Adsorption of Thiolates to Singly Coordinated Sites on Au(111) Evidenced by Photoelectron Diffraction

The adsorption structure of methylthiolate (${\mathrm{C}\mathrm{H}}_3\mathrm{S}$) adsorbed on Au(111), a long-standing controversial issue, has been unambiguously determined by scanned-energy and scanned-angle S $2p$ photoelectron diffraction. The methylthiolate molecules are found to occupy atop sites with a S-Au distance of $2.42\pm 0.03\AA$. The angular distribution of the S $2p$ photoelectrons due to forward scattering reveals that the S-C bond is inclined by approximately 50° from the surface normal towards both the $[\overline{2}11]$ and $[\overline{1}2\overline{1}]$ (nearest-neighbor thiolate) directions.

Figure 1

Schematic illustration for high-symmetry sites on the unreconstructed (111) surface of Au. The fcc-bridge site that is considered to be energetically most stable according to the recent theoretical studies [11, 12, 13, 14] is also indicated.

Figure 2

(a) Solid curves: scanned-energy S $2p$ PD modulation functions $\chi (k)s$ recorded in different emission directions for a saturated ${\mathrm{C}\mathrm{H}}_3\mathrm{S}$ monolayer adsorbed on Au(111) at room temperature. Dashed curves: theoretical simulations for the best-fit atop adsorption model. (b) $k^2$-weighted $R$ factors of the simulations for the normal-emission curve as a function of the S-Au distance. Simulations are for the $(\sqrt{3}\times \sqrt{3})R30°\mathrm{\text{−}}\mathrm{S}$ adlayer on Au(111) at five possible adsorption sites. Inset: $R$-factor change for a $(\sqrt{3}\times \sqrt{3})R30°\mathrm{\text{−}}{\mathrm{C}\mathrm{H}}_3\mathrm{S}$ structure bonding to the atop site with the S-C axis tilted by 50° from the surface normal and aligned along $[\overline{2}11]$ azimuth (see text) as a function of the S-Au distance. Horizontal arrows at the top indicate the possible range of S-Au distance for each site obtained from geometrical optimization in the previous DFT studies [11, 12, 13, 14].

Figure 3

Scanned-angle S $2p$ PD data for a ${\mathrm{C}\mathrm{H}}_3\mathrm{S}$ monolayer saturated on Au(111) at room temperature. (a) Polar angle ($\theta$) dependence of the S $2p$ intensity normalized by the C $1s$ one (solid circles) at a wave number of $k=9{\AA }^{-1}$ recorded along three different azimuths, $[\overline{2}11]$, $[\overline{1}2\overline{1}]$, and $[\overline{1}10]$. Solid lines are guides for the eye. Dashed lines show multiple scattering simulations for atop ${\mathrm{C}\mathrm{H}}_3\mathrm{S}$ structures with the S-Au distance of 2.42 Å and with the S-C axes tilted by 50° from the surface normal and aligned along $[\overline{2}11]$, $[\overline{1}2\overline{1}]$, and their symmetrically equivalent four azimuths. (b) Scanned-angle PD modulation functions $\chi (\theta )s$ (solid line) at a wave number of $k=3{\AA }^{-1}$ recorded along the three different azimuths. Simulated modulation functions are shown as dashed lines for the same geometries as used in the left panel but at $k=3{\AA }^{-1}$.

Figure 4

Schematic side and top view of the best-fit adsorption geometry of $(\sqrt{3}\times \sqrt{3})R30°\mathrm{\text{−}}{\mathrm{C}\mathrm{H}}_3\mathrm{S}$ on Au(111). The hydrogen atoms are indicated at tentatively assumed positions.