A Microscpoic Model for the Neutron Dynamic Structure Factor of Solid Methane in phase II
Abstract
We have constructed an approximate microscopic model for the neutron dynamic structure factor of solid methane in phase II. We expect our model to apply for neutron energies below 1eV at pressures near 1 bar and temperatures below 20K, where solid methane exists in a phase, called phase II, in which the tetrahedral hydrogens exist in a mixed phase of free rotors and hindered rotors. Certain matrix elements needed for the dynamic structure factor S(Q, ω) are adapted from a calculation by Ozaki et al.ozaki:3442ozaki2. The model treats the effects of molecular translations, intramolecular vibrations and the free and hindered rotational degrees of freedom as uncoupled ones. Total scattering cross sections calculated from the model agree with the cross section measurements of Griegergrieger:3161 and WhittemoreWittemore for the incident neutron energies of 0.5 meV 1eV. The effective density of states extracted from the model was compared with the Harker & Brugger frequency spectrum extracted from neutron scattering measurements conducted at 22KHARKER1967.