Nitrogen Afterglow Research Articles

Implementation of a single-shot LIF technique for 2-D imaging of metastable nitrogen molecules in a discharge afterglow at sub-atmospheric pressures

The nonequilibrium kinetics of nitrogen metastable N2 (A3Σu+) manifold is of a high interest for the non-thermal plasma diagnostics. A review of laser induced fluorescence (LIF) diagnostic techniques of N2 (A3Σu+) metastable molecules in nitrogen discharges indicates the need to introduce single-shot LIF measurements. The main obstacle in introducing the single-shot technique is the extremely weak fluorescence of N2 (A3Σu+) species, which monitoring requires a very sensitive and fast detector. In this paper, 2-D imaging technique of metastable nitrogen molecules in a nitrogen discharge via single-shot LIF is presented. The experiments involved a sub-atmospheric pressure nitrogen afterglow of a positive-pulsed streamer discharge that was generated in a modified wire-to-plate electrode system. Using 2-D single-shot LIF technique, instantaneous images of the spatial and temporal distributions of the metastable N2 (A3Σu+, v”=2) species and streamer optical emission in the nitrogen afterglow were captured for the first time. The LIF from N2 (A3Σu+, v”=2) metastable molecules and the streamer emission from the excited nitrogen molecules had similar spatial distribution. The key factors for successful single-shot LIF measurements were the intensive streamers produced by the modified wire-to-plate electrode system with a short gap distance, and the use of an intensified charge-coupled detector (ICCD) camera equipped with a two-stage microchannel plate (MCP). The single-shot LIF measurement has the advantages of offering the instantaneous distribution of N2 (A3Σu+) species in the discharge, short measuring time, and better spatial resolution. All of this enable a deeper insight into the nonequilibrium kinetics of nitrogen discharges, making the single-shot LIF technique a powerful diagnostic tool, especially when the LIF signal is weak in stochastic discharges.

Combined Application of Optical Emission Spectroscopy and Kinetic Numerical Modelling to Determine the Ions Densities in a Flowing N2 Post-Discharge

The combined application of optical emission spectroscopy (OES) and kinetic numerical modelling was employed to determine the N2+(X2 $$ \Sigma_{\text{g}}^{ + } $$ ), N3+, and N4+ densities in the post-discharge (pink afterglow; PA) of a nitrogen flowing DC discharge. We measured the relative densities of the N2(C3Πu) and N2+(B2 $$ \Sigma_{\text{u}}^{ + } $$ ) states along the post-discharge region by OES. The density values were attained as functions of the post-discharge residence time. We fitted the experimental densities with densities calculated from a kinetic numerical model developed to calculate the temporal density of several nitrogen species in the nitrogen afterglow. Analysis of the rate balance equations of these ions indicated that these densities can be determined from data generated from both the model and experimental N2+(B2 $$ \Sigma_{\text{u}}^{ + } $$ ) density. Thus, we determined the ions density profiles in the nitrogen post-discharge and observed that the N3+ density is dominant in the PA. This is followed by that of the N2+(X2 $$ \Sigma_{\text{g}}^{ + } $$ ) and N4+ ions. Such behaviour has been previously reported in a study that employed mass spectrometry to analyse the ions in the PA generated by a nitrogen high-frequency discharge. In our study, the DC discharge was operated at a gas flow rate of 0.9 Slm−1, a discharge current of 30 mA, and a gas pressure range of 400–700 Pa.

