On the Fresnel Factor Correction of Sum-Frequency Generation Spectra of Interfacial Water

Insights into the microscopic structure of aqueous interfaces are essential for understanding chemical and physical processes on the water surface, including chemical synthesis, atmospheric chemistry, and events in biomolecular systems. These aqueous interfaces have been probed by heterodyne-detected sum-frequency generation (HD-SFG) spectroscopy. To obtain the molecular response from the measured HD-SFG spectra , one needs to correct the measured ssp spectra for local electromagnetic field effects at the interface due to a spatially varying dielectric function. This so-called Fresnel factor correction can change the inferred response substantially, and different ways of performing this correction lead to different conclusions about the interfacial water response. Here, we compare simulated and experimental spectra at the air/water interface. We use three previously developed models to compare the experiment with theory: an advanced approach taking into account the detailed inhomogeneous interfacial dielectric profile, and the Lorentz and slab models two approximate the interfacial dielectric function. Using the advanced model, we obtain excellent quantitative agreement between theory and experiment, in both spectral shape and amplitude. Remarkably, we find that for the Fresnel factor correction of ssp spectra, the Lorentz model for the interfacial dielectric is equally accurate in the H-bonded region of the response, while the slab model underestimates this response significantly. The Lorentz model thus provides a straightforward method to obtain the molecular response from measured spectra of aqueous interfaces in the hydrogen-bonded region.