Absolute, time-resolved populations of N2(A3Σu   +, v = 0–5) vibrational levels in high-pressure ns pulse discharge plasmas are measured by Tunable Diode Laser Absorption Spectroscopy (TDLAS). The diffuse plasma is generated by a repetitively pulsed, double dielectric barrier, ns discharge across a 10 mm gap in a plane-to-plane geometry, at pressures of up to 400 Torr. The results of TDLAS measurements in nitrogen and in H2–N2, O2–N2, and NO–N2 plasmas are compared with kinetic modeling predictions, identifying the mechanisms of N2(A3Σu   +) generation and decay during the discharge pulses and in the afterglow. Comparison with the modeling predictions indicates that electron impact dissociation of N2 from the ground electronic state significantly underpredicts the yield of N atoms. The present data suggest that N2 dissociation in the plasma also occurs during the energy pooling process in collisions of two N2(A3Σu   +) molecules. The results also show that high-pressure, high repetition rate, volume-scalable ns pulse discharges can be used for efficient generation of atomic species for plasma chemical and plasma catalysis syntheses. In an NO–N2 mixture, it is shown that the N2(A3Σu   +) decay is controlled by the rapid energy transfer to NO, resulting in its electronic excitation and UV emission (NO γ bands). The diagnostics used in the present work can be used for the accurate characterization of both high-pressure, low-temperature gas discharge plasmas and high-temperature nonequilibrium flows generated in pulsed facilities such as shock tubes and expansion tunnels.

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.

Hybrid plasmas, sustained by a repetitive ns pulse discharge and a sub-breakdown RF waveform in N2 and its mixtures with H2, CO, and CO2, are studied using laser diagnostics and kinetic modeling. Plasma emission images show that adding the RF waveform to the ns pulse train does not result in a discharge instability development, since the RF field does not produce additional ionization. Unlike a ns pulse/DC discharge, the ns pulse/RF plasma is sustained using a single pair of electrodes external to the discharge cell. Measurements of electronically excited molecules, N2(A3Σu +), and vibrationally excited molecules in the ground electronic state, N2(X1Σg +, v), demonstrate that these species are generated selectively. N2(A3Σu +) molecules are produced predominantly by the ns pulse discharge waveform, while vibrational excitation of the ground electronic state N2 is mainly due to the RF waveform. Strong vibrational nonequilibrium is maintained at a low translational–rotational temperature. The ns pulse/RF discharge data demonstrate that the quenching of N2(A3Σu +) is not affected by N2 vibrational excitation. Kinetic modeling shows that the rate of N2(A3Σu +) quenching in a ns pulse discharge in nitrogen is underpredicted, and the modeling predictions agree with the data only if the rate of N atom generation by electron impact dissociation of N2 is increased by approximately an order of magnitude. This suggests a significant effect of excited electronic states on the net dissociation rate. Infrared emission spectra of ns pulse/RF hybrid plasmas in CO–N2 and CO2–N2 mixtures show that the present approach also generates strong vibrational excitation of CO and CO2, with the CO yield in the CO2–N2 mixture approximately a factor of two higher compared to that in a ns pulse discharge alone. This indicates a significant contribution of the vibrationally enhanced CO2 dissociation in the hybrid plasma. The present results demonstrate that sustaining the hybrid plasma in reacting molecular gas mixtures may isolate the plasma chemical reaction pathways dominated by vibrationally excited molecules from those of electronically excited molecules and atomic species.

Time-resolved CO2 and N2 vibrational populations and translational-rotational temperature are measured in a CO2-N2 plasma sustained by a ns pulse discharge burst in plane-to-plane geometry. Time-resolved, absolute number density of CO generated in the plasma is also inferred from the experimental data. CO2 and CO vibrational populations are measured by mid-IR, tunable Quantum Cascade Laser Absorption Spectroscopy, and N2 vibrational populations are measured by the ns broadband vibrational CARS. Transient excitation of N2 and CO2 asymmetric stretch vibrational energy modes is detected during the discharge burst. The time-resolved rate of CO generation does not correlate with N2 or CO2(ν3) vibrational temperatures, indicating that CO2 dissociation via the vibrational excitation is insignificant at the present conditions. The rate of CO generation decreases gradually during the discharge burst. The estimated specific energy cost of the CO product is close to that of N atoms in pure nitrogen, measured previously at similar operated conditions. Comparison of the experimental data with the kinetic modeling analysis indicates that CO2 dissociation in collisions with electronically excited N2 molecules is the dominant channel of CO generation at the present conditions, although the inferred CO yield in these processes is significantly lower than 1. The effect of vibrational energy transfer between N2 and CO2 on the plasma chemical processes is insignificant. The kinetic model underpredicts a rapid reduction of the N2 and CO2(ν3) vibrational temperatures during the later half of the discharge burst and in the afterglow. V-T relaxation of N2 by N and O atoms generated in the ns pulse discharge plasma does not affect the vibrational relaxation rate in a significant way. The reason for this difference remains not fully understood.

HO2 radicals are an important precursor in the formation of H2O2, a key species in plasma-liquid interactions, such that their formation and consumption pathways need to be understood. In this work, the generation and decay of HO2 have been studied in a controlled environment, in ns pulse discharge O2-He plasmas in contact with a liquid water surface. For this, time-resolved, absolute number densities of HO2 in O2-He mixtures excited by a repetitive ns pulse discharge are measured in situ by Cavity Ring Down Spectroscopy (CRDS). The discharge cell with external electrodes to generate the plasma and a water reservoir are integrated into the CRDS cavity. The high-reflectivity cavity mirrors are purged with helium to protect them from water vapor condensation. The experimental results are obtained at near room temperature, both during the discharge pulse burst and in the afterglow. The HO2 number density is inferred from the CRDS data using a spectral model exhibiting good agreement with previous measurements of absolute HO2 absorption cross sections. HO2 is generated during the discharge burst and decays in the afterglow between the bursts. The HO2 number density is also measured vs. the O2 fraction in the mixture. Comparison with the kinetic modeling predictions demonstrates good agreement with the data and identifies the dominant HO2 generation and decay processes. HO2 in the plasma is formed predominantly by the recombination of H atoms, generated by the electron impact dissociation of water vapor, with O2 molecules. Reactions with O atoms and OH radicals are among the main HO2 decay processes in the afterglow. HO2 is also detected when O2 is not present in the mixture. In this case, it is generated primarily by the recombination of OH radicals, via the formation of H2O2. The results demonstrate that CRDS can also be used for HO2 and other plasma chemical reaction product measurements in atmospheric pressure plasma jets impinging on a liquid water surface in ambient air.