COLLECTIVE DYNAMICS IN WATER BY HIGH-ENERGY RESOLUTION INELASTIC X-RAY-SCATTERING

A propagating excitation with a velocity of sound of 3200 +/- 100 m/s is measured in H2O at 294 K between 4 and 14 nm(-1), using inelastic x-ray scattering with 5 meV energy resolution. The existence of fast sound is therefore demonstrated in an energy-momentum region much wider than that of previous neutron measurements on D2O. The equivalence of the fast sound velocity in H2O and in D2O rules out models where this mode propagates only on the hydrogen network. These results show the ability of inelastic x-ray scattering to study the collective dynamics of liquids.