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