Evolution of the orientational and internal-state distribution of molecules in an expanding jet

Abstract

We report experiments on the internal state distribution and angular momentum alignment in an expanding jet of Na2 molecules seeded in Ar. The results show that even at a distance of one nozzle diameter from the nozzle exit, the distribution function is already far from equilibrium. It is found that the deviation from equilibrium can be expressed in a polynomial in the total internal energy of the molecules. This result is very analogous to the outcome of a first order Chapman–Enskog treatment for molecules with internal states. The transition to molecular flow marks the breakdown of the simple relationship mentioned above. This points to the fact that data taken far away from the nozzle are by themselves of limited value for a test of calculated distributions starting from nozzle conditions. Further we present measurements of alignment for two rovibrational states of Na2 molecules. Far downstream the nozzle the results are in agreement with previous experiments in molecular beams. In the vicinity of the nozzle reorienting collisions of excited-state molecules play an important role. It appears that these collisions strongly depend on the rotational state of the molecules: the cross sections are inversely proportional to J′2.