Adsorbate-geometry determination by measurement and analysis of angle-resolved-photoemission extended-fine-structure data: Application toc(2×2)S/Ni(001)

We describe our procedures for deducing adsorbate geometry from angle-resolved core-level photoemission measurements as they are applied to c(2×2)S/Ni(001). Extracting the energy-dependent, oscillating part of the sulfur (1s) photoemission partial cross section gives the angle-resolved-photoemission extended fine structure (ARPEFS). Fourier transformation of the ARPEFS yields peaks at distances characteristic of the local site geometry and in most cases closely related to geometrical path-length differences. Multiple-scattering, curved-wave calculations are fitted to Fourier-filtered data for quantitative geometry determination; the Fourier filtering reduces the size of the scattering cluster and the number of free parameters in the fit. Possible sources of error in this first ARPEFS measurement are discussed as a guide for future work. We find a S–Ni bond length of 2.19 Å ($d_{\perp }$=1.31 Å), a S–Ni second-layer bond length of 3.14 Å corresponding to a 4% expansion of the top Ni layer, and some evidence that those Ni atoms in the second Ni layer lying beneath sulfur atoms are pulled closer to the sulfur, leading to a buckled second layer.