Determination of non-Boltzmann vibrational distributions of N2(X, v″) in the He/N2 microwave-discharge afterglows

We have extended a technique for studying the vibrational distributions of ground-electronic-state, molecular nitrogen in the afterglow of a microwave discharge through mixtures of helium and nitrogen. The technique is based upon adding metastable helium atoms to the afterglow. The He*(2 3 S) excites the N2(X,v) to N+2(B 2Σ+u) in a Penning-ionization reaction. Since Penning ionization is a Franck–Condon process, the vibrational distribution of the N+2(B) product is determined by that of the N2 (X,v) from which it was produced. The measurements show that the ground-state nitrogen distribution is highly non-Boltzmann, with vibrationally hotter distributions being produced with lower mole fractions of nitrogen in the discharge. We have also observed the production of N+2(C 2Σ+u) from He* Penning ionization of molecular nitrogen. This process is energetically allowed only if the vibrational energy in the ground-electronic-state nitrogen exceeds 3.8 eV or 15 vibrational quanta.