Relaxation behavior of rovibrationally excited H2 in a rarefied expansion

The evolution of the rotational and vibrational distributions of mol. hydrogen in a hydrogen plasma expansion is measured using laser induced fluorescence in the vacuum-UV range. The evolution of the distributions along the expansion axis shows the relaxation of the mol. hydrogen from the high temp. in the upstream region to the low ambient temp. in the downstream region. During the relaxation, the vibrational distribution, which has been recorded up to v=6, is almost frozen in the expansion and resembles a Boltzmann distribution at T~2200 K. However, the rotational distributions, which have been recorded up to J=17 in v=2 and up to J=11 in v=3, cannot be described with a single Boltzmann distribution. In the course of the expansion, the lower rotational levels (J7) is measured to be frozen in the expansion at a temp. between 2000 and 2500 K. A model based on rotation-translation energy transfer is used to describe the evolution of the rotational distribution of vibrational level v=2 in the plasma expansion. The behavior of the low rotational levels (J