Slow-Neutron Inelastic Scattering from Low-Temperature Gaseous Methane, Liquid Methane, and Solid Methane

The partial differential cross sections for inelastic scattering of neutrons from samples of cold gaseous methane (T=125°K), liquid methane (T=99°K), and solid methane (T=83°K) were measured with the materials testing reactor (MTR) phased chopper velocity selector. These results are compared to the theory reported by Griffing for methane gas, which fits the cold-gas data very well over the entire range of the experiment and also fits the liquid and solid data at the large momentum transfers. The results of these comparisons indicate that the rotations are free in all three phases and that, in the liquid and solid phases, there are oscillations of the center of mass of the molecule with dominant transitions having an energy change near 0.017 eV. The range of agreement of the gas theory with the liquid and solid data indicates that the liquid and solid act according to the gas model out to a time of roughly 5×10−13 sec. The similarity of the liquid and solid data over the range of the experiment suggest further that they act similarly out to even longer times. The zeroth and first moments of energy transfer were calculated and for the gas compare favorably with a molecular theory which treats the rotations classically. For the liquid and solid data, the moments give further indications of the molecular oscillations.