Onset of a transverse dynamics in the THz spectrum of water

This paper reports on a high resolution/high contrast measurement of the spectrum of heavy water achieved by two complementary techniques, Inelastic Neutron and X-Ray Scattering. The mutual consistency between the spectral shapes measured by the two methods is subjected to thorough scrutiny and the results of their combined best fit are discussed. In particular, the presence of a low frequency mode related to shear propagation is studied in connection with the structural relaxation active in water at lower frequencies. The best fit analysis leads to the conclusion that the onset of a shear mode can only be observed at timescales approaching the solid-like regime, i.e. before internal rearrangements of the structure are fully accomplished. Finally, it is shown that the lifetime of the low frequency mode is fully consistent with the one predicted for a shear wave approaching the macroscopic limit.