The reaction kinetics in argon flowing afterglow (post-discharge) with the air addition was studied by optical emission spectroscopy. The optical emission spectra were measured along the~post-discharge flow tube. A zero-dimensional kinetic model for the reactions in the afterglow was developed. This model allows to calculate the time dependencies of particle concentrations

Krčma, F.

Mazánková, V.

Petrová, K.

Trunec, D.

English

CTU Open Journal Systems (Czech Technical University, Prague / České vysoké učení technické v Praze)

Plasma Physics and Technology 3(3):136–139, 2016 © Department of Physics, FEE CTU in Prague, 2016STUDY OF ARGON AFTERGLOW WITH THE AIR ADDITIONMazánková V.a,∗, Trunec D.b, Petrová K.a, Krčma F.aa Faculty of Chemistry, Brno University of Technology, Purkyňova 118, 612 00 Brno, Czech Republicb Faculty of Science, Masaryk University, Kotlářská 2, 611 37 Brno, Czech Republic∗ mazankova@fch.vut.czAbstract. The reaction kinetics in argon flowing afterglow (post-discharge) with the air additionwas studied by optical emission spectroscopy. The optical emission spectra were measured alongthe post-discharge flow tube. A zero-dimensional kinetic model for the reactions in the afterglow wasdeveloped. This model allows to calculate the time dependencies of particle concentrations.Keywords: argon afterglow, optical emission spectroscopy, kinetic model.1. IntroductionBesides different types of plasma argon plasma gener-ated by electrical discharges has an exceptional posi-tion for its exploitation. A lot of treatment techniquesand production of thin films and multilayer systemsare arranged in argon plasma atmosphere [1]. Specialposition has also argon afterglows containing differentimpurities. Afterglow systems containing oxygen ornitrogen have a wide range of applications in differentfields due to the presence of reactive oxygen or nitro-gen species (oxygen atoms, excited metastable oxygenand nitrogen molecules) [2, 3]. The afterglows of Ar-O2 or Ar-N2 plasmas have been studied experimentallyor theoretically by numerous groups. The publishedstudies mainly aimed to determine the concentrationsof reactive species in the treatment area. Theoreticalkinetic model of Ar-N2 flowing afterglow was devel-oped by Loiseau et al. [4] and recently a model forAr-N2 mixture was also developed by Shneider et al.[5] in order to study population inversion in this gasmixture. A zero dimensional kinetic model in Ar-O2surface-wave microwave discharges was developed byKutasi et al [6, 7] to investigate electron and heavyparticle kinetics and dissociation of O2 molecules inthe discharge and the flowing afterglow. The presentwork is focused on the experimental study of air ad-dition directly to the argon flowing afterglow. Theintensity of the arising nitrogen second positive system(SPS) was measured in dependence on decay time byoptical emission spectroscopy. A simple kinetic modelwas developed in order to explain the experimentalresults.2. Experimental setupThe flowing configuration of argon DC discharge withthe air admixture was used for this experimental study.The experimental set-up was already used for our pre-vious studies of argon afterglow with nitrogen admix-ture [8] and nitrogen afterglow with mercury vapouradmixture [9]. A simplified schematic drawing of theexperimental set-up is given in Figure 1.The activedischarge was created in a quartz discharge tube withthe inner diameter of 12 mm at the constant total gaspressure of 1 000 Pa and the discharge power of 50 W.Hollow molybdenum electrodes were placed in theside arms (at the interelectrode distance of 120 mm)of the main discharge tube. The argon gas was of99.99 % purity and it was further cleaned by Oxi-clear. The reactor system was pumped continuouslyby a rotary oil pump. The argon flow of 1 400 sccmwas automatically controlled by the Bronkhorst massflow controller. The synthetic air was prepared bymixing of nitrogen and oxgen (8 sccm N2 + 2 sccmO2) and flow was also automatically controlled bythe Bronkhorst mass flow controllers. The moveablecapillary tube for air addition was made of Pyrex andit was immersed upstream from the discharge intothe quartz tube at its axis. Its external diameter was2 mm, the inner diameter was 0.5 mm and the lengthwas 400 mm. The position of the output end of cap-illary tube was fixed at 50 mm from the end of theactive discharge (nearest edge of the side arm withcathode). The flow analysis was performed in thesame way as in previous study on argon metastablequenching [8]. The input gas temperature was 300 K.The optical spectra were measured by JobinYvon monochromator TRIAX 550 with 300 gr/mmand 1 200 gr/mm grating and with CCD detector.The emitted light was led to the entrance slit of themonochromator by the multimode quartz optical fibremovable along the discharge tube.3. Results and DiscussionThe optical emission spectra were measured in theactive discharge with grating 1200 gr/mm and inthe afterglow with grating 300 gr/mm. The argonmetastable concentration in the active discharge wasdetermined from the spectra using the self-absorptionmethod [10]. However, no self-absorption was ob-served, so the mean (across the tube diameter)metastable concentration was below 109 cm−3. Onlythe SPS and argon lines were found in the spectra.The N2(C) state, which is the upper state of SPS, is136vol. 3 no. 3/2016 Study of argon afterglow with the air additionFigure 1. Scheme of the experimental setup. 