A Microscpoic Model for the Neutron Dynamic Structure Factor of Solid Methane in phase II

TL;DR

This paper develops an approximate microscopic model for the neutron dynamic structure factor of solid methane in phase II, applicable for low-energy neutrons and specific temperature and pressure conditions, incorporating molecular motions and rotational effects.

Contribution

The paper introduces a new microscopic model that accounts for molecular translations, vibrations, and rotational degrees of freedom in solid methane phase II, aligning well with experimental data.

Findings

Model agrees with experimental cross sections for 0.5 meV to 1 eV neutron energies.

Effective density of states matches neutron scattering spectra at 22K.

Applicable for neutron energies below 1 eV at near 1 bar and below 20K.

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 1\textit{eV} 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 $S(Q, \omega)$ are adapted from a calculation by Ozaki \textit{et al.}\cite{ozaki:3442}\cite{ozaki2}. 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 Grieger\cite{grieger:3161} and Whittemore\cite{Wittemore} for the incident neutron energies of 0.5 \textit{meV} $\sim$ 1\textit{eV}. The effective density of states extracted from the model was compared with the Harker $&$ Brugger frequency spectrum extracted from neutron scattering measurements conducted at 22K\cite{HARKER1967}.