Spectroscopic Measurement of Atomic Hydrogen and Hydrogen

Low-pressure H_2 discharges have been used for some time as sources of H^ - ions. These discharges contain many different species of particles which interact with each other and with the walls of the discharge chamber. Models exist that predict the populations of the various species for given macroscopic discharge parameters. However, many of the cross sections and wall catalyzation coefficients are unknown or somewhat uncertain. Therefore, it is of interest to measure the populations of as many of these species as possible, in order to determine the validity of the models. These models predict that H^ - is created predominantly by the two-step process of vibrational excitation of hydrogen molecules followed by dissociative attachment of slow electrons to these vibrationally -excited hydrogen molecules. Many different collisional processes must be included in the models to explain the dependence of the various populations upon macroscopic parameters. This work presents results of spectroscopic measurements of the density and translational temperature of hydrogen atoms and of specific rotationally- and vibrationally-excited states of electronic ground-state H_2, in a discharge optimized for H^- production, as well as conventional measurements of the various charged species within the plasma. The spectroscopic measurements are performed directly by narrowband, single -photon absorption in the vacuum ultraviolet. The highest vibrational and rotational quantum numbers observed are 8 and 15 respectively. The results for the atomic concentration are in agreement with conventional models. The inferred atom recombination coefficient can vary with wall conditions, but is reproducible and has a value of approximately 0.6 on tungsten-covered copper walls for normal operating conditions. The experimental results on the H_2 vibrational distribution are in good agreement with the results of a model similar to those found in the literature. The vibrational distribution is found to be characterized by a temperature and to be primarily excited by the thermal electrons. The wall relaxation rates of the vibrationally -excited molecules correspond to 10-40 wall collisions. Finally, the H^- density was measured and compared to a model that uses the neutral species measurements as input. The results are consistent, but the role played by the superthermally populated molecules of high rotational excitation is still ambiguous. Suggestions for future research are given.