The dynamics of supercritical water: A quasielastic incoherent neutron scattering study

Results of quasielastic and inelastic incoherent neutron scattering experiments of light water at several thermodynamic states (200 < T < 400 degrees C and 184 < P < 400 bar) corresponding to densities ranging from 0.2 to 0.9 g/cm(3) are presented. The results have been analyzed using a jump diffusion model and the two parameters of this model, namely, tau(0), the residence time and D, the translational diffusion coefficient have been determined as a function of the density of supercritical water. The values of the diffusion coefficient D reported here are in very good agreement with those measured by NMR and increases strongly as the density of the medium decreases. The residence time tau(0) slightly increases as the density of supercritical water decreases, with a value between 0.14 and 0.2 ps (ten times shorter than that measured in ambient liquid water). Moreover, we observe an almost linear increase in the mean jump distance L = root 6D tau(0) from 1.5 Angstrom at 0.9 g/cm(3) to 4 Angstrom at 0.2 g/cm(3). Finally, the value of the mean square displacement [u(2)](1/2) increases from 0.6 Angstrom at 0.9 g/cm(3) to 1 Angstrom at 0.2 g/cm(3). (C) 2000 American Institute of Physics. [S0021-9606(00)51330-0].