Understanding structure and dynamics of interfacial water is of great importance in many fundamental and applied sciences. Vibrational sum frequency generation (VSFG) spectroscopy has intrinsic interface-selectivity and hence has become a unique powerful tool to probe water at interfaces. Furthermore, VSFG spectroscopy can be combined with the pump-probe technique to study the dynamics of interfaces. However, VSFG spectroscopy had an essential problem: the conventional method only detects the intensity of the signal and hence cannot provide the phase information. To solve this problem, we have developed multiplex heterodyne-detected VSFG (HD-VSFG). HD-VSFG enables us to measure VSFG spectra with complete phase information and hence separately provide spectra of the real and imaginary parts of the second-order susceptibility (x~((2))). Moreover, as an extension of the HD-VSFG technique, we have developed time-resolved heterodyne-detected VSFG (TR-HD-VSFG) and two-dimensional (2D-) HD-VSFG spectroscopies to investigate ultrafast dynamics of interfacial water. Using these novel techniques developed, we have studied the structure and dynamics of water at various interfaces. This review describes technical development of steady-state and time-resolved HD-VSFG spectroscopies and overviews typical applications to the water interfaces.
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