Recombination and dissociation of H2 + and H3 + ions on surfaces to form H2(vscript): Negative-ion formation on low-work-function surfaces

The recombination and dissociation of H⁺₂ and H⁺₃ ions incident upon metal surfaces leads to H, H₂(v`), and H<sup>-</sup> products rebounding from the surface. A four-step model for H<sup>+</sup><sub>2</sub> -ion recombination generates H<sub>2</sub>(v`) via resonant electron capture through the b ³Σ⁺_u and X ¹Σ⁺_g states. A molecular trajectory analysis provides final-state H₂(v`) distributions for incident energies of 1, 4, 10, and 20 eV. The calculated H<sub>2</sub>: H<sup>+</sup><sub>2</sub> yields compare favorably with the observed yields. A similar four-step model for incident H<sup>+</sup><sub>3</sub> proceeds via resonant capture to form the H<sub>3</sub>(2p <sup>2</sup>E'→2p <sup>2</sup>A<sub>1</sub>) ground state, in turn dissociating into H+H<sub>2</sub>(v_`), with the fragment molecule rebounding to give the final H₂(v`) distribution. Comparing the final populations v`≥5 for incident H⁺₂ or H⁺₃ shows that the H⁺₃ ion will be more useful than H⁺₂ for H^- generation via dissociative attachment. Molecular ions incident upon low-work-function surfaces generate additional H₂(v`) via resonant electron capture through excited electronic states and provide two additional sources of H<sup>-</sup> production: Direct H<sup>-</sup> production by H dissociation products rebounding from the surface and H<sup>-</sup> production through the formation of H<sup>-</sup><sub>2</sub> in the surface selvage that in turn dissociates into H+H<sup>-</sup>. The H<sup>-</sup><sub>2</sub> in the selvage is formed by resonant capture to the low-lying vibrational levels of H<sub>2</sub>(v`), and complements dissociative attachment to high-lying levels in the discharge. The H, H₂(v`), and H^- yields are inventoried for H⁺₃ incident upon barium surfaces.