Neutron Scattering and Nuclear Spin Species Conversion in the Solid Methanes

Abstract

<p>An understanding of the interconversion between the nuclear spin symmetry species in the solid methanes is essential for characterising the unusual low temperature behaviour of these solids. In this thesis, the measurement of the total neutron scattering cross-sections of the methanes is used to study quantitatively the extent and kinetics of the spin conversion process.</p> <p>A method of accurately measuring total cross-sections of molecular solids, at low temperatures (0.75 K</p> <p>Three of the solid isotopic methanes were studied (CH₄, CH₃D and CH₂D₂). In CH₄, extensive measurements were made on solid in phase II, in which spin conversion occurs at different rates on the two different kinds of sublattice. The conversion lifetime of the molecules on the ordered sublattice sites was found to be 75 hours. On the disordered sublattices, the lifetimes were much shorter and strongly temperature dependent. The equilibrium cross-section data were used to critically test a theoretical description of solid CH₄. It was found to be satisfactory.</p> <p>Spin conversion in CH₃D was studied quantitatively and found to occur primarily below T = 4 K. In contrast to CH₄, the spin conversion process in CH₃D was very fast. The cross-section data were also used to guide the analysis of some existing thermodynamic data for CH₃D, and to derive the characteristics of the low-lying tunnelling levels. Some inferences about the nature of the lowest temperature phase (phase III) are made.</p> <p>Little is known about CH₂D₂, and the cross-section measurements were made to determine if spin conversion occurs, especially below T = 4 K. A change in cross-section was found in this temperature region and an argument is presented which could ascribe such a change to spin conversion.</p>