Development of Time-Resolved Heterodyne-Detected Vibrational Sum Frequency Generation Spectroscopy and Its Application to Interfacial Dynamics

chemistry, electrochemistry, biochemistry, and atmospheric chemistry. However, molecular-level understanding of structure and dynamics of their interfaces is still limited due to technical difficulties. Vibrational sum frequency generation (VSFG) spectroscopy is based on a second-order nonlinear optical process and provides interface-specific information. While conventional homodyne-detected VSFG measures the square of second-order nonlinear susceptibility ( |χ (2) | 2 ), heterodyne-detected (HD-) VSFG enables us to directly measure the imaginary part of χ (2) (Im χ (2) ), which corresponds to Im χ (1) obtained with absorption spectroscopy in bulk. We realized the first time-resolved (TR-) HD-VSFG measurement by combining HD-VSFG spectroscopy and pump-probe technique. This review introduces the principle of TR-HD-VSFG spectroscopy and its applica-tions to interfacial dynamics at the metal, water, and lipid monolayer interfaces.