Molecular Dynamics of Methane at Low and Moderate Densities Studied by Cold Neutron Scattering

Abstract

In order to study the molecular dynamics of methane at low and moderate densities cold neutron scattering experiments were undertaken with the following target samples:gas: Temperature 295 K and pressures 15, 70 and 130 atm.Temperature 198 K and pressure 60 atm.liquid: Temperature 184 K and pressure 45 atm.To interpret the experimental data it is necessary to assume that the rotational and translational motions of the molecule scatter neutrons independently. The following result is then obtained:1. The rotational motion is fairly well reproduced by models close to the free rotation behaviour for all the investigated states.2. The translational motion at 295 K is described by the ideal gas model only for very dilute systems. Already at the pressure 15 atm the collision narrowing effect is visible. For pressures 15 to 130 atm and the temperature 295 K the rigid sphere model gives the best agreement with the experimental data, while for states around the critical point no available model can satisfactorily reproduce the measured spectra. The observed discrepancies are manifested in a somewhat larger width of the experimental, quasielastic peak than predicted by the theoretical models based on the picture of freely streaming particles making binary collisions.