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Ultrafast Dynamics at Water Interfaces Studied by Vibrational Sum Frequency Generation Spectroscopy.

We present an overview of studies on the ultrafast dynamics of water at aqueous interfaces carried out by time-resolved vibrational sum frequency generation (VSFG) spectroscopies. This research field has been growing rapidly, stimulated by technical developments achieved recently. In this review, first, the principles and instrumentations are described for conventional VSFG, heterodyne-detected VSFG, and various IR-pump/VSFG-probe techniques, namely, time-resolved conventional VSFG, time-resolved heterodyne-detected VSFG, and their extension to two-dimensional spectroscopy. Second, the applications of these time-resolved VSFG techniques to the study of the femtosecond vibrational dynamics of water at various interfaces are discussed, in the order of silica/water, charged monolayer/water, and the air/water interfaces. These studies demonstrate that there exists water dynamics specific to the interfaces and that time-resolved VSFG spectroscopies can unambiguously detect such unique dynamics in an interface-selective manner. In particular, the most recent time-resolved heterodyne-detected VSFG and two-dimensional heterodyne-detected VSFG unveil the inhomogeneity of the hydrogen bond and relevant vibrational dynamics of interfacial water through unambiguous observation of hole-burning in the OH stretch band, as well as the subsequent spectral diffusion in the femtosecond time region. These time-resolved VSFG studies have also left several issues for discussion. We describe not only the obtained conclusive physical insights into interfacial water dynamics but also the points left unclear or controversial. A new type of experiment that utilizes UV excitation is also described briefly. Lastly, the summary and some future perspectives of time-resolved VSFG spectroscopies are given.