The role of the internal energy in the distribution function of a heat conducting gas

Using a state-selective optical technique, we have measured directly the nonequilibrium distribution function f in heat conducting iodine gas as a function of the rotational and vibrational quantum numbers. The results show that, contrary to the case for translational energy, no rapid convergence exists for the expansion of f in polynomials (Wang-Chang-Uhlenbeck polynomials) containing the rotational or vibrational energy. The weight of the polynomials quadratic in the internal energy relative to polynomials of the first degree in the internal energy is found to be about 9% for the rotational energy and about 27% for the vibrational energy.