Non-dipole effects in photoelectron-monitored X-ray standing wave experiments: characterisation and calibration

The results are reported of a series of experiments designed to characterise and calibrate the role of non-dipolar (notably electric quadrupole–dipole interference) effects in the angular distribution of photoelectron emission on structural studies of surfaces using normal incidence X-ray standing wavefield absorption. The values of the resulting backward/forward asymmetry parameter (relative to the photon propagation direction) have been measured for 1s emission at photon energies around 3 keV for first and second row elements from C to Cl, and are compared with recent theoretical determinations. The influence of additional non-dipole interference terms in the coherent X-ray standing wave has also been determined for the case of Al1s emission and found to be weak, typically introducing systematic errors in atomic positions of only around 0.02–0.04 Å.