Working on a dream: bringing up the level of interface spectroscopy to the bulk level

Abstract

Liquid interfaces are unique environments in which a variety of fundamental phenomena occur. Therefore, it is important to obtain a molecular-level understanding of liquid interfaces for both basic science and industrial applications. However, it is not an easy task to investigate molecules in the interface region that only has nanometer thicknesses. Second-order nonlinear spectroscopy, or even-order nonlinear spectroscopy more generally, is intrinsically interface-selective because the relevant nonlinear signal is generated only in the region in which the inversion symmetry is broken under the dipole approximation. In the past 2 decades, we have been developing and applying new interface nonlinear spectroscopic methods, aiming to bring up the level of knowledge on interfacial molecules to that on molecules in solution. During this attempt, we developed electronic sum-frequency generation spectroscopy, heterodyne-detected electronic sum-frequency generation spectroscopy, and heterodyne-detected vibrational sum-frequency generation spectroscopy, as well as fourth-order Raman spectroscopy. We also extended the methods to femtosecond time-resolved measurements. Using these methods, we are now able to study the structure and dynamics at liquid interfaces, in particular exposed interfaces such as air/liquid interfaces, at a similar level to the study for solution. I overview our interface research while describing thoughts we had at each turning point.