IR-visible sum-frequency generation (SFG) spectroscopy has been used in a total internal reflection geometry to study the molecular structure of polystyrene (PS) at PS/sapphire and PS/air interfaces, simultaneously. The symmetric vibrational modes of the phenyl rings dominate the SFG spectra at the PS/air interface as compared to the antisymmetric vibrational modes at the PS/sapphire interface. This indicates approximately parallel orientation of the phenyl rings at the PS/air interface while nearly perpendicular orientation at the PS/sapphire interface, with respect to the surface normal.

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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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We report the first observation of sum frequency generation (SFG) photons on high-surface-area powders, critically important materials in heterogeneous catalysis. We utilize SFG in total internal reflection (TIR) geometry and show that the TIR-SFG approach markedly reduces the destructive interference associated with nonlinear optical spectroscopy of small particle surfaces, making SFG studies of high-surface-area powders possible. The index of refraction of materials and their distance from the TIR-SFG prism are key parameters in generating and detecting the sum frequency signal. We find that TIR-SFG is highly sensitive to capillary condensation. To demonstrate the capability of the TIR-SFG technique, we measure the thermodynamics of methanol adsorption and desorption on high-surface-area SiO2.