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) (3×3) R30° -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 σ * S-C resonance but also into a σ* 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 σ* S-Au orbital that lies near parallel to the surface has a higher energy that overlaps that of the σ* S-C resonance. The new NIXSW data show the S-C bond 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 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. © 2009 American Institute of Physics.