X-Ray Absorption Spectra of Water from First Principles Calculations

We present a series of ab initio calculations of the x-ray absorption cross section (XAS) of ice and liquid water at ambient conditions. Our results show that all available experimental data and theoretical results are consistent with the standard model of the liquid as comprising molecules with approximately four hydrogen bonds. Our simulations of ice XAS including the lowest lying excitonic state are in excellent agreement with experiment and those of a quasitetrahedral model of water are in reasonable agreement with recent measurements. Hence we propose that the standard, quasitetrahedral model of water, although approximate, represents a reasonably accurate description of the local structure of the liquid.

Figure 1

Left: NEXAFS spectrum of crystalline ice I from Ref. 6 (thick, solid) and calculated XAS of hexagonal ice $\mathrm{I}h$ (solid line) and cubic ice $\mathrm{I}c$ (dotted line). Right: NEXAFS (thick solid line) and XRS (thick dashed line) spectra of water from Ref. 6, and calculated XAS of water (gray line) from the TIP4P MD simulation at 300 K. Vertical thickness of the calculated curve is the associated standard error from a sample of 32 water molecules.

Figure 2

Left: Comparison of the calculated XAS of water and crystalline ice $\mathrm{I}h$ indicating qualitative spectral differences between liquid and solid phases. Right: A similar comparison between ice $\mathrm{I}h$ and a model of amorphous ice. Also indicated in both panels are the characteristic spectra from water molecules with at least one broken donor hydrogen bond (see text).

Figure 3

Comparison of DFT XAS within various approximations for the x-ray absorption process, viz XCH, FCH, and HCH. (a) hexagonal ice; (b) amorphous ice; (c) liquid water; (d) liquid water sampling only broken-hydrogen-bonded species.