SEXAFS)”, the excitation of the core hole is monitored by using either the Auger yield or the true secondary emission. The high intensity of a synchrotron X-ray source has thus far been essential to these studies. If the excitation of the core hole is by electron bombardment, the technique is termed “extended appearance potential fine structure (EAPFS)”. The analysis in the two cases is quite similar. For K-shell excitation by electron bombardment, however, it can be shown that the phase shifts are dominated by s partial waves as contrasted to p waves in the case of X-ray excitation. At this stage in its development extended fine structure analysis of surfaces is capable of providing accurate inter-atomic spacings for selected systems. Unlike the other structural probes of the surface, such as LEED and ion backscattering, it can provide this information for even highly disordered surfaces.