Determination of atomic and molecular particle densities and temperatures in a low-pressure hydrogen hollow cathode discharge

Atomic (H) and molecular (H-2) hydrogen densities and temperatures have been determined in a magnetized hollow cathode are 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(H2) (v, J) - n(H2)(v + 1, J). The CARS intensity of many rotational states (J less than or equal to 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(H2)) approximate to 0.4 and about one third of the molecular hydrogen is found in vibrationally excited states. Here, the vibrational temperature is about T-vib approximate to 5000 K, which far exceeds the gas temperature of T-gas approximate to 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.