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Vibrational sum-frequency generation (VSFG) spectroscopy is a powerful tool to study interfaces. Recently, multiplex heterodyne-detected VSFG (HD-VSFG) has been developed, which enables the direct measurement of complex second-order nonlinear susceptibility χ((2)). HD-VSFG has remarkable advantages over conventional VSFG. For example, the imaginary part of χ((2)) Imχ((2)) obtained with this interferometric technique is the direct counterpart to the infrared Imχ((1)) and Raman Imχ((3)) spectra in the bulk, and it is free from the spectral deformation inevitable in conventional VSFG |χ((2))|(2) spectra. The Imχ((2)) signal is obtained with a sign that contains unambiguous information about the up/down orientation of interfacial molecules. Furthermore, HD-VSFG can be straightforwardly extended to time-resolved measurements when combined with photoexcitation. In this review, we describe the present status of experiments and applications of multiplex HD-VSFG spectroscopy, in particular with regard to the orientation and structure of interfacial water at charged, neutral, and biorelevant water interfaces.
Nihonyanagi et al. (Sat,) studied this question.