Kinetics of excited metastable species and ionization in the afterglow of ns pulse discharge in nitrogen

Time-resolved, absolute populations of metastable molecules, N2(A3Σu+,v=0,1), number densities of metastable atoms, N(2D) and N(2P), and the number density of molecular ions, N2+, are measured in nitrogen and N2–NO mixtures in a heated plasma flow reactor, at temperatures up to T = 1000 K. The flow in the reactor is excited by the ns pulse discharge bursts, generating a diffuse plasma with well-defined boundaries. The measurements are made by the tunable diode laser absorption spectroscopy, atomic resonance absorption spectroscopy, and pulsed UV cavity ring down spectroscopy. The results are compared to the kinetic modeling predictions. The results illustrate the generation and decay of N2(A), a precursor for metastable excited atoms, during the discharge burst and in the afterglow. The N2(A) populations decay significantly during the burst, mainly due to quenching by the ground state N atoms. The excited atoms, N(2D) and N(2P), decay in the afterglow is due to their quenching by N2 molecules and N atoms, respectively. The apparent yield of metastable N atoms during the quenching of N2(A), inferred from comparison with the modeling predictions, is significantly lower than expected, below 1%. Time-resolved N2+ measurements in the afterglow exhibit its gradual decay, dominated by the conversion to N4+ and electron–ion recombination. At the present conditions, the effect of associative ionization of excited metastable species on the electron and ion densities in the afterglow appears negligible. Enhancing the metastable N atom yield may require the use of different precursors, such as Ar metastables generated in N2–Ar plasmas.