Structure ofp(2×2)andc(2×2)Oxygen on Ni(100): A Surface Extended-X-Ray Absorption Fine-Structure Study

In contrast to recent theoretical electronic structure calculations and analysis of electron energy-loss (ELS) spectra, surface extended-x-ray-absorption fine-structure measurements establish the local structural equivalence of $p(2\mathrm{}\times 2)$ and $c(2\mathrm{}\times 2)$ oxygen on Ni(100). In both cases the oxygen atoms chemisorb in the fourfold hollow site with an O-Ni bond length of 1.96 ± 0.03 Å (i.e., $d_{\perp }=0.86\mathrm{}\pm 0.07$ Å). In the light of the present results the large ELS frequency shift (14 meV) between the $p(2\mathrm{}\times 2)$ and $c(2\mathrm{}\times 2)$ phases remains a puzzle.