Calculation of Near-Edge X-Ray Absorption Fine Structure at Finite Temperatures: Spectral Signatures of Hydrogen Bond Breaking in Liquid Water

We calculate the near-edge x-ray-absorption fine structure of H2O in the gas, hexagonal ice, and liquid phases using heuristic density-functional based methods. We present a detailed comparison of our results with experiment. The differences between the ice and water spectra can be rationalized in terms of the breaking of hydrogen bonds around the absorbing molecule. In particular the increase in the pre-edge absorption feature from ice to water is shown to be due to the breaking of a donor hydrogen bond. We also find that in water approximately 19% of hydrogen bonds are broken.