Water spectra vibrational state mechanisms

For ice, unlike for water, we use slightly different lifetimes xq and x which pertain to elastic vibrations and are ascribed, respectively, to the mechanisms (b) and (c), namely, xq = 0.23 ps and x = 0.34 ps. If xq = t, the calculated spectrum shows worse agreement with experiment. The proximity of the lifetimes xq and xfl indicates that elastic translation and reorientation of H-bonded molecules are determined by the same (or by a very similar) state of molecular structure, characterized by elastic constants k and cTOt. 

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