Relaxation behavior of rovibrationally excited H2 in a rarefied expansion

Abstract

The evolution of the rotational and vibrational distributions of molecular 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 molecular hydrogen from the high temperature in the upstream region to the low ambient temperature in the downstream region. During the relaxation, the vibrational distribution, which has been recorded up to vϭ6, is almost frozen in the expansion and resembles a Boltzmann distribution at T Ϸ2200 K. However, the rotational distributions, which have been recorded up to Jϭ17 in vϭ2 and up to Jϭ11 in vϭ3, cannot be described with a single Boltzmann distribution. In the course of the expansion, the lower rotational levels (JϽ5) adapt quickly to the ambient temperature (Ϸ500 K), while the distribution of the higher rotational levels (JϾ7) is measured to be frozen in the expansion at a temperature 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 vϭ2 in the plasma expansion. The behavior of the low rotational levels (JϽ5) is described satisfactory. However, the densities of the higher rotational levels decay faster than predicted.

Nonequilibrium rovibrational energy distributions of hydrogen isotopologues in an expanding plasma jet

“…4͒. Compared to previous results on H 2 , 26 the relaxation of rotational states of HD is increasing with rotational states J and becomes almost constant for J Ն 8.…”

“…26 In the latter case, the redistribution of the internal energy is due to molecule-molecule collisions, since atom, electron, and ion densities are orders of magnitude lower than that of the neutrals. The same effect has been measured for rotational states in vibrational state v = 4 of HD molecules ͑Fig.…”

“…With a simple model 26 including cross sections calculated by Flower and Roueff, 27 the relaxation of a H 2 gas at 5500 K and 6000 Pa via collisions of H 2 molecules with ground state H 2 ͑0,0͒ has been calculated ͑Fig. 5͒.…”

Lyman transitions of high rovibrationally excited H2, HD and D2 molecules

“…The AS beam focus is positioned 8 mm downstream of the nozzle exit, i.e., in the supersonic part of the hydrogen jet, where the internal excitation of molecules is still very high compared with positions further downstream, where relaxation of hydrogen molecules occurs [39].…”

Inflow and shock formation in supersonic, rarefied plasma expansions

“…Note that ␥ =5/3 is a good approximation, even for molecular plasmas containing molecules, atoms, ions, and electrons, since during the expansion the vibrational and rotational degrees of freedom are frozen in the expansion, 25 certainly in the case of hydrogen and to a lesser extent also for nitrogen. In the stationary shock the density increases as a result of the decreasing velocity, ensuring continuity of the flow.…”