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Title:N And H Talif Measurements, N2(a3Σu+) Tdlas Measurements, And Kinetic Modeling Of Nanosecond Pulse Discharge Plasmas In N2-h2 Mixtures
Author(s):Yang, Xin
Contributor(s):Adamovich, Igor V.; van den Bekerom, Dirk; Raskar, Sai; Jans, Elijah R.; richards, caleb
Subject(s):Dynamics and kinetics
Abstract:Time-resolved, absolute number densities of ground state N atoms in nitrogen and H$_2$-N$_2$ plasmas, as well as ground state H atoms in H$_2$-N$_2$ plasmas, excited by a ns pulse discharge burst at P = 150 Torr, are measured by Two-Photon Absorption Laser-Induced Fluorescence (TALIF). Metastable N$_2$(A$^3\Sigma_u^+$,v=0,1) molecules and gas temperature in the discharge are measured at the same conditions by Tunable Diode Laser Absorption Spectroscopy (TDLAS). Both in pure nitrogen and in H$_2$-N$_2$ mixtures, the results show that the N atom number density does not decay on the time scale of 1 ms and is controlled only by the rate of N$_2$ dissociation in the plasma during the discharge burst. Both N$_2$(A$^3\Sigma_u^+$,v) populations and the rate of N atom generation decrease significantly during the ns pulse discharge burst (by a factor of 3-5), although the pulse energy coupled to the plasma and the number densities of N$_2$(C$^3\Pi_u$) and N$_2$(B$^3\Pi_g$) molecules remain approximately the same during the burst. Comparison of the measurement results and the modeling predictions indicates an additional major channel of N$_2$ dissociation in the plasma, by energy pooling in collisions of two N$_2$(A$^3\Sigma_u^+$) molecules: N$_2$(A$^3\Sigma_u^+$) + N$_2$(A$^3\Sigma_u^+$) $\rightarrow$ N + N + N$_2$. In H$_2$-N$_2$ mixtures, the N atom number density decreases significantly with the hydrogen mole fraction in the mixture, while the H atom number density is very high, up to 10$^{16}$ cm$^{-3}$ in a 1\% H$_2$-N$_2$ mixture. Kinetic modeling suggests that a significant fraction of N atoms generated by electron impact in the plasma are formed in the excited electronic state, N($^2$D). In pure nitrogen, the electronically excited N($^2$D) atoms are quenched to the ground state, N($^4$S), within $\sim$ 10 $\mu$s, while in H$_2$-N$_2$ mixtures, they react with H$_2$ to produce NH and H atoms, N($^2$D) +H$_2$ $\rightarrow$ NH + H. Another major channel of H atom generation in the plasma is the reactive quenching of electronically excited nitrogen molecules by H$_2$, N$_2$(C$^3\Pi_u$) + H$_2$ $\rightarrow$ N$_2$ + H + H. The rate of H atom generation remains nearly constant during the discharge pulse burst. The present work provides quantitative insight into the mechanism of radical species generation in N$_2$-H$_2$ plasmas, which is critical for understanding the kinetics of ammonia synthesis by non-thermal plasma-assisted catalysis.
Issue Date:2021-06-23
Publisher:International Symposium on Molecular Spectroscopy
Genre:Conference Paper / Presentation
Date Available in IDEALS:2021-09-24

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