Effects on H− production in a multicusp ion source by mixtures of H2 with H2O, NH3, CH4, N2H4, and SF6

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Abstract
Effects of H− production in a multicusp ion source are measured by separately mixing with hydrogen small amounts (0.33%–10%) of water, ammonia, methane, and hydrazine—molecules which produce large amounts of H− via dissociative attachment (DA) resonances at higher electron energies. The mixing was done in a separate reservoir, with careful measurement of individual pressures. Experimental enhancements of 1.4 and less were observed, whereas calculated enhancements, using accurate DA cross sections for ground‐state H2, should have produced factors of 1.5, 3.0, 1.3, and 2.4 enhancements for water, ammonia, methane, and hydrazine, respectively, at a mean electron energy of 1.0 eV in the extraction region. The difference is accounted for by including, in the enhancement calculation, vibrationally and rotationally excited H2 molecules, with v‘=5–11, and J‘=0–5, and the large DA cross sections for the excited H2(v‘,J‘). The relative populations of H2(v‘,J‘) thus obtained are found to be substantially smaller than those predicted by theoretical calculations. The effect on H− current was also studied by mixing small amounts of SF6 with H2. A 1.5% mixture was found to reduce the H− output by one half.