Photoelectron-diffraction measurements of sulfur and selenium adsorbed on Ni(001)

Photoelectron-diffraction data are presented for the systems sulfur-Ni(001) and selenium-Ni(001). A combination of all normal emission data leads to a clearer understanding of photoelectron diffraction from an adsorbate core level as a two-step process. Experimental observation of diffraction effects from lowcoverage, disordered overlayers of selenium demonstrates that the diffraction is clearly due to backscattering off the nickel substrate, and also that normal photoelectron diffraction is potentially quite useful in the study of disordered systems. Certain criteria are established which any photoelectron-diffraction technique must satisfy to be a viable tool for studying surface structure. A comparison of $c(2\mathrm{}\times 2)$ sulfur and selenium data and presentation of off-normal data for the selenium system leads to the conclusion that normal photoelectron diffraction satisfies these criteria. Normal photoelectron diffraction was observed for the nickel $3p$ shell and valence band, showing that both diffraction and multiple-scattering processes affect angle-resolved photoemission intensities.