Intermolecular vibrations and diffusion in water investigated by scattering of cold neutrons

Abstract

Abstract The energy distributions of neutrons scattered by H 2 O, D 2 O and some aqueous solutions were measured at temperatures between ‐ 5 °C and + 95 °C. Quasi‐elastic and inelastic scattering data were analysed to study the diffusion mechanism and the frequency distribution of the intermolecular vibrations, respectively. The diffusion coefficients were derived from the line widths of the quasi‐elastic scattering law at small momentum transfers Q . Near the boiling point they agree within 10% with the spin‐echo results. In the low‐temperature range, however, the neutron scattering results are greater up to 70%. The energy dependence of the quasi‐elastic scattering law shows a Lorentzian shape. The Q ‐dependence of the integral quasi‐elastic scattering law and of the line widths were analysed by application of the jump diffusion model and the globule model. The temperature dependence of the mean‐square displacement of the protons shows a minimum at about 30 °C. For heavy water an influence of the pair correlation of the molecules upon the quasi‐elastic line width has been observed. The frequency distribution in H 2 O ice at ‐ 5 °C shows peaks at excitation energies of 6.7 meV and 87 meV. The main peaks occur in the water spectra at nearly the same energies, but they are increasingly broadened with increasing temperature.