Molecular dynamics by neutron scattering in pure hydrogen and a mixture of hydrogen and argon gases

The inelastic scattering of 2.4 A neutrons by two states of pure hydrogen and two states of a mixture of hydrogen and argon (at a similar density) have been studied. From these data relaxation times for the velocity auto-correlation function of each state are obtained and are compared to the predictions of a simple hard sphere kinetic theory. It is found that, although the relaxation time depends on the momentum transfer, for pure hydrogen the prediction is in general agreement with experiment. For the mixture the prediction is about 2.5 times larger than the measured values, which is attributed to the reduction in the 'persistence' of the velocity after a collision. (auth)