Non-dipolar contributions in XPS detection of X-ray standing waves

The analysis of X-ray standing wave (XSW) data for the determination of structural parameters of adsorbates (coherent position, P-C(h), and coherent fraction, F-C(h) is usually done in the dipole approximation. Using carbon and oxygen containing molecular adsorbates on Ag(1 1 1), we show how the non-dipolar contributions can be quantified and that they can have a strong impact on the resulting P-C(h) and F-C(h). The results are particularly important for XPS detection of XSW intensities from light elements, for which fluorescence detection does not work. Moreover, we discuss how the XSW technique allows determining both the amplitude and the phase from quadrupole contributions in photoelectric scattering. (C) 2001 Elsevier Science B.V. All rights reserved.