A modeling analysis of molecular orientation at interfaces by polarization-dependent sum frequency generation vibrational spectroscopy

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Sum frequency generation (SFG) vibrational spectroscopy has been proven an excellent tool to measure the molecular structures, symmetries and orientations at surfaces/interfaces because of its strong polarization dependence. However, a precise quantitative analysis of SFG spectral intensity and molecular orientation at interfaces must be carefully performed. In this work, we summarized the parameters and factors that are often ignored and illustrated them by evaluating studies of CO adsorption on the (111) facet of platinum (Pt) and palladium (Pd) single crystals at the gas (ultra-high vacuum, UHV)/solid interfaces and methanol (water) adsorption at the air/liquid (solid/liquid) interfaces in the presence of sodium iodide (chloride) salts. To intuitively estimate the influence of incidence angles and refractive indices on the SFG intensity, solely a defined factor of | F yyz | was discussed, which can be individually separated from the macroscopic second-order non-linear susceptibility term and represents the SSP intensity. Moreover, effects of refractive indices and the molecular hyperpolarizability ratio ( R ) were discussed in the orientational analysis of interfacial CO and methanol molecules. When I PPP / I SSP was identical, molecules with a larger R had smaller tilting angles ( θ ) on Pt (assuming θ < 51°), and CO molecules on Pd would tilt much closer to the surface than they did on Pt. A total internal reflection (TIR) geometry enhanced the SFG intensity, but it also amplified the influence of refractive index on SFG intensity at the solid (silica)/liquid interface. The refractive index and R-value had similar influence on the methanol orientation in the presence of sodium iodide salts at air/liquid and solid/liquid interfaces. This work should provide a guideline for analyzing the orientation of molecules with different R , which are adsorbed on catalysts or located at liquid interfaces involving changes of refractive indices. Even slight changes of refractive index (medium 2) and hyperpolarizability ratio ( R = βaac/βccc), which are usually rather ignored, have significant effects on the SFG spectral intensity and molecular orientation of gas (UHV)/solid, air/liquid and solid/liquid interfaces.