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 (H2O, D2O, and H218O). 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 D2O is 6.5±1% slower than that of H2O, and both can be mapped on each other by introducing an effective temperature shift of ΔT=4.5±1 K. In contrast, the temperature-dependent echo lifetimes of H218O and H2O are the same within error. D2O and H218O have identical masses, yet H218O is much closer to H2O 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.