Dynamics of liquid CH₄ from cold-neutron scattering

Abstract

The scattering of 4.1 Å neutrons by liquid CH₄ at 98°K has been studied at several angles using a rotating-crystal spectrometer. The spectra at all angles show a broad inelastic hump associated primarily with the rotational motions and a quasi-elastic peak which is related to the translational motions. The width of the quasi-elastic peak follows the simple diffusion behavior over almost the entire range of present experiments, and leads to a value of 2.2×10⁻⁵ cm²/sec for the diffusion coefficient. A more detailed analysis suggests that the delay time in the onset of simple diffusion behavior is similar to that in argon. The data clearly show that if the translations are viewed in terms of the Langevin model, then it is not necessary to consider a wavelength-dependent damping factor such as has been recently proposed by Griffing. High-resolution data taken at 15° scattering angle do not reveal any sharp inelastic peaks, which should be present if the rotations are free. From this it is concluded that the rotations are hindered in the liquid state, in accord with earlier spectroscopic work, but in disagreement with the neutron work of Hautecler and Stiller. Calculations based on a model in which the rotations are free and the translations follow the simple diffusion model show poor agreement with experiments. Consideration of a more realistic model for the translations, based on the results of the computer experiments of Nijboer and Rahman for argon, yields only a slight improvement. It is suggested that a proper treatment of the hindered rotations is necessary to obtain agreement with experiment.