Abstract
The chemistry of low pressure H(2) + O(2) discharges with different mixture ratios has been studied in a hollow cathode DC reactor. Neutral and positive ion distributions have been measured by mass spectrometry, and Langmuir probes have been used to provide charge densities and electron temperatures. A simple zero order kinetic model including neutral species and positive and negative ions, which takes into account gas-phase and heterogeneous chemistry, has been used to reproduce the global composition of the plasmas over the whole range of mixtures experimentally studied, and allows for the identification of the main physicochemical mechanisms that may explain the experimental results. To our knowledge, no combined experimental and modelling studies of the heavy species kinetics of low pressure H(2) + O(2) plasmas including ions has been reported before. As expected, apart from the precursors, H(2)O is detected in considerable amounts. The model also predicts appreciable concentrations of H and O atoms and the OH radical. The relevance of the metastable species O((1)D) and O(2)(a(1)Δ(g)) is analysed. Concerning the charged species, positive ion distributions are dominated by H(3)O(+) for a wide range of intermediate mixtures, while H(3)(+) and O(2)(+) are the major ions for the higher and lower H(2)/O(2) ratios, respectively. The mixed ions OH(+), H(2)O(+) and HO(2)(+) are also observed in small amounts. Negative ions are shown to have a limited relevance in the global chemistry; their main contribution is the reduction of the electron density available for electron impact processes.