Inelastic neutron scattering from liquid and solid hydrogen at high momentum transfer

Solid and liquid para-hydrogen and a solid mixture of ortho- and para-hydrogen have been studied by inelastic neutron scattering at momentum transfers from 5 to 30 A${̊}^{\mathrm{-}1}$. The observed scattering function can be interpreted in terms of scattering from independent hydrogen molecules using the impulse approximation when internal excitations of the molecules are included. Scattering peaks are observed at the recoil energy, shifted by the energy of the internal rotational transitions, and broadened by the translational motion of the molecules. The peaks can be fitted by Gaussian functions, showing that, to within the accuracy of the measurement, the translational momentum distribution of the hydrogen molecules in the condensed phases has a Gaussian form. The kinetic energy of the para-hydrogen molecules at 1 bar is 76±9 K in the solid at 10 K and 63±6 K in the liquid at 17 K. The kinetic energy for the ortho component in the solid ortho-para mixture is estimated to be the same as for the para solid within the experimental error, although extraction of quantitative information is more difficult because of the complicated spectra.