Collective Dynamics in Water by High Energy Resolution Inelastic X-Ray Scattering

A propagating excitation with a velocity of sound of $3200\pm 100$ $\mathrm{m}/\mathrm{s}$ is measured in ${\mathrm{H}}_2\mathrm{O}$ at 294 $\mathrm{K}$ between 4 and 14 ${\mathrm{nm}}^{-1\mathrm{}}$, using inelastic x-ray scattering with 5 $\mathrm{meV}$ energy resolution. The existence of fast sound is therefore demonstrated in an energy-momentum region much wider than that of previous neutron measurements on ${\mathrm{D}}_2\mathrm{O}$. The equivalence of the fast sound velocity in ${\mathrm{H}}_2\mathrm{O}$ and in ${\mathrm{D}}_2\mathrm{O}$ 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.