Hydrogen—Helium Ion—Molecule Reactions

The reactions producing HeH+ from gaseous mixtures of H2 and He subjected to electron impact in the ion source of a mass spectrometer have been studied. Ionization efficiency curves show practically all of the HeH+ to be formed by reaction of H2+ and He. Phenomenological cross sections were compared with values for the formation of H3+ from the H2+–H2 present in the ion source. The maximum value of the ratio of rate constants for HeH+ and H2+ formation computed from the experimental data was 28% of the theoretical ratio. If the heat of the endothermic reaction between H2+ and He is supplied from internal energy in H2+, then the rate constant should be computed in terms of the fraction vibrationally excited H2+ with sufficient internal energy to meet the needs of the reaction rather than total H2+ in the ion source. An estimate of the population of the excited vibrational state in H2+ produced from H2 by electron impact gives approximately 29% with enough energy to react. The translational energy dependenc...

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