Molecular motions in liquid and solid hydrogen and deuterium

Molecular motions in liquid and solid hydrogen and deuterium have been investigated by the inelastic scattering of cold neutrons. In all cases discrete peaks are observed corresponding to δJ = 0 and J = 1 → 0 rotational transitions. In solid hydrogen there is no broadening of the elastic peak (J = 1 → 1), but the inelastic peak (J = 1 → 0) is appreciably broader than the experimental resolution. The peak widths are in fair agreement with the theoretical predictions of Elliott and Hartmann, confirming that quadrupole-quadrupole interactions are dominant. In liquid hydrogen both elastic and inelastic peaks are broadened by diffusive motion of the molecules, consistent with macroscopic measurements of the diffusion coefficient. In both solid and liquid hydrogen, scattering by intermolecular vibrations is observed; the liquid hydrogen results are fitted approximately by a simple model containing one arbitrary parameter. Measurements on solid and liquid deuterium show the same qualitative features of rotational and diffusive motions, but are complicated by the appearance of peaks due to coherent scattering.