Two-Component Dynamics and the Liquidlike to Gaslike Crossover in Supercritical Water

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.