Synthesis of cyanides in N2–CH4 discharge afterglow

A microwave discharge (2.45 GHz) is used to study the conversion of methane in a nitrogen afterglow. Investigations are performed both by means of emission spectroscopy and mass spectrometry.We show that methane is injected in the ‘early afterglow’ of the nitrogen discharge where the energy transfer between and is the dominant process producing . Comparing experimental to theoretical results obtained for different vibrational levels v′ of the state at different pressures, we determined the reaction rate constant values corresponding to the energy transfer between and , assuming a Treanor distribution (Tr = 400 K, Tv0–1 = 2500 K) for the vibrational levels of . The reaction rate constant values range from 1.63 × 10−18 m3 s−1 for v′ = 0 to 3.62 × 10−11 cm3 s−1 for v′ = 8. The mean value is equal to 1.93 × 10−17 m3 s−1 when v′ ranges from 0 to 10.The emission intensity decay of the first positive system is studied for bands corresponding to Δv = 3 versus methane concentration. The reaction rate constant value measured for the quenching of by CH4 is close to values proposed in literature in the case of collisions between and CH4. We studied the formation of CN, HCN and C2N2 species in the afterglow, comparing experimental to theoretical results and we measured the reaction rate constant value corresponding to:(1) The production of HCN, by reaction of CH3 with N, k9 = 8.7 × 10−18 m3 s−1.(2) The production of CN, by reaction of CHx<4 with N, γk10 = 1.2 × 10−17 m3 s−1, with γ = .(3) The production of C2N2, we show that it is probably due to the reaction between gaseous and adsorbed cyanogen radicals on the reactor wall. The product of the reaction rate constant by the surface density of CN adsorbed is equal to k14(CNs) = 55 ms−1.(4) The global destruction of CN and C2N2 when CH4 in injected in the afterglow, k11 < 1 × 10−19 m3 s−1 and k15 = 9 × 10−20 m3 s−1, respectively.

Quenching rate of N(2P) atoms in a nitrogen afterglow at atmospheric pressure

We present the results of an experimental determination of the quenching rate of the metastable 2P state of atomic nitrogen by nitrogen molecules. 2D density profiles of N(2P) and N(4S) atoms were measured by absolute optical emission spectroscopy inside a tube placed downstream of an atmospheric pressure pulsed discharge in nitrogen. The results are analyzed by means of a detailed kinetic model taking into account the major processes of production and depletion of N(2P). The quenching rate constant of N(2P) by N2 molecules determined with this method at atmospheric pressure and T = 300 K is k = (3 ± 1) × 10−17 cm3 s−1.

Effect of extractives in plasma modification of wood surfaces

This paper presents data on wettability of freshly sanded black spruce (Picea mariana) wood surfaces after treatment in the flowing afterglow of nitrogen (N2) and nitrogen–oxygen (N2/O2) dielectric barrier discharges at atmospheric pressure. Water contact angle measurements showed that plasma-treated wood samples became more hydrophobic and less hygroscopic, with the more prominent changes observed in nitrogen–oxygen plasma mixtures. Natural ageing experiments over a time period of 14 days indicated a change in plasma-treated wood surfaces to contact angles approaching those of untreated samples. On the other hand, when lower molecular mass molecules were removed from black spruce by various solvent extraction methods, plasma-induced modification seems much less pronounced. In addition, the latter samples were much more stable over time, indicating that wood extractives play a very critical role in such instability phenomenon.

Escherichia coli morphological changes and lipid A removal induced by reduced pressure nitrogen afterglow exposure.

Lipid A is a major hydrophobic component of lipopolysaccharides (endotoxin) present in the membrane of most Gram-negative bacteria, and the major responsible for the bioactivity and toxicity of the endotoxin. Previous studies have demonstrated that the late afterglow region of flowing post-discharges at reduced pressure (1-20 Torr) can be used for the sterilization of surfaces and of the reusable medical instrumentation. In the present paper, we show that the antibacterial activity of a pure nitrogen afterglow can essentially be attributed to the large concentrations of nitrogen atoms present in the treatment area and not to the UV radiation of the afterglow. In parallel, the time variation of the inactivation efficiency quantified by the log reduction of the initial Escherichia coli (E. coli) population is correlated with morphologic changes observed on the bacteria by scanning electron microscopy (SEM) for increasing afterglow exposure times. The effect of the afterglow exposure is also studied on pure lipid A and on lipid A extracted from exposed E. coli bacteria. We report that more than 60% of lipid A (pure or bacteria-extracted) are lost with the used operating conditions (nitrogen flow QN2 = 1 standard liter per minute (slpm), pressure p = 5 Torr, microwave injected power PMW = 200 W, exposure time: 40 minutes). The afterglow exposure also results in a reduction of the lipid A proinflammatory activity, assessed by the net decrease of the redox-sensitive NFκB transcription factor nuclear translocation in murine aortic endothelial cells stimulated with control vs afterglow-treated (pure and extracted) lipid A. Altogether these results point out the ability of reduced pressure nitrogen afterglows to neutralize the cytotoxic components in Gram-negative bacteria.

