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 Al 1s emission and found to be weak, typically introducing systematic errors in atomic positions of only around 0.02–0.04 Å.