Development of Electronic Sum Frequency Generation Spectroscopies and Their Application to Liquid Interfaces

Abstract

This feature article describes our recent work on the development of interface-selective second-order nonlinear electronic spectroscopies and their application to molecular structure and dynamics at liquid interfaces. We describe three methods that measure electronically resonant second-order nonlinear optical susceptibility (χ(2)) of interfacial molecules without contribution from the bulk: electronic sum-frequency generation (ESFG) provides high-quality interfacial electronic spectra by employing the combination of narrow-band and broad-band visible/near-infrared femtosecond pulses. Time-resolved ESFG (TR-ESFG) is the extension of ESFG to time-resolved measurements and provides femtosecond time-resolved electronic spectra at interfaces. Heterodyne-detected ESFG (HD-ESFG) is the ultimate second-order nonlinear electronic spectroscopy that directly provides the real and imaginary parts of χ(2). In particular, the imaginary χ(2) spectra obtained with HD-ESFG can be directly compared to the absorption spectra of bulk solution, and their signs contain information about the absolute orientation of the interfacial molecules. These interface-selective electronic spectroscopies enable rigorous comparison between the electronic spectra of liquid interfaces and those of bulk liquids, and the application to the following topics is discussed in this article: effective polarity at the air/water interface, interfacial ultrafast dynamics, absolute alignment of interfacial molecules, pH of the air/water interface, and the origin of nonresonant background. These studies reveal unique properties of molecules at air/liquid interfaces, which is markedly different from that in the bulk from a microscopic viewpoint.