Rotational excitations in CH4 II: Effect of Kr impurities

The perturbation of the partial orientational order in CH4II caused by dilution with Kr(xKr≤0.20) has been studied with inelastic incoherent neutron scattering. Both, the lines assigned as rotational tunneling and almost free molecular rotation, display a characteristic asymmetric broadening which gets more pronounced with increasing Kr concentration. At low Kr concentrations (xKr≤0.05) the tunneling spectra are well explained by a model which is based on a statistical distribution of local rotational potentials. In this model only the number of Kr atoms replacing orientationally ordered CH4 nearest neighbors is relevant for the local potential at the sites of tunneling molecules. The different local symmetries are not considered in the model. It was not possible to formulate a quantitative model for the scattering from molecules at nearly free rotor sites.