Study of first electronic transition and hydrogen bonding state of ultra-thin water layer of nanometer thickness on an α-alumina surface by far-ultraviolet spectroscopy

Abstract

The first electronic transition (A˜←X˜) and the hydrogen bonding state of an ultra-thin water layer of nanometer thickness between two α-alumina surfaces (0.5–20nm) were studied using far-ultraviolet (FUV) spectroscopy in the wavelength range 140–180nm. The ultra-thin water layer of nanometer thickness was prepared by squeezing a water droplet (~1μL) between a highly polished α-alumina prism and an α-alumina plate using a high pressure clamp (~4.7MPa), and the FUV spectra of the water layer at different thicknesses were measured using the attenuated total reflection method. As the water layer became thinner, the A˜←X˜ bands were gradually shifted to higher or lower energy relative to that of bulk water; at thicknesses smaller than 4nm, these shifts were substantial (0.1–0.2eV) in either case. The FUV spectra of the water layer with thickness <4nm indicate the formation of structured ice-like hydrogen bond (H-bond) layers for the higher energy shifts or the formation of slightly weaker H-bond layers as compared to those in the bulk liquid state for lower energy shifts. In either case, the H-bond structure of bulk liquid water is nearly lost at thicknesses below 4nm, because of steric hydration forces between the α-alumina surfaces.