X-Ray photoelectron and auger electroo forward scattering: A new tool for surface crystallography

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Abstract
A critical review is given of a new technique, developed during the past several years, for surface crystallography. The technique is based on the observation that X-ray photoelectrons and Auger electrons with kinetic energies of a few hundred electron volts and above exhibit enhanced intensities along internuclear axes, or bond directions, connecting the emitting atom with its nearest and next-nearest neighbor atoms. This affect is a result of forward scattering, or forward focusing, of the outgoing electron wave by the potential of atoms overlying the emitter. As a corollary, atoms present in the top atomic layer do not exhibit such forward-scattering enhancements. Consequently, this effect is an excellent diagnostic of whether or not an atom is in the top layer, and if not, it is an excellent diagnostic of the local structural environment around the emitting atom. As a new probe of short-range order, this technique finds important applications in areas such as the orientation of adsorbed molecules, epitaxial growth morphology, surface segregation, interdiffusion at interfaces, core-level binding-energy shifts, and electron escape depths.