Intermediate-energy azimuthal photoelectron diffraction fromc(2×2)S(1s)on Ni(001) in a geometry emphasizing substrate backscattering

Azimuthal x-ray photoelectron diffraction (XPD) for S $1s$ emission from $c(2\mathrm{}\times 2)\mathrm{S}$ on Ni(001) at electron kinetic energies of 200-300 eV and with an experimental geometry emphasizing backscattering from the substrate is found to show large intensity modulations of ~ 30-45% as a function of azimuthal angle. The results are also well predicted by single-scattering cluster calculations, and are found to be consistent with S bonded above the fourfold hollow sites at a distance of 1.35±0.1 Å, in good agreement with previous work. Such azimuthal measurements are also compared with fixed-geometry swept-$h\nu$ photoelectron diffraction as to advantages and disadvantages. In general, azimuthal XPD measurements are predicted to have a high sensitivity to both adsorption site type and perpendicular adsorbate distance.