Investigation of the effect of additional electrons originating from the ultraviolet radiation on the nitrogen memory effect

The influence of ultraviolet radiation on memory effect in nitrogen has been investigated. The spectrum of the radiation which passes through the walls of the experimental sample was obtained by the spectrometer. A detailed comparison of experimental results of electrical breakdown time delay as a function of afterglow period with and without ultraviolet irradiation was performed. These studies were done for such product of gas pressure and inter-electrode distance when both breakdown initiation mechanisms exist. The research has shown that ultraviolet radiation leads to the decrease in ion concentration in early nitrogen afterglow due to recombination of nitrogen ions with electrons released from the tube walls and electrodes. Meanwhile, it has been cofirmed that this radiation has a negligible influence on the breakdown initiation in late nitrogen afterglow when a significant nitogen atom concentration is persistent. When the concentration of nitrogen atoms decreases enough, the breakdown initiation is caused by cosmic rays but UV photons have an important influence because of the rise of the electron yield.

Study of nitrogen flowing afterglow with mercury vapor injection.

The reaction kinetics in nitrogen flowing afterglow with mercury vapor addition was studied by optical emission spectroscopy. The DC flowing post-discharge in pure nitrogen was created in a quartz tube at the total gas pressure of 1000 Pa and discharge power of 130 W. The mercury vapors were added into the afterglow at the distance of 30 cm behind the active discharge. The optical emission spectra were measured along the flow tube. Three nitrogen spectral systems--the first positive, the second positive, and the first negative, and after the mercury vapor addition also the mercury resonance line at 254 nm in the spectrum of the second order were identified. The measurement of the spatial dependence of mercury line intensity showed very slow decay of its intensity and the decay rate did not depend on the mercury concentration. In order to explain this behavior, a kinetic model for the reaction in afterglow was developed. This model showed that the state Hg(6 (3)P1), which is the upper state of mercury UV resonance line at 254 nm, is produced by the excitation transfer from nitrogen N2(A ³Σ(u)⁺) metastables to mercury atoms. However, the N2(A ³Σ(u)⁺) metastables are also produced by the reactions following the N atom recombination, and this limits the decay of N2(A ³Σ(u)⁺) metastable concentration and results in very slow decay of mercury resonance line intensity. It was found that N atoms are the most important particles in this late nitrogen afterglow, their volume recombination starts a chain of reactions which produce excited states of molecular nitrogen. In order to explain the decrease of N atom concentration, it was also necessary to include the surface recombination of N atoms to the model. The surface recombination was considered as a first order reaction and wall recombination probability γ = (1.35 ± 0.04) × 10(-6) was determined from the experimental data. Also sensitivity analysis was applied for the analysis of kinetic model in order to reveal the main control parameters in the model.

Kinetics of excited states and radicals in a nanosecond pulse discharge and afterglow in nitrogen and air**This paper is closely related to ‘Nitric oxide kinetics in the afterglow of a diffuse plasma filament’ by D Burnette et al (doi:10.1088/0963-0252/23/4/045007) published in this volume, which focuses on the kinetic modelling of the experiments. This paper presents the results of the experiments.

The present kinetic modelling calculation results provide key new insights into the kinetics of vibrational excitation of nitrogen and plasma chemical reactions in nanosecond pulse, ‘diffuse filament’ discharges in nitrogen and dry air at a moderate energy loading per molecule, ∼0.1 eV per molecule. It is shown that it is very important to take into account Coulomb collisions between electrons because they change the electron energy distribution function and, as a result, strongly affect populations of excited states and radical concentrations in the discharge. The results demonstrate that the apparent transient rise of N2 ‘first level’ vibrational temperature after the discharge pulse, as detected in the experiments, is due to the net downward V–V energy transfer in N2–N2 collisions, which increases the N2(X 1Σ, v = 1) population. Finally, a comparison of the model's predictions with the experimental data shows that NO formation in the afterglow occurs via reactive quenching of multiple excited electronic levels of nitrogen molecule, , by O atoms.

