Low-frequency Raman study of water isotopes

Depolarized low-frequency Raman spectra of H2O, D2O, H2 18O, and D2 18O are measured from 266 to 350 K. The reduced Raman spectra below 250 cm−1 are reproduced by a superposition of one relaxation mode and two damped harmonic oscillators. The multiple random telegraph model, which takes into account the inertia and memory effects is applied to analyze the central relaxation component. The 180 cm−1 mode corresponds to vibration of the oxygen atom. The hydrogen isotopes affect the relaxation time causing its variation. The relaxation time is considered to correspond to the duration of fluctuation of the local structure in liquid water. The intensity of the 50 cm−1 mode decreases with increasing temperature and finally vanishes above about 300–310 K. The 50 cm−1 mode can be distinguished from the relaxation mode when the relaxation time is more than five times longer than the period of the 50 cm−1 vibration mode.