Atom and ion chemistry in low pressure hydrogen dc plasmas

The chemical composition of a low-pressure hydrogen dc plasma produced in a hollow cathode discharge has been measured and modeled. The concentrations of H atoms and of H(+), H(2)(+) and H(3)(+) ions were determined with a combination of optical spectroscopic and mass spectrometric techniques, over the range of pressures (p approximately 0.008-0.2 m bar) investigated. The results were rationalized with the help of a zero-order kinetic model. A comparatively high fraction ( approximately 0.1+/-0.05) of H atoms, indicative of a relatively small wall recombination, was observed. Low ionization degrees (<10(-4)) were obtained in all cases. In general, the ionic composition of the plasma was found to be dominated by H(3)(+), except at the lowest pressures, where H(2)(+) was the major ion. The key physicochemical processes determining the plasma composition were identified from the comparison of experimental and model results, and are discussed in the paper.