Molecular-scale dynamics in sub-to supercritical water is studied with inelastic X-ray scattering and molecular dynamics simulations. The obtained longitudinal current correlation spectra can be decomposed into two main components: A low-frequency (LF), gaslike component and a high-frequency (HF) component arising from the O-O stretching mode between hydrogen-bonded molecules, reminiscent of the longitudinal acoustic mode in ambient water. With increasing temperature, the hydrogen-bond network diminishes and the spectral weight shifts from HF to LF, leading to a transition from liquid-to gaslike dynamics with rapid changes around the Widom line.
P. Sun, J.B. Hastings, D. Ishikawa, A.Q.R. Baron, G. Monaco, Two-Component Dynamics and the Liquidlike to Gaslike Crossover in Supercritical Water, Phys. Rev. Lett. 125, 256001 (2020), https://doi.org/10.1103/PhysRevLett.125.256001.