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 S2p and C1s 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 Å giving a Cu–S bondlength of 2.27±0.03 Å. The methanethiolate adsorbed in the fcc hollow site has a Cu–S layer spacing of 1.73±0.04 Å, corresponding to the same bondlength of 2.27±0.04 Å. The S–C bondlength was found to be 1.92±0.10 Å. 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.