On the role of Fresnel factors in sum-frequency generation spectroscopy of metal-water and metal-oxide-water interfaces

We performed sum-frequency generation (SFG) spectroscopic measurements on water in contact with supported thin metal and metal-oxide films. We employed an internal reflection configuration and varied the angles of incidence of the visible and infrared beams and measured the SFG signals using different polarization combinations. While SFG is a surface-specific vibrational spectroscopy, the shape of the SFG spectra can be fully accounted for by the bulk response of the materials through the frequency-dependent enhancement of the local incident infrared fields at the interface, i.e., Fresnel effects. We find that the dispersion of the refractive index of the bulk water phase leads to a strong enhancement of the electric field at the interface at specific infrared frequencies. These local, frequency-dependent fields act on the frequency-independent, nonresonant SFG response of the electrons at the surfaces of the metal or metal-oxide films. As a result, the measured SFG spectra closely follow this infrared frequency dependence. Hence, we conclude that the nonresonant SFG signal from the electrons in the metal or metal-oxide film strongly dominates over the resonant SFG signal of the interfacial water vibrations. This work demonstrates the importance of Fresnel factors in SFG spectroscopy of metal(-oxide)−liquid interfaces and shows that the spectral dependence and magnitude of the Fresnel factors can be calculated, making corrections to the data in principle possible. Finally, this work provides practical recommendations for the selection of suitable experimental conditions for future SFG studies on metal(-oxide)−liquid interfaces aimed at elucidating interfacial water structures at these interfaces.

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