Molecular Motion of Water in Real Space and Time observed with Inelastic X-ray Scattering*

We report on real-space and real-time molecular motions of water and its temperature variation at pico-second and sub-nm scales in time and space using van Hove function of water molecules. Measurement of high-resolution inelastic x-ray scattering spectra, S(Q, E), over wide Q (momentum transfer) and E (energy), enables us to determine the van Hove function by Fourier transform. The results show that the dynamics of first and second nearest neighbor molecules are strongly coupled, and that the topological rearrangement of configurations shows slowing-down with decreasing temperature. By comparing the results with molecular dynamics simulation, we find that the time-scale of local topological changes in molecular configuration corresponds to the Maxwell relaxation time, which is determined by macroscopic measurement of viscosity and shear modulus. This supports our earlier findings that the topological changes in molecular connections is the origin of viscosity in liquid.