An Examination of Nitric Oxide Kinetics in a Plasma Afterglow with Significant Vibrational Loading

Abstract

Laser-induced fluorescence measurements (LIF) of nitric oxide and two photon absorption laser-induced fluorescence (TALIF) of oxygen and nitrogen atoms are performed in a diffuse plasma filament with significant vibrational loading. The results are compared with kinetic modeling calculations. The experimental data shows that significant NO concentrations are achieved within a few microseconds after the pulse, after which the NO evolution is controlled by the reverse Zel'dovich reactions. The modeling calculation results show that the dominant formation channel is through collisions of oxygen atoms with multiple electronically-excited nitrogen states. Aside from acting as an energy reservoir, the large concentration of vibrationally-excited nitrogen in the ground electronic state seems to have no effect on NO formation. The present experimental results are accurately reproduced by incorporating all electronically excited * levels. I. Introductionespite the wide-spread interest in research in low temperature N2-O2 plasmas, a complete understanding of the partitioning of energy in such systems and the afterglow chemistry has remained somewhat elusive. Of particular interest to us is the role of nitric oxide (NO) in these non-equilibrium systems. The chemistry of NO has long been an important issue since nitrogen oxides are regulated pollutants linked to serious public health issues. Consequently, one application of plasma chemistry has been in NOx reduction [1]. In contrast, plasmas have also been used as a means of producing NO at low temperatures for biomedical applications such as wound healing and tool sterilization [2]. Applications involving generation or reduction of NO are obviously limited by the fact that the formation and consumption mechanisms in plasmas are not completely understood.At thermal equilibrium conditions, NO production in dry air is described by the Zel'dovich mechanism reactions, + 2 ↔ + and + 2 ↔ +O. The first reaction has a barrier in the forward direction of 3.27 eV, and the second reaction has an activation barrier in the reverse direction of 1.64 eV. The rate constants for these two reactions, as well as that for the "third Zel'dovich reaction", + ↔ + are plotted in Fig. 1 vs. temperature. It is apparent from the figure that the reverse process of the first reaction is dominant at low temperatures. Therefore, production of atomic radicals in a plasma lacking hydrogen (i.e. dry air) would be expected to mitigate rather than to enhance NO production. However, an atom or a molecule created in one of excited quantum states, e.g. via excitation by electron impact, may possess an internal energy higher than the activation barrier of these processes, thus leading to rapid NO formation. Rates of individual reactions for many excited states are still largely unknown. Also, concentrations of such species, which are difficult to measure, are functions of the discharge coupled energy and reduced

Experimental and numerical study of fast gas heating and O atom production in a capillary nanosecond discharge

“…quenching by atomic oxygen 22 and were taken to be k ≈ 7.5⋅10 -11 cm 3 /s. In a later paper 20 , higher values of the rate constants, k 8 = k 9 = 3⋅10 -10 cm 3 /s, were suggested on the basis of O( 3 P), N( 4 S) and NO densities, measured in the afterglow of a pulsed nanosecond discharge in air at P = 100 Torr. In the present calculations, data suggested in Ref.…”

“…In the present calculations, data suggested in Ref. 20 were used (see Table 1). To underline the importance of quenching of excited nitrogen molecules by atomic oxygen, Fig.15 b gives the results of calculations of O-atoms density for two cases: taking into account reactions (R8, R9, R10), curve 1; and without taking into account the aforementioned reactions, curve 2.…”

Plasma for nitrogen fixation by using N2/O2 mixture: Reaction pathway, energy flow, and plasma reactor

“…Burnette et al [ 60 ] and Shkurenkov et al's [ 61 ] simulation results further show that when only N 2 (A,B,C) states are considered in the model, the obtained NO concentration is about five times lower than what is obtained in the experiment. Only when N 2 (W), N 2 (B′), N 2 (E), N 2 (a′), N 2 (a), N 2 (w), and N 2 (a″) are all included in the model, the simulation results agree with the experiment results very well.…”

“…However, under the present condition, the N concentration is higher than the NO concentration; thus, it was believed that N( 2 P, 2 D) + O 2 → NO + N might also contribute to NO forming, which is not the case in the previous study. [ 60 ] In addition, they also believe that NO decays mainly through NO + N → N 2 + O reaction.…”

Active Particles Production by Pulsed Nanosecond Discharge in Ambient Air. Quenching of Electronically Excited States of Nitrogen by O2 Molecules and O(3P) Atoms

“…Rate constants of these reactions were proposed using the analysis of the experimental data on N 2 (A 3  u  , v=7) quenching by atomic oxygen [46] and were taken to be k  7.510 -11 cm 3 /s. In a later papers [47,48], higher values of the rate constants (k 7,8,9 = 310 -10 cm 3 /s) were suggested on the basis of treatment of experimental data on temporal evolution of O( 3 P), N( 4 S) and NO number densities in the afterglow of a pulsed nanosecond discharge in air at P = 100 Torr and T = 300 -350 K. In the present calculations, data suggested in [47,48] were used (see Table 1). …”