Electron-Hole Theory of the Effect of Quantum Nuclei on the X-Ray Absorption Spectra of Liquid Water

Electron-hole excitation theory is used to unveil the role of nuclear quantum effects on the x-ray absorption spectral signatures of water, whose structure is computed via path-integral molecular dynamics with the MB-pol intermolecular potential model. Compared to spectra generated from the classically modeled water, quantum nuclei introduce important effects on the spectra in terms of both the energies and line shapes. Fluctuations due to delocalized protons influence the short-range ordering of the hydrogen bond network via changes in the intramolecular covalence, which broaden the preedge spectra. For intermediate-range and long-range ordering, quantum nuclei approach the neighboring oxygen atoms more closely than classical protons, promoting an “icelike” spectral feature with the intensities shifted from the main edge to the postedge. Computed spectra are in nearly quantitative agreement with the available experimental data.

Figure 1

(a) XAS spectra from MD (dashed blue line) and PIMD (solid red line) simulations at 298 K. Experimental data [16] is shown in shade. Joint probability distribution of the covalent bond length as a function of the excitation energy for state with $4a_1$ character, from (b) MD and (c) PIMD simulations. Probability distribution of the excitation energies for states with $4a_1$ (green) and $2b_2$ (orange) character, from (d) MD and (e) PIMD simulations.

Figure 2

(a) Distribution of proton-transfer coordinate ($v$) from MD (dashed blue line) and PIMD (solid red line) trajectories. XAS spectra are decomposed into contributions from excited water molecules within two $v$ regions (b),(c). Green circles represent absorption cross sections (ACS) of excitation within each $v$ region. Insets represent excited states with $b_2$ character. The excited oxygen atom is shown in black in the insets.

Figure 3

XAS spectra calculated using configurations from MD (dashed line) and PIMD (solid line) simulations at 270 and 360 K.

Figure 4

Distributions of ring size at 270, 298, and 360 K obtained from MD (dashed line) and PIMD (solid line) simulations.