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

• Published in: Plasma sources science & technology Vol. 31; no. 1
• Main Authors: Yang, Xin, Jans, Elijah, Richards, Caleb, Raskar, Sai, van den Bekerom, Dirk, Wu, Kai, Adamovich, Igor V
• Format: Journal Article
• Language: English
• Published: United States IOP Publishing 31.01.2022
• Subjects: 70 PLASMA PHYSICS AND FUSION TECHNOLOGY; hydrogen dissociation; kinetics; N2-H2 plasmas; nanosecond pulse discharge; nitrogen dissociation; plasmas; tunable diode laser absorption spectroscopy; tunable diode laser adsorption spectroscopy; two photon absorption laser-induced fluorescence
• DOI: 10.1088/1361-6595/ac3053
• ISSN: 0963-0252

We report time-resolved, absolute number densities of metastable N2(A3Σu+, 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(A3Σu+) 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(A3Σu+) 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(A3Σu+, 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(A3Σu+) molecules may be initially formed in the highly vibrationally excited states. The reduction of the N2(A3Σu+) 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(A3Σu+, 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