Medium energy ion scattering investigation of methylthiolate-induced modification of the Au(111) surface

100 keV H⁺ scattering has been used to investigate the structure of the methylthiolate/Au(111) interface in the Au(111)(√3 × √3)R30° phase. Adsorption of the thiolate onto the clean Au(111) surface leads to a large drop in the scattered ion yield due to the lifting of the clean surface ‘herring-bone’ reconstruction, but the thiolate-covered surface shows an ion yield higher than that of an unreconstructed Au(111) surface, providing direct evidence of a significant number of Au atoms that are displaced from their bulk-terminated positions at the buried interface. Simulations for two different Au adatoms models at the interface, namely, the Au-adatom-monothiolate (AAM) and Au-adatom-dithiolate (AAD) models, show significant sensitivity to the exact values of interlayer spacings and atomic vibrational amplitudes, but the comparison with experimental results appears to favour the AAD model with 0.17 ML Au adatoms in bridging sites at the interface.