Surface extended X-ray absorption fine structure: critical appraisal of a structural tool for adsorbates

Abstract
The availability of intense, tunable, polarized X-radiation from electron storage rings has led to the successful application of surface extended X-ray absorption fine structure (s.e.X.a.f.s.) measurements to the determination of inter-atomic spacings and, by using multishell analyses, complete structures for adsorbates on surfaces. Recent work on a surface iodide structure on Ni{100}, Ag{100}—c(2 x 2) Cl, Ag{111}—(/ 3 x /3 ) R 30°—Cl (0.7 fractional monolayer coverage) and disordered 0.4 monolayer Ag{111}—Cl structure are reviewed. The results for Cl on Ag{100} confirm a previous exhaustive I.e.e d. intensity analysis for the same system, but yield greater precision in the Cl—Ag bond length (2.69 + 0.03 A). Studies of Cl on Ag{111} were extended to low crystal temperatures, and demonstrate: (i) a significant improvement in signal quality; (ii) removal of anomalies due to anharmonicity at room temperature; and (iii) the ability of s.e.X.a.f.s., by using a multishell data analysis procedure including four atomic shells, to provide an unambiguous structure determination from both a poorly ordered and a disordered structure. The Cl—Ag bond length is found to be 2.70 + 0.01 A at both 0.4 and 0.7 fractional monolayer coverages.