Recombination and dissociation of H2+ and H3+ ions on surfaces to form H2(v″): 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− products rebounding from the surface. A four-step model for H+2 -ion recombination generates H2 (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 H2: H+2 yields compare favorably with the observed yields. A similar four-step model for incident H+3 proceeds via resonant capture to form the H3 (2p 2E′→2p 2A1) ground state, in turn dissociating into H+H2 (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− production: Direct H− production by H dissociation products rebounding from the surface and H− production through the formation of H−2 in the surface selvage that in turn dissociates into H+H−. The H−2 in the selvage is formed by resonant capture to the low-lying vibrational levels of H2 (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.