Reactivity of metastable helium molecules in atmospheric pressure afterglows

This work focuses upon previously encountered problems that have impeded the understanding of the role of He2(3Σu) metastable molecules in the afterglows of electrical discharges into atmospheric pressures of helium containing admixtures of reactants. New data are reported that generally confirm the literature values for the reactivities of this species at high pressures and continue the disagreement with earlier measurements in low pressure plasmas. Results are presented for reactions with HCl and HBr for the first time. A study of systematic dependencies of rates upon molecular parameters is reported which shows that the ratio for bimolecular reactions of vibrationally excited and unexcited He2(3Σu) molecules is largely independent of the identity of the reacting partner. This provides a means for using data on the pressure dependent part of the effective rate of reaction of He(2 3S) atoms to estimate the rate coefficients for the bimolecular reactions of the more elusive He2(3Σu) molecule.