Contributions of multipole terms to the photoelectric yield in X-ray standing-wave measurements

The influence of higher-order multipole terms in the photoelectron yield on the result of an X-ray Standing Wave (XSW) measurement is discussed. The contributions of dipole, quadrupole and octupole term are analyzed for the simple model of a single atom within the XSW field. It is shown that each multipole term shows a different dependence on the polarization vectors and wave vectors of the direct and diffracted beam. The model is generalized for the case of a monolayer of atoms adsorbed on the surface of a crystal. They outline how multipole contributions can be determined by an XSW measurement