A Hreels study of benzenethiol adsorption on Au(111) and Au(322)

Adsorption of benzenethiol on a Au(111)-(root 3 x 22)-reconstructed surface at 300 K leads to formation of molecular long range disordered overlayers without lifting of the surface reconstruction. Absence of the S-H stretching vibration, both in and out of specular scattering geometry, along with identification of modes typical of benzene-like systems, indicates formation of a thiolate intermediate at both saturation and submonolayer coverage. Comparison of HREELS band intensities as a function of coverage in the specular direction indicates that no major coverage-dependent reorientation occurs on Au(111). Consideration of relative band intensities, use of dipole selection rules and comparison with previous studies of benzene and benzenethiol adsorption on transition metal surfaces indicate a largely flat-lying bonding geometry at both monolayer and submonolayer coverages. In contrast, adsorption on Au(322), a surface consisting of monoatomic (100) steps separating (111) microterraces five unit cells wide, leads to formation of an ordered monolayer. The adsorbed intermediate on Au(322) is benzenethiolate at both low and high coverage. At low coverages the molecule is strongly inclined with respect to the (111) micro-terraces while at higher coverages adsorbates adopt a more flat-lying geometry. This unusual behavior may be due to coverage-dependent occupation of step and terrace sites with a strongly inclined species formed at step sites, preferentially occupied at low coverage, and a second flat-lying state becoming occupied within the monolayer. Alternatively, both step- and terrace-bound thiolate are strongly inclined at low exposures, with the two species undergoing an orientational transition beyond a critical coverage to form a more flat-lying intermediate. This study shows clear evidence for step-induced ordering of the thiolate intermediate and the ability of steps to promote a molecular reorientation.