X-Ray Absorption Signatures of the Molecular Environment in Water and Ice

The x-ray absorption spectra of water and ice are calculated with a many-body approach for electron-hole excitations. The experimental features, including the effects of temperature change in the liquid, are reproduced from configurations generated by ab initio molecular dynamics. The spectral difference between the solid and the liquid is due to two major short-range order effects. One, due to breaking of hydrogen bonds, enhances the pre-edge intensity in the liquid. The other, due to a nonbonded molecular fraction in the first coordination shell, affects the main spectral edge in the conversion of ice to water. This effect may not involve hydrogen bond breaking as shown by experiment in high-density amorphous ice.

Figure 1

Calculated and experimental [2] XAS spectra. From bottom to top. (a) Ice: Full line—theory, ice $\mathrm{I}c$; dashed line—experiment, ice $\mathrm{I}h$, at 95 K. (b) Water: Full line—theory at 363 K; dashed line—experiment at 290 K. (c) Theoretical spectra for water at two different temperatures: blue line, at 330 K, and red line, at 363 K. (d) Difference spectra: Theory and experiment (data points with error bars). Details on the alignment and on the theoretical error bars (red) are given in the supporting online material [14]. As the temperature difference in the experiment ($\Delta T=65\mathrm{K}$) was nearly double that in the calculation ($\Delta T=33\mathrm{K}$), the theoretical data are magnified by a factor of 10, while the experimental data are magnified by a factor of 5 instead of by the factor of 10 used in Ref. 2. Linear dependence on temperature is to be expected for a temperature change small on the energy scale of the structural changes.

Figure 2

Excited electron state in the presence of a core hole on the central oxygen atom. The calculation is at the Hartree-Fock level for the monomer ($m$) and at the COHSEX level for ice $\mathrm{I}c$ ($i$) and water ($w$). 1: Pre-edge; 2: near edge; 3: post-edge. Molecules up to the second coordination shell are shown for ice and water. The color difference in the plots corresponds to a difference in the sign of the wave function.

Figure 3

Electronic DOS of ice ($\mathrm{I}c$) and water, calculated with a $4\times 4\times 4$ $k$-point mesh (ice) and at $k=0$ (water) with Gaussian broadening of 0.3 eV. The conduction states have positive energy. The insets show a central tagged molecule and two adjacent molecules in ice (top) and water (bottom). Also plotted is the integrated local DOS in correspondence with the DOS peaks. The bottom inset shows overlap, indicated by an arrow, between the antibonding density of the nonbonded molecule on the left and the central molecule, while no overlap occurs between the latter and the bonded molecule at its right. See also Ref. 14.