Atomic (H) and molecular hydrogen densities and temperatures have been determined in a magnetized hollow cathode arc plasma burning at low pressure . Rayleigh scattering measurements are used to derive the sum of atomic and molecular densities, each weighted with its scattering cross section. Coherent anti-Stokes Raman scattering (CARS) has been used to determine the population density differences of rovibrational molecular states . The CARS intensity of many rotational states of can be detected and these levels are found to be populated according to a Boltzmann distribution. In the low-pressure plasma only the fundamental vibrational band of can be found experimentally owing to the low particle densities. In order to evaluate the density properly from the measured CARS data, the vibrational population for v0 > 0 is calculated from a spatially one-dimensional diffusion reaction model. Within the plasma centre the dissociation degree and about one third of the molecular hydrogen is found in vibrationally excited states. Here, the vibrational temperature is about , which far exceeds the gas temperature of . The dissociation degree and the vibrational distribution are mainly determined by electron-impact processes in the inner plasma region and recycling processes at the vessel walls, whereas the influence of inelastic neutral - neutral collisions is rather marginal.
Plasma kinetics in molecular plasmas and modeling of reentry plasmas