Atomic (H) and molecular (H 2 ) hydrogen densities and temperatures have been determined in a magnetized hollow cathode arc plasma burning at low pressure (p = 4-40 Pa). 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 H 2 states n H 2 (v, J ) − n H 2 (v + 1, J ). The CARS intensity of many rotational states (J 9) of H 2 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 H 2 can be found experimentally owing to the low particle densities. In order to evaluate the H 2 density properly from the measured CARS data, the H 2 vibrational population for v > 0 is calculated from a spatially one-dimensional diffusion reaction model. Within the plasma centre the dissociation degree d = n H /(n H + 2n H 2 ) ≈ 0.4 and about one third of the molecular hydrogen is found in vibrationally excited states. Here, the vibrational temperature is about T vib ≈ 5000 K, which far exceeds the gas temperature of T gas ≈ 1000-3000 K. 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.
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