Viscoelastic behavior of water in the terahertz-frequency range: An inelastic X-ray scattering study

High-resolution, inelastic x-ray scattering measurements of the dynamic str ucture factor S(q,omega) of Liquid water in the THz frequency range have be en performed as a function of wave vector q (1-7 nm(-1)) and temperature T (273-473 K), using pressure (0-1.5 kbar) to keep the density at rho approxi mate to 1 g/cm(3). We show that, for q less than or equal to 2 nm(-1), the S(q, omega) spectra can be consistently explained in terms of a hydrodynami c formalism which includes a viscoelastic, q-independent contribution to th e memory function for the density fluctuations. This allows us to extract v alues for the infinite frequency sound velocity and for the structural rela xation time which are found to compare favorably with those obtained using techniques which probe a lower-frequency range. As a consequence, the atomi c dynamics in water is shown to have a homogeneous character down to a leng th scale of approximate to 3 nm. At q>2 nm(-1), we find that the viscoelast ic contribution to the memory function becomes q dependent. Thus this work provides a view on the evolution of the collective dynamics of water across the q region where the continuum approximation inherent in the hydrodynami c formulation begins to fail. The physical consequences of such a result ar e discussed in some detail. [S1063-651X(99)03911-2].