The water interface provides a unique environment where various important fundamental processes take place. In particular, chemical reactions occurring at the water interface are very important in organic/inorganic chemistry, electrochemistry, environmental science, biological science, and so on. Several experimental and theoretical studies have suggested that chemical reactions at the water interfaces are different from those in the bulk phase. However, it was not possible to directly investigate chemical reactions that proceed at the interface because of the lack of suitable experimental methods.<br/>Heterodyne-detected vibrational sum-frequency generation (HD-VSFG) spectroscopy is a powerful technique to study liquid interfaces. This interface-selective nonlinear spectroscopy enables us to directly measure the phase and amplitude of the nonlinear optical signal generated only from the interface region with a few monolayer thickness, providing interface-selective vibrational spectra that can directly be compared with infrared and Raman spectra in the bulk. A critical advantage of the spectra obtained with HD-VSFG is that they are linear to the response of the interfacial molecules and hence they are the simple sum of the constituent spectral components. This characteristic is critical for time-resolved measurements because we can obtain time-resolved spectra that can be interpreted simply as the spectral change induced by photoexcitation. We realized UV-excited time-resolved HD-VSFG for direct observation of photochemical processes at the interface and succeeded in tracking a photochemical reaction of phenol that occurs at the water surface. The obtained data clearly indicated that the reaction of phenol was drastically accelerated at the interface, compared to the reaction in solution. This marked difference in the reaction dynamics is considered to arise from the unique solvation environment at the interface. Largely different solvation at the interface is expected not only for phenol but also for all kinds of molecules, implying generality of the great difference in chemical reactions between at the interface and in the bulk.

Ultrafast chemical dynamics at the water surface revealed by femtosecond time-resolved phase-sensitive nonlinear spectroscopy

ACS Spring 2022

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