High-resolution, inelastic x-ray scattering measurements of the dynamic structure factor $S(Q,\omega )$ of liquid water have been performed for wave vectors $Q$ between 4 and $30{\mathrm{nm}}^{-1}$ in distinctly different thermodynamic conditions ($T=263–420\mathrm{K}$; at, or close to, ambient pressure and at $P=2\mathrm{kbar}$). In agreement with previous inelastic x-ray and neutron studies, the presence of two inelastic contributions (one dispersing with $Q$ and the other almost nondispersive) is confirmed. The study of their temperature and $Q$ dependence provides strong support for a dynamics of liquid water controlled by the structural relaxation process. A viscoelastic analysis of the $Q$-dispersing mode, associated with the longitudinal dynamics, reveals that the sound velocity undergoes a complete transition from the adiabatic sound velocity $\left(c_0\right)$ (viscous limit) to the infinite-frequency sound velocity $\left(c_{\infty }\right)$ (elastic limit). On decreasing $Q$, as the transition regime is approached from the elastic side, we observe a decrease of the intensity of the second, weakly dispersing feature, which completely disappears when the viscous regime is reached. These findings unambiguously identify the second excitation to be a signature of the transverse dynamics with a longitudinal symmetry component, which becomes visible in $S(Q,\omega )$ as soon as the purely viscous regime is left.

• Received 24 September 2004

DOI:https://doi.org/10.1103/PhysRevE.71.011501