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 [(2)]. HD-VSFG has remarkable advantages over conventional VSFG. For example, the imaginary part of (2) [Im(2)] obtained with this interferometric technique is the direct counterpart to the infrared [Im(1)] and Raman [Im(3)] spectra in the bulk, and it is free from the spectral deformation inevitable in conventional VSFG [|χ(2)|2] spectra. The Im(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.


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  • Article Type: Review Article
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