The influence of step sites on the bonding of benzenethiol on Au surfaces

The influence of step sites on benzenethiol adsorption on Au surfaces has been studied using high resolution electron energy loss spectroscopy, X-ray photoelectron spectroscopy and thermal desorption spectroscopy. At all exposures, the absence of a SH stretching vibration indicates that the sulphydryl hydrogen is lost during adsorption on Au(322) at 300 K. The formation of benzenethiolate is confirmed by C 1s and S 2p core level data. In contrast to the Au(111) surface, the introduction of defects consisting of monatomic (100) steps separating (111) microterraces five atoms wide, leads to the thiolate surface intermediate adopting a coverage dependent bonding geometry. At low exposures, the phenyl ring is tilted away from the (111) microterraces while at saturation coverage a more flat-lying orientation is preferred. Consideration of core level and vibrational data, supported by thermal stability studies, suggests that this unusual behaviour can be attributed to the existence of inequivalent adsorption sites.