Peculiar Hydrogen-Bond Structure, Physical Properties and Function of Interfacial Water Molecules Elucidated by Nonlinear Laser Spectroscopy

Abstract

Interfacial phenomena, in which water molecules are deeply involved, are critical subjects in basic and applied research in a wide range of fields, including physics, chemistry, and biology. In the symmetry-breaking interfacial systems, molecular orientation is key structural parameter that determines the functional properties of water molecules. However, orientation of water molecules, i.e. configuration of hydrogens, in the interfacial hydrogen-bond network has been extremely difficult to be investigated by traditional structure-analysis techniques such as electron diffraction, grazing X-ray scattering and even scanning probe microscopy, because hydrogen has only a single electron and responds extremely weakly to the probes of these techniques; thus, the determination of molecular orientation of interfacial water systems has been an experimental challenge. We have recently tackled the challenges of elucidating the orientational structures of water molecules on metal surfaces by using cutting-edge nonlinear optical spectroscopy, i.e. heterodyne-detected sum-frequency generation (HD-SFG) spectroscopy. In this review article, brief descriptions of the basic concept of SFG spectroscopy as a hydrogen-configuration sensitive probe are followed by overviews of our current understanding of the adsorption geometry and rotational ordering of water molecules on representative metal surfaces and ice-film surfaces.