Infrared-visible sum-frequency generation (SFG) is used to spectroscopically probe the interfacial structure of a self-assembled monolayer of alkyl thiol on a Au(111) surface. The SFG spectra of the CH3 group measured as a function of azimuthal angle indicates that sulfur atoms cannot be situated at sites of a single type, e.g. , hollow or bridge, but must be in a mixed arrangement. SFG is also used to study the reconstruction of the gold substrate by alkyl thiol.PACS numbers: 68.45.v, 42.65.k, 61.16.d Nearly 10 years ago Nuzzo and Allara [1] discovered that when gold is immersed in a dilute organosulfur solution an organic monolayer is spontaneously chemisorbed on the surface of the gold, producing a self-assembled monolayer (SAM). Such coatings are potentially useful for corrosion inhibition, wear protection, and organic sensors. Perhaps the best characterized SAM system is CH3(CH2)"~SH (C,), adsorbed on a Au surface. The C, /Au system, specifically its surface and chain orientation, has been widely studied. He [2] and x-ray [3] scattering studies have shown that the C H3 outer surface has a rectangular unit cell which includes four tilted hydrocarbon chains. Infrared spectroscopy [4], in agreement with Raman scattering [5] and ellipsometry [6], as well as an earlier sum-frequency generation (SFG) study [7], suggests that the chains are densely packed and the number of the gauche defects is insignificant. Despite all the available experimental results, the location of sulfur chemisorption sites is known only by inference. The conventional belief has been that the S atoms reside at hollow sites of the gold substrate, generating a simple (~3 X~3)R30 structure. The location of these sites is essential for a detailed evaluation of Au-S, Au-Au, and S-S bonds at the interface. These in turn affect the monolayer orientation and stability.Single frequency second-order nonlinear optical measurement has proven to be a unique technique for studying interface structural symmetry [8]. In this work infraredvisible SFG was used to probe the structural symmetry of the C~s/Au (111) interface. The present measurements in which the frequency dependence of the signal was used to differentiate the substrate and adlayer structures demonstrate the sensitivity of SFG spectroscopy to the structural symmetry of an interface. Our experiments reveal an unexpected threefold rotational symmetry at the sulfur adsorbate sites of the C~s/Au(111) system. This result precludes the sulfur atoms being located solely at single-type hollow sites of the gold substrate. Such a configuration would give rise to an isotropic SFG signal. Recently, we learned of an elegant x-ray experiment that independently produced results leading to the same conclusion [9]. The experimenters suggest a disulfide configuration with sulfur in the hollow and bridge sites.This arrangement would produce SFG results consistent with ours.In addition to adsorbate site geometry, the effect of the thiol chemisorption on the surface stress of the gold substrate was also exami...

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Infrared-visible sum-frequency generation (SFG) is employed in an internal reflection geometry to investigate, in situ, the adsorption of acetonitrile on ZrO2 from liquid and vapor. This configuration induces large field enhancements near the critical angle that increase signal levels by more than 100-fold compared with reflection from the liquid or vapor side of the interface. Acetonitrile is a widely used, highly polar, chromatographic solvent that has simple structure and strong stretching modes with large frequency separation, making it an ideal liquid for SFG studies. The C≡N and C–H stretching modes are observed at submonolayer and neat liquid coverages on ZrO2. The relative polarization dependent SFG oscillator strengths are determined providing estimates of the binding energy, 4.7±0.2 kcal/mol, and the mean orientation of the molecular axis to the surface, 22±2°. Spectra of CD3CN show resolvable structure in the C≡N and C–D stretching regions, suggesting the presence of aggregates on the surface. SFG measurements as a function of CH3CN concentration in CCl4, at equilibrium, is shown to be consistent with the formation of dimers and higher order surface species.

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