Exchange Between Rotational and Translational Energy in Gas Molecules

Rotational relaxation times for the gases H2 and D2 have been calculated with the quantum mechanical method of distorted waves using a one-dimensional collision model and an exponential repulsive interaction potential. The potential constants were determined by fitting to the data of Hirschfelder, et al. and certain theoretical potential calculations for H2 by Margenau. The calculations indicate that more than one relaxation time is required to describe rotational relaxation phenomena in these gases. The theoretical relaxation times are 15 to 35 times shorter than the experimental values when one averages with the velocity distribution for a Maxwellian beam of particles. The relative values of the calculated relaxation times compare favorably with those obtained from experiments. Averaging with a one-dimensional velocity distribution instead of that of a Maxellian beam of prticles at temperature T yields calculated values of the relaxation times that are a factor 7 to 19 shorter than the experimental values. One expects the calculated values to be shorter than the experimental values in view of the one-dimensional collision model. The method used in treating H2 collisions is not applicable to N2 and O2. Results of another approximation method for these gases indicate that more complicated techniques must be used.