Slow Neutron Inelastic Scattering Study of Light Water and Ice

Abstract

The results of an extensive slow neutron inelastic scattering study of H2O at 268° and 299°K are reported. A high-energy resolution spectrometer was used to obtain the double differential scattering cross sections over a wide range of energy and momentum transfers. Energy transfers to 0.6 eV and momentum wavevector transfers (k = ΔP / ℏ) to 32 Å−1 were measured for the liquid and solid phases of H2O. The differential cross sections exhibit considerable structure corresponding to hindered rotational motions of the H2O molecule as well as the intramolecular bending and stretching modes of vibration. In the case of ice the angular dependence of the scattering under the hindered rotation band is in good first-order agreement with the distribution calculated for a proton assumed to be bound in an isotropic elastic force field. The scattering properties of water and ice are presented in terms of the scattering law. In the latter form the results are compared to the calculated scattering using the McMurry–Russell modification of the Nelkin model, and the Egelstaff–Schofield theory for an incoherent scatterer with a Gaussian self-correlation function. Spectral densities for the modes of H2O, in water and ice, have been obtained and used with program leap to regenerate scattering functions, which are in good agreement with the experimental results. The rms amplitude of the proton motions has been obtained from the spectral density.