Publications

Measurements of atoms and metastable species in N₂ and H₂–N₂ nanosecond pulse plasmas

Yang, Xin Y.; Jans, E.R.; Richards, Caleb J.; Raskar, Sai R.; van den Bekerom, Dirk C.; Wu, Kai W.; Adamovich, Igor V.

We report time-resolved, absolute number densities of metastable N₂(A³Σ_u⁺, v = 0, 1) molecules, ground state N₂ and H atoms, and rotational–translational temperature have been measured by tunable diode laser absorption spectroscopy and two-photon absorption laser-induced fluorescence in diffuse N₂ and N₂ –H₂ plasmas during and after a nanosecond pulse discharge burst. Comparison of the measurement results with the kinetic modeling predictions, specifically the significant reduction of the N₂(A³Σ_u⁺) populations and the rate of N atom generation during the burst, suggests that these two trends are related. The slow N atom decay in the afterglow, on a time scale longer than the discharge burst, demonstrates that the latter trend is not affected by N atom recombination, diffusion to the walls, or convection with the flow. This leads to the conclusion that the energy pooling in collisions of N₂(A₃Σ_u⁺) molecules is a major channel of N₂ dissociation in electric discharges where a significant fraction of the input energy goes to electronic excitation of N₂. Additional measurements in a 1% H₂ –N₂ mixture demonstrate a further significant reduction of N₂(A³Σ_u⁺, v = 0, 1) populations, due to the rapid quenching by H atoms accumulating in the plasma. Comparison with the modeling predictions suggests that the N₂(A³Σ_u⁺) molecules may be initially formed in the highly vibrationally excited states. The reduction of the N₂(A³Σ_u⁺) number density also diminishes the contribution of the energy pooling process into N2 dissociation, thus reducing the N atom number density. The rate of N atom generation during the burst also decreases, due to its strong coupling to N₂(A³Σ_u⁺, v) populations. On the other hand, the rate of H atom generation, produced predominantly by the dissociative quenching of the excited electronic states of N₂ by H₂, remains about the same during the burst, resulting in a nearly linear rise in the H atom number density. Comparison of the kinetic model predictions with the experimental results suggests that the yield of H atoms during the quenching of the excited electronic state of N₂ by molecular H₂ is significantly less than 100%. The present results quantify the yield of N and H atoms in high-pressure H₂ –N₂ plasmas, which have significant potential for ammonia generation using plasma-assisted catalysis