Structure determination of methanethiolate on unreconstructed Cu(111) by scanned-energy mode photoelectron diffraction

The local structure of methanethiolate, CH3S-, on an unreconstructed Cu(111) surface at low temperature, has been investigated by S 2p and C is scanned-energy mode photoelectron diffraction, with chemical state sensitivity. 71(+14/-16)% of the methanethiolate was found to occupy bridge sites, 29 +/- 14% to occupy fcc hollow sites and 0 + 19% to occupy hcp hollow sites. In the bridge site the layer spacing of the sulphur atom to the outermost substrate layer is 1.87 +/- 0.03 A giving a Cu-S bondlength of 2.27 0.03 Angstrom. The methanethiolate adsorbed in the fcc hollow site has a Cu-S layer spacing of 1.73 +/- 0.04 Angstrom, corresponding to the same bondlength of 2.27 +/- 0.04 Angstrom. The S-C bondlength was found to be 1.92 +/- 0.10 Angstrom. These conclusions are consistent with the results of previous X-ray standing wave and scanning tunnelling microscopy studies for a common model involving co-occupation of bridge and hollow sites, although differing relative occupations and long-range ordering are thought to arise from different preparation conditions. The new data favour a model in which the S-C bond axis of the bridge-bound thiolate is tilted by 45 +/- 12 away from the surface normal in the azimuth directed towards the fcc hollow site. (C) 2002 Elsevier Science B.V. All rights reserved