TERMINAL ROVIBRATIONAL DISTRIBUTION OF NA2 IN A SODIUM SUPERSONIC BEAM

We have measured the terminal rovibrational distribution of Na2 in a m olecular beam produced by a supersonic expansion of pure sodium at sou rce pressure X nozzle diameter products p0d=0.02 and 0.32 Torr cm. The rovibrational distribution was determined by the laser-induced fluore scence technique and covers the 550-1200 and 550-2700 cm-1 internal en ergy ranges, respectively. The highest vibrational state for which mol ecules were detected is v''=15. For the lower stagnation pressure, the internal energy distribution is well relaxed and can be described by a temperature of about 350-400 K. For the p0d=0.32 Torr cm experiment, the distribution is nonthermal with a large population excess for the levels with a high internal energy. The local temperature exceeds 460 K at 2000 cm-1. A striking result is that the rovibrational distribut ion depends almost exclusively on the internal energy (i.e., it does n ot depend much on the partitioning of this energy between vibration an d rotation). This new result does not seem to be in contradiction with previous experiments which tested only a small set of levels. We disc uss the mechanisms of the rovibrational relaxation during the sodium b eam expansion. Recent experiments have shown that this relaxation is a very efficient process and we believe that it can explain our observa tions. We attribute the population excess of the rovibrational levels with a high internal energy to the process of dimerization in the expa nsion. Finally, a nonstatistical behavior of the Na2 ortho/para popula tion was observed for the vibrational levels v''=10-15. Such an effect for hydrogen molecules formed by three-body recombination has been pr edicted by Roberts within the resonance complex theory of this process