The Dynamics of Liquid H2O and D2O and Solid H2O from the Inelastic Scattering of Epithermal Neutrons

An extensive series of neutron inelastic scattering measurements has been made on liquid H2O and D2O at 299 °K and on solid H2O at 268°K. Double differential cross-sections for H2O were obtained for energy transfers to 0.6 eV and momentum wave vector transfers (Δp/n) to 32 Å-1 while for D2O the energy and momentum transfer range was 0.2 eV and 9.5 Å-1, respectively. All measurements were obtained with the Battelle rotating crystal spectrometer using initial neutron energies of 0.101 to 0.616 eV . Scattering angles ranged from 15 to 155°. The measured differential cross-sections exhibit considerable structure in energy corresponding to the torsional motions and the intramolecular bending and stretching modes of the H2O molecule. Instrumental resolution was adequate to obtain information about the widths of some of these transitions. The angular dependence of scattering associated with the excitation of a definite mode, in a solid or liquid, can be expected to depend upon the shape of the potential well in which the dynamical unit moves. In the case of the present measurements on ice, it was possible to determine the angular dependence of the inelastic scattering from the torsional band. Th e experimental differential cross-section for excitation of these modes is found to be in good agreement with the angular dependence calculated for inelastic scattering from a symmetric harmonic oscillator potential. Spectral density functions for the proton and deuteron have been obtained using an extrapolation technique. These density functions extend to higher energy transfers than previously published neutron results, and include the intramolecular bending modes of the water molecule. The mean squared amplitudes of the proton and deuteron clouds are derived from the spectral density functions. These amplitudes are found to be in agreement with neutron diffraction studies of D2O but are smaller than values obtained by other investigators who measured the Debye-Waller factor for quasi-elastically scattered slow neutrons. (author)