Impact of nuclear quantum effects on the structural inhomogeneity of liquid water

Abstract

The 2D Raman–terahertz (THz) response of liquid water is studied in dependence of temperature and isotope substitution ( ${\mathrm{H}}_2$ O, ${\mathrm{D}}_2$ O, and ${\mathrm{H}}_2^{18}$ O). In either case, a very short-lived (i.e., between 75 and 95 fs) echo is observed that reports on the inhomogeneity of the low-frequency intermolecular modes and hence, on the heterogeneity of the hydrogen bond networks of water. The echo lifetime slows down by about 20% when cooling the liquid from room temperature to the freezing point. Furthermore, the echo lifetime of ${\mathrm{D}}_2$ O is $6.5\pm 1\%$ slower than that of ${\mathrm{H}}_2$ O, and both can be mapped on each other by introducing an effective temperature shift of $\mathrm{\Delta }T=4.5\pm 1$ K. In contrast, the temperature-dependent echo lifetimes of ${\mathrm{H}}_2^{18}$ O and ${\mathrm{H}}_2$ O are the same within error. ${\mathrm{D}}_2$ O and ${\mathrm{H}}_2^{18}$ O have identical masses, yet ${\mathrm{H}}_2^{18}$ O is much closer to ${\mathrm{H}}_2$ O in terms of nuclear quantum effects. It is, therefore, concluded that the echo is a measure of the structural inhomogeneity of liquid water induced by nuclear quantum effects.