Photoelectron Emission Excited by a Hard X-ray Standing Wave

The following chapter describes the basics and some applications for the case that photoelectrons are emitted from a hard X-ray interference field instead of by a travelling wave. The dipole approximation holds astonishingly well even for hard X-rays as far as the magnitude of the transition matrix element is concerned. However, the forward-backward asymmetry caused by higher order multipole terms needs to be considered when the photoelectron is emitted by the coherent action of two X-ray waves travelling in different directions. This has implications on the chosen experimental set-up which will be briefly discussed together with other experimental aspects. Finally, some examples of X-ray standing wave analysis using hard X-ray photoelectron spectroscopy will be presented yielding the geometric and electronic structure of (crystalline) materials with pm resolution.