Bacterial Decontamination of the Inner Wall of Narrow Tubes by a Nitrogen Afterglow at Atmospheric Pressure and its Relation to Local Atomic Nitrogen Concentration

AbstractA quartz tube with inner wall contaminated by Escherichia coli bacteria is treated using an atmospheric pressure nitrogen afterglow flowing at 20 SLM. The biocidal efficacy is measured locally at both the input and at the output of a 57 cm tube, giving a reduction of survivors of around 5 and 4 log for a 40 min treatment time, respectively. A weak contribution of gas heating is observed and delineated from the effect uniquely due to the plasma afterglow. Based on previous work, a model allows the estimation of the variation of the axial concentration of N atoms, assumed to be the main active species, and its correlation to the biocidal effect of the afterglow. Extrapolation of the model opens the perspective of the decontamination of longer tubes with smaller diameters such as catheter and endoscope ducts.

The local dissociation phenomenon in a nitrogen afterglow

We used the optical emission spectroscopy diagnostic to study the nitrogen afterglow of a pure N2 flowing dc discharge operating under particular experimental conditions to facilitate the simultaneous occurrence of the pink afterglow (PA) and the Lewis–Rayleigh afterglow. The PA is a special kind of nitrogen plasma occurring outside the direct influence of an external electric field. The phenomenon results from the flux of energy, introduced in the nitrogen molecules by the electrons in the discharge region, from the lower to the higher vibrational levels due to vibrational–vibrational (V–V) and vibrational–translational (V–T) exchange reactions. We studied the following set of experimental conditions: discharge electric current (I = 15–50 mA), gas pressure (p = 200–1070 Pa) and gas flow rate (Q = 400–1000 sccm). The emissions of the first positive system of the nitrogen molecules were monitored from the end of the discharge down to the end of the post-discharge tube. A kinetic numerical model developed to investigate the nitrogen afterglow generated a calibrating factor for the 580.4 nm band in such a way that the relative density of the N(4S) atoms could be measured along the afterglow. The experimental results indicated that N(4S) atoms are created locally in the afterglow producing atomic density profiles that follow the behaviour of the other species studied experimentally in the PA, such as , N2(B 3Πg), N2(C 3Πu), , , N+, , , N(2D) and N(2P). The numerical model was also used to fit the N2(B 3Πg), and the N(4S) experimental density profiles and to evaluate the participation of several kinetic pathways capable of producing local dissociation in the N2 afterglow. It was found that the dominant dissociation channel in the PA is the reaction . Its rate constant was estimated, being approximately 5 × 10−12 cm3 s−1.

Fourier transform emission spectroscopy of the A2Π–X2Σ+ (red) system of 13C14N (II)

High resolution emission spectra of the A2Π–X2Σ+ transition of 13C14N have been measured in the 15000–24000cm−1 region. Molecules were produced by the reaction of 13CH4 and 14N2 in an active nitrogen afterglow discharge. The spectra were recorded using the Fourier transform spectrometer associated with the McMath-Pierce Solar Telescope of the National Solar Observatory. A rotational analysis of 27 bands involving the excited state vibrational levels v′=9–22 and the ground state vibrational levels up to v″=12 has been obtained. An improved set of spectroscopic constants has been determined for the v=0–22 levels of the A2Π state by combining the present measurements with those reported previously for the v=0–8 vibrational levels of the A2Π state [Ram et al., Astrophys. J. Suppl. Ser. 188 (2010) 500] and existing infrared and millimeter-wave measurements of 13C14N. The 6–3, 7–4, 8–5 and 9–6 bands of the B2Σ+–A2Π transition were also identified in the 23300–24000cm−1 region and were included in the final analysis. An experimental line list and calculated term values are provided.

