Collective dynamics of supercritical water probed by inelastic X-ray scattering

Inelastic X-ray scattering experiments on sub- and supercritical water were performed to investigate collective dynamics of this unique solvent. Analysis within a generalized Langevin formalism shows that the positive dispersion of the inelastic excitations, as compared to the hydrodynamic value, first decreases (1.0 < ρ < 0.8 g cm−3) at all measured momentum transfers, Q = 1.3-10.7 nm−1, and then increases (0.7 < ρ < 0.26 g cm−3) again only at higher Q. The latter increase may be related to a fast breaking and reforming of the hydrogen bonds in sub- and supercritical water.