Atomic and molecular surface and volume processes in the analysis of negative hydrogen discharges

The principal source of negative ion generation in hydrogen discharges is now recognized to be low-energy electron attachment to H2(nu prime prime) molecules excited to the middle portion of the vibrational spectrum. Electron excitation processes are generally taken to be the principal source of H2(nu prime prime) generation, with high-energy excitations through the singlet spectrum being the principal excitation process populating the active portion of the vibrational spectrum. A description of the collisional re-excitation from level (nu)(prime)(prime), to level (nu)(prime)(prime), requires 15 x 15 matrix of cross sections linking all initial and final levels. These cross sections were evaluated and incorporated into the modelling code. An additional source of vibrational excitation may be derived from recombination of H2(+) and H3(+) ions on the surfaces of the discharge. In this case the molecular ions will impinge with kinetic energies given by the plasma potential, 1 to 10 eV. The evaluation of H2(nu prime prime) resulting from the surface recombination process is reported. The use of low work function materials for the discharge surfaces makes possible two additional sources of negative ions: the direct dormation of negative ions by hydrogen atoms rebounding from the surface, and the dissociation of H2(-) ions formed in the surface selvage.