The structure of the Au(111)/methylthiolate interface: New insights fromnear-edge x-ray absorption spectroscopy and x-ray standing waves

摘要

The local structure of the Au(111)(√,13 X√3)R30°-methylthiolate surface phase has been investigatedby S K-edge near-edge s-ray absorption fine structure (NEXAFS) both experimentally andtheoretically and by experimental normal-incidence x-ray standing waves (NIXSW) at both the Cand S atomic sites. NEXAFS shows not only excitation into the intramolecular S—C resonancebut also into a o-* S—Au orbital perpendicular to the surface, clearly identifying the local Sheadgroup site as atop a Au atom. Simulations show that it is not possible, however, to distinguishbetween the two possible adatom reconstruction models; a single thiolate species atop a hollow-siteAu adatom or a dithiolate moiety comprising two thiolate species bonded to a bridge-bonded Auadatom. Within this dithiolate moiety a second c·* S—Au orbital that lies near parallel to the surfacehas a higher energy that overlaps that of the CI* S—C resonance. The new NIXSW data show the S—Cbond to be tilted by 61° relative to the surface normal, with a preferred azimuthal orientation in(211), corresponding to the intermolecular nearest-neighbor directions. This azimuthal orientation isconsistent with the thiolate being atop a hollow-site Au adatom, but not consistent with theoriginally proposed Au-adatom-dithiolate moiety. However, internal conformational changes withinthis species could, perhaps, render this model also consistent with the experimental data.