Theory of vibrational relaxation in mixtures of ortho- and para-hydrogen

Abstract

A numerical study of the vibrational relaxation at 500 K of a mixture of ortho-H2 and para-H2 is described. The required state-to-state rate constants were calculated from the quantum results of Rabitz and co-workers, and missing pieces of data were estimated by interpolation.It is concluded that only one relaxation time will be observed in any mixture of ortho-H2 and para-H2 and that (except at very high dilutions in a third inert gas) the relaxation rate constant will be close to the mean of the individual rate constants for relaxation, weighted according to the respective mole fractions of ortho-H2 and para-H2 present in the mixture.We find that the relaxation process can be modelled very accurately as an electrical RC network, whose time constants can be written down quite easily as sums of the appropriate microscopic rate constants, and by using this model, it is a simple matter to explore the conditions required for a mixture of two gases to exhibit two distinct vibrational relaxation times.