X-ray scattering and spectroscopy of supercooled water and ice

Abstract: This thesis presents experimental studies of water and ice at near-atmospheric pressures using intense x-rays only accessible at synchrotrons and free-electron lasers. In particular, it focuses on the deeply supercooled, metastable state and its implications on ice nucleation.The local structure of the liquid phase was studied by x-ray scattering over a wide temperature range extending from 339 K down to 227 K. In order to be able to study the deeply supercooled liquid, micron-sized water droplets were evaporatively cooled in vacuum and probed by ultrashort x-ray pulses. This is to date the lowest temperature at which measurements of the structure have been performed on bulk liquid water cooled from room temperature. Upon deep supercooling, the structure evolved toward that of a low-density liquid with local tetrahedral coordination. At ~230 K, where the low-density liquid structure started to dominate, the number of droplets containing ice nuclei increased rapidly. The estimated nucleation rate suggests that there is a “fragile-to-strong” transition in the dynamics of the liquid below 230 K, and its implications on water structure are discussed.Similarly, the electronic structure of deeply supercooled water was studied by x-ray emission spectroscopy down to 222 K, but the spectral changes expected from the structural transformation remained absent and explanations are discussed. At high fluence, the non-linear dependence of the x-ray emission yield indicated that there were high valence hole densities created during the x-ray pulse length due to Auger cascades, resulting in reabsorption of the x-ray emission.Finally, the hydrogen-bonded network in water was studied by x-ray absorption spectroscopy and compared to various ices. It was found that the pre-edge absorption cross-section, which is associated with distorted hydrogen bonds, could be minimized for crystalline ice grown on a hydrophobic BaF2(111) surface with low concentration of nucleation centers.