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+, H2+ and H3+ ions were determined with a combination of optical spectroscopic and mass spectrometric techniques, over the range of pressures ( p similar to 0.008-0.2 mbar) investigated. The results were rationalized with the help of a zero-order kinetic model. A comparatively high fraction (similar to 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.