Phonon Relaxation of Light and Heavy Water

It is clear no w why cooling softens the coh and stiffens the (Oj of liquid H2O (region I in Fig. 36.2) [28, 32, 34—38] other than the otherwise as happened in other usual materials. The cooling shortening and stiffening of the master 0 H bond is 

36 Thermally Driven Density and Phonon-Stiflhess Oscillation 

Using an attenuated total reflection (ATR) IR spectroscopy, Marechal [41] measured the phonon spectra of ordinary (H2O) and heavy (D2O) water, as shown in Fig. 36.9. The isotope has two effects. One is the intensity attenuation of the absorption peaks and the other is the softening of the frequencies of all the detectable vibration modes of H2O [41]. 

For intramolecular vibration, rrii is the mass of H or D (2 atomic unit), m2 is the mass of O (16 unit) for intermolecular vibration, mi — m2 is the mass of 2H + O (18 unit) or 2D -I- O (20 unit). Based on the derived ratios and the measurements, one can find 

Devotion of co arising from the Coulomb coupling will play a certain role, but it is different from other effects, such as quantum fluctuation and polarization, are negligible. It is thus justified that the first-order approximation is sufficient to describe the isotopic effect on the phonon relaxation dynamics of coh and cob- The large deviation of the co arises from the polarization and Coulomb repulsion. Therefore, the addition of isotope softens all the phonons by mediation of the effective mass of the dimer. Isotope also lowers the peak intensities because of the enhanced scattering by the low-frequency vibrations. 

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