Isotope and Temperature Effects in Liquid Water Probed by X-Ray Absorption and Resonant X-Ray Emission Spectroscopy

High-resolution x-ray absorption and emission spectra of liquid water exhibit a strong isotope effect. Further, the emission spectra show a splitting of the $1b_1$ emission line, a weak temperature effect, and a pronounced excitation-energy dependence. They can be described as a superposition of two independent contributions. By comparing with gas phase, ice, and $\mathrm{NaOH}/\mathrm{NaOD}$, we propose that the two components are governed by the initial state hydrogen bonding configuration and ultrafast dissociation on the time scale of the O $1s$ core hole decay.

Figure 1

O $K$-edge XAS spectra of liquid ${\mathrm{H}}_2\mathrm{O}/{\mathrm{D}}_2\mathrm{O}$ and $\mathrm{NaOH}/\mathrm{OD}$ solutions, acquired at $T=277\mathrm{K}$.

Figure 2

From top to bottom, left: nonresonant ($h\nu =550.1\mathrm{eV}$) XES spectra of amorphous ${\mathrm{H}}_2\mathrm{O}$ and ${\mathrm{D}}_2\mathrm{O}$ ice, nonresonant ($h\nu =550.1\mathrm{eV}$, labeled a), and resonant ($h\nu =536.3$ and 534.6 eV, labeled b and c) XES spectra of liquid ${\mathrm{H}}_2\mathrm{O}$ and ${\mathrm{D}}_2\mathrm{O}$; right: nonresonant spectra of ${\mathrm{H}}_2\mathrm{O}$ and ${\mathrm{D}}_2\mathrm{O}$, differences between these spectra ($d_1$: ${\mathrm{D}}_2\mathrm{O}\mathrm{\text{−}}0.62$ ${\mathrm{H}}_2\mathrm{O}$, $d_2$: ${\mathrm{H}}_2\mathrm{O}\mathrm{\text{−}}0.87$ ${\mathrm{D}}_2\mathrm{O}$), and resonant spectra of $\mathrm{NaOH}/\mathrm{OD}$ solutions ($h\nu =533.5\mathrm{eV}$). The spectra of the liquids were acquired at $T=277\mathrm{K}$, those of ice at $T\sim 100\mathrm{K}$.

Figure 3

Nonresonant ($h\nu =550.1\mathrm{eV}$) XES spectra of liquid ${\mathrm{H}}_2\mathrm{O}$ at different temperatures, normalized to the maximum of the high-energy component of the $1b_1$ peak. Inset: relative contribution of $d_2$ (see Fig. 2) as a function of temperature.