Effects of Hminus;production in a multicusp ion source are measured by separately mixing with hydrogen small amounts (0.33percnt;ndash;10percnt;) of water, ammonia, methane, and hydrazinemdash;molecules which produce large amounts of Hminus;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 groundhyphen;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 H2molecules, withvlsquo;=5ndash;11, andJlsquo;=0ndash;5, and the large DA cross sections for the excited H2(vlsquo;,Jlsquo;). The relative populations of H2(vlsquo;,Jlsquo;) thus obtained are found to be substantially smaller than those predicted by theoretical calculations. The effect on Hminus;current was also studied by mixing small amounts of SF6with H2. A 1.5percnt; mixture was found to reduce the Hminus;output by one half.
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