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

The analysis of X-ray standing wave (XSW) data for the determination of str uctural parameters of adsorbates (coherent position, P-C(h), and coherent f raction, F-C(h) is usually done in the dipole approximation. Using carbon a nd 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 im pact on the resulting P-C(h) and F-C(h). The results are particularly impor tant for XPS detection of XSW intensities from light elements, for which fl uorescence detection does not work. Moreover, we discuss how the XSW techni que allows determining both the amplitude and the phase from quadrupole con tributions in photoelectric scattering. (C) 2001 Elsevier Science B.V. All rights reserved.