title: The structure of the Au(111)/methylthiolate interface: new insights from near-edge x-ray absorption spectroscopy and x-ray standing waves.
abstract: The local structure of the Au(111)(square root(3) x square root(3))R30 degrees-methylthiolate surface phase has been investigated by S K-edge near-edge s-ray absorption fine structure (NEXAFS) both experimentally and theoretically and by experimental normal-incidence x-ray standing waves (NIXSW) at both the C and S atomic sites. NEXAFS shows not only excitation into the intramolecular sigma(*) S-C resonance but also into a sigma(*) S-Au orbital perpendicular to the surface, clearly identifying the local S headgroup site as atop a Au atom. Simulations show that it is not possible, however, to distinguish between the two possible adatom reconstruction models; a single thiolate species atop a hollow-site Au adatom or a dithiolate moiety comprising two thiolate species bonded to a bridge-bonded Au adatom. Within this dithiolate moiety a second sigma(*) S-Au orbital that lies near parallel to the surface has a higher energy that overlaps that of the sigma(*) S-C resonance. The new NIXSW data show the S-C bond to be tilted by 61 degrees relative to the surface normal, with a preferred azimuthal orientation in <211>, corresponding to the intermolecular nearest-neighbor directions. This azimuthal orientation is consistent with the thiolate being atop a hollow-site Au adatom, but not consistent with the originally proposed Au-adatom-dithiolate moiety. However, internal conformational changes within this species could, perhaps, render this model also consistent with the experimental data.
authors.affiliation: Department of Physics, University of Warwick, Coventry CV4 7AL, United Kingdom.
authors.forename: A
authors.initials: A
authors.lastname: Chaudhuri
authors.forename: M
authors.initials: M
authors.lastname: Odelius
authors.forename: R G
authors.initials: RG
authors.lastname: Jones
authors.forename: T-L
authors.initials: TL
authors.lastname: Lee
authors.forename: B
authors.initials: B
authors.lastname: Detlefs
authors.forename: D P
authors.initials: DP
authors.lastname: Woodruff