Evidence of catalytic production of hot hydrogen in RF generated hydrogen/argon plasmas

This is the third in a series of papers by our team on apparently anomalous Balmer series line broadening in hydrogen containing RF generated, low-pressure (< 600 mTorr) plasmas. In this paper the selective broadening of the atomic hydrogen lines in pure H-2 and Ar/H-2 mixtures in a large general electronics conference (GEC)-type cell (36cm length x 14 cm ID) was mapped as a function of position, H-2/Ar ratio, time, power, and pressure. Several observations regarding the selective line broadening were particularly notable as they are unanticipated on the basis of earlier models. First, the anomalous broadening of the Balmer lines was found to exist throughout the plasma, and not just in the region between the electrodes. Second, the broadening was consistently a complex function of the operating parameters, particularly gas composition (highest in pure H-2) position, power, and pressure. Clearly, not anticipated by earlier models were the findings that under some conditions the highest concentration of "hot" (> 10 eV) hydrogen was found at the gas entry end, and not in the high-field region between the electrodes, and that in other conditions, the hottest H was at the (exit) pump (also grounded electrode) end. Third, excitation and electron temperatures (measured optically) were less than 1 eV in all regions of the plasma not directly adjacent (> 1 mm) to the electrodes, providing additional evidence that the energy for broadening, contrary to standard models, is not obtained from the electric field. Fourth, in contrast to our earlier studies of hydrogen/helium and water plasmas, we found that in some conditions 98% of the atomic hydrogen was in the "hot" state throughout the GEC-type cell. Virtually every operating parameter studied impacted the character of the hot H atom population. Clearly, second and third order effects exist, indicating a need for experimental design. Some non-field mechanisms for generating hot hydrogen atoms are outlined. (c) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.