The structure of the Au(111)/methylthiolate interface : new insights from near-edge X-ray absorption spectroscopy and X-ray standing waves

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Chaudhuri, Anindita ; Odelius, Michael ; Jones, R. G. ; Lee, Tien-Lin ; Detlefs, B. ; Woodruff, D. P. (2009)

The local structure of the Au(111)([square root of]3×[square root of]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 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° 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.
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    A. Chaudhuri1, M. Odelius2, R.G. Jones3, T.-L. Lee4, B. Detlefs5, D.P.Woodruff1* 1 Physics Department, University of Warwick, Coventry CV4 7AL, UK 2 Fysikum, AlbaNova University Center, Stockholm University,S-10691 Stockholm, Sweden 3 School of Chemistry, University of Nottingham, Nottingham NG7 2RD, UK 4 Diamond Light Source Ltd, Didcot, Oxfordshire, OX11 0DE,, UK 5ESRF, BP 220, 38043 Grenoble, Cedex 9, France

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