Biological Decontamination Using High and Reduced Pressure Nitrogen Afterglows

AbstractTypical results quantifying the antibacterial efficiencies of high and reduced pressure nitrogen afterglows are presented, using the same microbiological protocol. In parallel, the diffusion of the nitrogen atoms through different polymer membranes is studied.magnified image

Analysis of processes responsible for the memory effect in air at low pressures

The memory effect in air at 0.7 and 6.6 mbar pressures, due to post-discharge survival of some species that affect subsequent breakdowns, was analysed using memory curves. In order to complete the analysis of the processes, memory curves for nitrogen have also been monitored, because air is 78% nitrogen. In an early afterglow, the memory effect in air, as well as in pure nitrogen, is a consequence of the same processes, i.e. the presence of and ions, formed by the collision between nitrogen metastable molecules from the previous discharge. The concentrations of nitrogen ions in an air-filled tube are lower than the concentration in a nitrogen-filled tube for a given afterglow period because of their recombination with O, O2 and NO particles which are also present in the early afterglow in air. De-excitation of N2(A) and N2(a′) metastable molecules due to their collision with O2, O and O− particles also contributes to the reduction in the concentration of and ions. In the late afterglow in air and pure nitrogen, N(4S) atoms created during the previous discharge are responsible for the memory effect. In the early air afterglow N(4S) atoms can be formed by the collision of O+ ions with N2(X) molecules. However, because of the relatively small concentration of these ions the most probable process is the recombination of N(4S) atoms with O2, O, NO and NO2 particles. The net effect is that the concentration of N(4S) atoms in the late afterglow is less in air than in nitrogen. When the concentration of N(4S) atoms is sufficiently reduced, the breakdown is initiated by cosmic rays.

Study of an atmospheric pressure flowing afterglow in N2/NO mixture and its application to the measurement of N2(A) concentration

The transportation of species in a high velocity (∼103 cm s−1) flowing corona afterglow of molecular nitrogen at atmospheric pressure with added NO as impurity (<10−5) is studied. The variation of species densities along 50 cm downstream of the discharge in an 8 mm inner diameter tube is modelled and compared with emission spectroscopy measurements. It is shown that, according to a quite simple O and NO kinetic mechanism for such a gas composition in the post discharge, N2(A) concentration can be obtained from the emissions of the NOγ band at 247.9 nm and the NOβ band at 320.7 nm using a low resolution spectrometer (1 nm). The validity of both the measurement technique and of the assumed creation and loss mechanisms of N2(A) are demonstrated. An empirical relationship using the ratios of two line intensities has been plotted for several hundred points. The N2(A) concentration thus obtained, around 1011 cm−3, depending on the experimental conditions, is consistent with the results of direct measurements using the N2 Vegard–Kaplan system at 260.4 nm.

Erratum to: “Associative ionization reactions involving excited atoms in nitrogen plasma”

A model of kinetic processes in gas-discharge plasmas of pure nitrogen and its mixtures with nitrogen oxide and oxygen is presented. A distinctive feature of the model is that it includes associative ionization reactions involving N(2P) electronically excited atoms. Taking into account these processes allows one to explain both the anomalously slow decay of gas-discharge nitrogen plasma and the increase in the electron density in the region of the so-called pink afterglow in nitrogen. The possibility of substantially accelerating secondary ionization by adding NO molecules to a partially dissociated nitrogen is demonstrated. It is shown that such acceleration is caused by the associative ionization reaction N(2P) + O(3P) → e + NO+. The calculated results agree well with available experimental data.

