Rotational excitations and orientational disorder in solid mixtures of CH4 and Kr

The orientational disorder in a solid solution of methane and krypton (27% Kr) has been studied with inelastic neutron scattering. The spectra display a transition from diffusive high temperature rotations to rotational excitations of molecules with frozen-in orientational disorder at low temperatures. Of particular interest is a relatively sharp feature at an energy transfer of 0.78 meV which both broadens and shifts with increasing temperature. A simple model is based on a statistical distribution of local potentials due to the presence of Kr atoms and the state of excitation of the molecules. Finally, the isotope effect of the rotational excitations has been investigated.