Quantum effects in rotational relaxation of a freely expanding gas

The distributions of rotational and translational energy along the axis of a jet of nitrogen expanding into a vacuum are analyzed by taking into account the noncontinuous nature of the rotational spectrum of a molecule. The system of gasdynamic equations for a model problem of expansion from a spherical source is solved. At low kinetic temperatures the rotational relaxation time of the gas is considered in the framework of quantum mechanics. Experimental curves representing the upper and lower limits of the distribution of rotational energy along the jet manifest a fork into which only curves obtained by taking quantum effects into account fit.