Raman spectra of water in the translational and librational region

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Abstract
Polarized and depolarized Raman spectra in the translational and librational region of liquid water and heavy water have been measured as a function of temperature. A detailed analysis of the isotropic spectrum is performed. This component, which cannot arise from any ‘allowed’ molecular contribution in this frequency region, shows two distinct spectral features which can be obtained from an analysis of temperature dependent lineshapes. The first one, which dominates the spectrum up to 500 cm-1, shows on the basis of a H2O-D2O comparison a translational behaviour at very low frequency and a librational one at higher frequencies. It is assigned on the basis of its frequency profile, of its absolute intensity and of its temperature dependence to rototranslational spectra due to long range induction mechanisms already found in many molecular liquids. The second one, which is peaked at roughly 800 cm-1 in water and extends up to the bending mode region suggests the existence of a short range interaction induced polarizability.