X-ray and Raman spectroscopy of water and aqueous solutions.

摘要

Understanding the unique physical and chemical properties of liquid water remains an active area of current research. In this dissertation I present an in-depth study of the structural properties of liquid water, using a combination of experimental and theoretical methods. These studies serve to enhance our understanding of the role of hydrogen bonding in determining the unique properties of liquid water.;In Chapter 2, I investigate the essential question of whether continuum or multi-component ("intact" or "broken bond", etc.) models best describe the hydrogen bond (HB) network in liquid water. The temperature dependence of water's Raman spectrum has long been considered to constitute the strongest evidence for a multi-component distribution. Here I show, using a combined experimental and theoretical approach, that many of the features of the Raman spectrum considered to be hallmarks of a multi-state system, including the asymmetric band profile, the isosbestic (temperature invariant) point, and van't Hoff behavior, actually result from a continuous distribution.;In Chapter 3, I present a detailed comparison of experimental Raman spectroscopy measurements with classical MC simulations, which indicates that the strong perturbations to the Raman spectrum of liquid water, effected by the addition of potassium halides is a direct result of the electric fields existing near the anion. Furthermore, we find that the anion has a clear effect on the hydrogen bonding structure beyond the first solvation shell.;In Chapter 4, I present an experimental and theoretical investigation of the local structure of liquid water using X-ray absorption spectroscopy (XAS). By investigating the temperature dependence of the XAS, we find that the specific spectral features are highly sensitive to relatively minor hydrogen bond distortions. These experimental measurements, in conjunction with a series of spectral calculations on model clusters, indicate that local structure of liquid water is indeed locally tetrahedral. This is in stark contrast to the recent suggestion that liquid water instead comprises rings and chains.;In Chapter 5, I investigate the evaporative cooling rates of a droplet train of liquid water injected into vacuum studied via Raman thermometry. The resulting cooling rates are fit to an evaporative cooling model based on Knudsen's maximum rate of evaporation, in which we explicitly account for surface cooling and potential temperature gradient. We have determined that the value of the evaporation coefficient (gammae) of liquid water is 0.62 +/- 0.09, confirming that a rate-limiting energetic barrier impedes the evaporation rate. Such insight will help in the formulation of a microscopic mechanism for the evaporation of liquid water.