Production of negative ions under non-equilibrium conditions

The results of experimental and theoretical studies have shown that the dissociative attachment cross-sections for electron impact of molecular hydrogen are very strong functions of the initial vibrational state of the molecule. The existence of this comprehensive set of cross-sections, which establish the vibrational level and rotational level dependencies of dissociative attachment from the ground electronic state, permits the evaluation of the effect of non-equilibrium level populations on the production rate of H- in discharges. In this work a self-consistent analysis of the effects of vibrational excitation on the electron energy distribution and on the dissociation attachment rates was performed for typical conditions in hydrogen discharges. The electron kinetics and excitation rates, obtained by numerically solving the electron Boltzmann transport equation, were coupled to the first order time derivative population equations for the vibrational levels. The dissociative attachment rate was calculated as a function of E/N, (where E is the applied electric field and N is the total gas density), gas temperature and fractional ionization. Under conditions of high vibrational excitation there is a significant increase in the energy deposited into dissociative attachment as compared to that for a gas at a low vibrational temperature. Calculations have been made which show that the cooling of the gas to 2000K,coupled with efficient vibrational pumping, results in the total rate of negative ion formation exceeding that from the ground state by over three orders of magnitude. The possibilities of achieving these conditions are considered