N 2( C3Π u)→N 2( B3Π g)+ hν fluorescence increase due to collisional intermolecular energy transfer induced by discharged O 2 in active nitrogen and oxygen mixtures

This work is a further investigation of increases in violet and UV fluorescence in various flowing nitrogen afterglows, when oxygen discharged in a microwave ( μ− w) cavity is added to active nitrogen and oxygen mixtures downstream of the μ− w discharge zones of nitrogen and oxygen. The present N 2 second positive band system (pbs) fluorescence (i.e. N 2( C 3 Π u , υ′→ B 3 Π g , υ″)+ hν) increases also occur, when an orange flame (previously reported) is visually observed in nitrogen afterglows of active nitrogen and oxygen, by adding μ− w discharged O 2. The orange flame is reported to result from a collisional energy transfer between excited molecular oxygen and molecular nitrogen species. (Nitrogen was activated by two ways: (i) with or without argon in a μ− w discharge cavity and (ii) by reacting gaseous nitrogen with metastable argon, Ar( 3P 0,2), generated in μ− w discharged Ar.) The present, as well as previous, results indicate that the well-known metastable energy donor, N 2 ( A 3 Σ u + ) , is also an efficient energy acceptor from non-nitrogen species, namely from excited O 2, most probably O 2( α 1 Δ g ). In addition, by analyzing experimental results using two different conventional chemical kinetics approximations, a lower limit estimate (∼1×10 −10 molecule −1 cm 3 s −1) is deduced of the pseudo-unimolecular chemical reaction rate constant, k αA , for the energy transfer between O 2( α 1 Δ g ), as the energy donor, and N 2 ( A 3 Σ u + ) , as the energy acceptor. Further, enhanced intensities of background N 2 + first negative band system (nbs) emissions (i.e. N 2 ( B 2 Σ u + → X 2 Σ g + ) + h ν ) are observed along with enhanced background N 2 second pbs emissions intensities caused by discharged oxygen, near a nitrogen pink afterglow. The energy transfer, responsible for the enhanced N 2 first and second pbs emissions intensities and N 2 + first nbs emissions intensities, may contribute to the corresponding upper atmospheric and space emissions, in particular to the various visible and UV short-lived (lasting a few to ∼ a thousand ms) emissions discovered since 1989, termed upper atmospheric “transient luminous events”.

Nitrogen Atmospheric Pressure Post Discharges for Surface Biological Decontamination inside Small Diameter Tubes

AbstractA nitrogen afterglow at atmospheric pressure has recently been described as able to transport active species over long distances in small diameter tubes, with a biocidal effect. For a discharge gas composed of nitrogen, either of high purity or with some controlled ppm of oxygen, survival curves are presented. The afterglow, flowing at 40 slm in a cylindrical quartz tube with 8 mm internal diameter is studied using emission spectroscopy. Fundamental or excited states of atomic or molecular species of parent gases are detected and evaluated. Their absolute concentration is measured along the tube axis. Correlated to transport equations, results give information on the creation and destruction reactions of these species, especially of the O(1S) metastable state of O, the species that has been shown to boost the biocidal effect.magnified image

Luminous Activity of Nitrogen and Argon Afterglows Issued From Dielectric Barrier Discharges at Atmospheric Pressure

In this paper, a "long" atmospheric pressure N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> dielectric-barrier-discharge afterglow is presented. It is a rather spectacular experiment obtained when the (under high gas flow) discharge effluents are channeled in long tubes of small diameter. The snapshots of the long afterglow are shown in pure N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -Ar afterglows. The luminosity of the afterglow denotes that active species created during the discharge can be transported in long distances, which could be useful in a variety of applications.

Emission and shock visualization in nonequilibrium nitrogen afterglow plasma

Kinetic modeling of propagating and stationary normal shocks in nonequilibrium nitrogen afterglow plasma is used to simulate the results of shock emission measurements in nitrogen afterglow. Emission intensity overshoot behind the shock predicted by the model is in satisfactory agreement with the experimental results and is consistent with previous analytic estimates. The model demonstrates that the first and the second positive band emission overshoot behind the shock are produced by energy transfer processes among the triplet electronic states of nitrogen generated in the electric discharge. On the other hand, charge separation and ambipolar electric field produced across the shock layer do not result in electron heating and additional electron impact excitation of electronic states. The calculations show that emission overshoot makes possible accurate detection of a stationary shock layer in supersonic flowing afterglow experiments.