Surface Extended X-Ray-Absorption Fine Structure of Low-ZAdsorbates Studied with Fluorescence Detection

Comparison of x-ray-fluorescence-yield and electron-yield surface extended x-ray-absorption fine-structure spectra above the S $K$ edge for $c(2\mathrm{}\times 2)\mathrm{S}$ on Ni(100) reveals an order-of-magnitude higher sensitivity for the former technique. Thiophene (${\mathrm{C}}_4$ ${\mathrm{H}}_4$S) chemisorption on Ni(100) is studied with S coverages down to 0.08 monolayer. The molecule dissociates at temperatures as low as 100 K by interaction with fourfold hollow Ni sites. Blocking of these sites by oxygen leaves the molecule intact.