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

The 2D Raman-terahertz (THz) response of liquid water is studied in dependence of temperature and isotope substitution (H2O, D2O, and (H2O)-O-18). 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 Delta T = 4 : 5 +/- 1 K. In contrast, the temperature-dependent echo lifetimes of (H2O)-O-18 and H2O are the same within error. D2O and (H2O)-O-18 have identical masses, yet (H2O)-O-18 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.