NONEQUILIBRIUM DISTRIBUTIONS OF ROTATIONAL AND VIBRATIONAL ENERGIES IN A FREE-JET EXPANSION

The occupations of various rovibrational levels of Na-2 molecules in a free jet of Ar have been determined as functions of position. For the first part of the expansion, the internal state distributions are fou nd to be close to equilibrium; the temperatures agree with the continu um model. Further downstream, appreciable deviations from equilibrium arise; an analysis of these distributions in terms of rotational and v ibrational temperatures is found to be ambiguous. However, the average rotational and vibrational energies of the molecules, determined as f unctions of position, are in reasonable agreement with the thermal con duction model, assuming 26 Angstrom(2) for the effective rotational in elastic cross-section and 11 Angstrom(2) for the vibrational one. The need for a more detailed analysis is argued. (C) 1997 Elsevier Science B.V.