1 — catalyzer Oxiclear; 2 — mass flow controllers; 3 — quartz dischargetube; 4 — capacitance gauge; 5 — quartz optical fibre; 6 — monochromator Jobin Yvon Triax 550; 7 — CCD; 8 —PC; 9 — movable Pyrex capillary tube.produced by excitation transfer in collisions of argonmetastable Ar(3P2) with nitrogen molecules.As it can be seen from Figure 2, no transitions fromthe upper state with v > 2 were observed, which istypical for the population of N2(C) state by excitationtransfer from Ar(3P2) state.The SPS intensity wasmeasured as a function of position along the flow tube.The SPS was identified 20 millimeters before the addi-tion point. An example of the recorded post-dischargespectrum after air addition is shown in Figure 2. Thedependence of SPS intensity on the distance from ac-tive discharge is shown in Figure 3. This dependencecan be divided into two parts. The first part repre-sents a rapid increase of SPS intensity to the maximalvalue due to the mixing of air with flowing argon.Figure 2. Overview spectrum of argon post-dischargewith air titration.In the second part the SPS intensity decreases dueto the decrease of argon metastable concentration andthus due to the decrease of excitation transfer rate. Azero-dimensional kinetic model for the argon — airafterglow was developed. The initial concentrationsof argon [Ar]0, molecular oxygen [O2]0 and molecularnitrogen [N2]0 in ground states were calculated fromthe equation of state for ideal gas for T = 300 K andfrom corresponding flow rates. The concentration ofargon ground state was considered to be constant. Atour experimental conditions the electrons are thermal-Figure 3. The dependence of SPS intensity (0-1 tran-sition) on distance from active discharge.ized due to collisions with argon atoms. The timeneeded for the electron temperature decrease within10% of neutral gas temperature was calculated usingdata published by Trunec et al. [11]. This calculatedtime for estimated initial electron energy of 4 eV was1.2 ms, which corresponds to the distance of 2.4 cm inthe flow tube. So, the electron temperature was equalto neutral gas temperature (300 K) in our experimentand this electron temperature was also used in the ki-netic model. The reactions involved in the model andtheir rate coefficients were taken from [8, 9, 12]. Theresulting system of differential equations was solvednumerically. The results of the kinetic model areshown in Figures 4, 5 and 6.The Figure 4 shows the time dependence of elec-tron and ion concentrations. During the first hun-dred µs, the Ar+ ions are converted to Ar+2 molecularions, however both these ions react with atomic andmolecular oxygen and also with molecular nitrogen incharge transfer reactions producing O+, O+2 and N+2ions. Thereafter the O+ ions and all nitrogen ions are137Mazankova V., Trunec D., Petrová K., Krčma F. Plasma Physics and TechnologyFigure 4. The calculated time dependence of electronand ion concentrations in the afterglow.Figure 5. The calculated time dependence of neutralparticle concentrations in the afterglow.converted to O+2 ions by exothermic charge transferreaction. So, after the first millisecond all positiveions are converted to O+2 ions. Thus, after the firstmillisecond only electrons and O+2 ions are the remain-ing charged particles, which recombine mutually andtheir concentrations further decrease and also the O+2ions react again in exothermic charge transfer reactionwith arising NO.The Figure 5 shows the time dependence of neutraloxygen species and excited argon and oxygen speciesconcentrations.The Ar∗ metastables produced in thedischarge are converted to excimer Ar∗2 molecules,both Ar∗ and Ar∗2 react with molecular oxygen pro-ducing atomic oxygen in ground or excited state. Afterthe first millisecond, the concentrations of argon ex-cited species are negligibly small. Mutual reactionsof O and O2 produce ozone and metastable oxygenmolecules O2(a) and O2(b). So after the first mil-lisecond the remaining neutral particles with signif-icant concentrations in afterglow are atomic oxygenin ground state O(P), metastable molecular oxygenstates O2(a) and O2(b) and ozone together with argonand molecular oxygen in ground states. The concen-tration of metastable atomic oxygen O(D) is very lowFigure 6. The calculated time dependence of nitrousoxides concentrations in the afterglow.Figure 7. The comparison of calculated N2(C) con-centration with measured SPS intensity. Blue line –calculated N2(C) concentration, black line – convolu-tion of calculated N2(C) concentration with apparatusfunction, triangles – measured SPS intensities.(around 104 cm−3).The Figure 6 shows the time dependence of nitrogenand nitrous oxides species concentrations. The N2(C)states produced in excitation transfer reaction decaysto N2(B) which decays further to N2(A). All thesethree nitrogen excited states are also quenched byO2 producing oxygen atoms. The nitrogen atoms areproduced in collisions of two N2(A) molecules. Theatomic oxygen and nitrogen then react and nitrousoxides are produced.The results from the kinetic model are comparedwith measured SPS intensities in Figure 7.In order to compare directly the results of modelwith experimental results the convolution of kineticmodel results with an apparatus function of experimen-tal setup was calculated [8]. The apparatus functionaccounts for the light propagation in flow tube wall.This light propagation in the wall leads to the slowerdecrease of SPS intensity in the experiment. So calcu-lated SPS intensity decay is in good agreement withexperimental data except the times longer than 5 ms,when there is still very low nonzero experimentally138vol. 3 no. 3/2016 Study of argon afterglow with the air additionmeasured SPS intensity. The origin of this very lowSPS intensity is not clear.4. ConclusionsThe flowing argon afterglow with air addition wasstudied using optical emission spectroscopy. The op-tical emission spectroscopy was used to measure thenitrogen SPS along the flow tube, which arises inexcitation transfer between argon metastables andnitrogen molecules. The kinetic model shows timedependences of different species in the afterglow. Theargon and nitrogen excited states decay very quicklymainly by quenching with molecular oxygen. Alsothe argon and nitrogen ions decay very quickly dueto charge transfer reactions to molecular oxygen. Soafter first millisecond only oxygen species are impor-tant. Then atomic oxygen reacts with atomic nitrogenproducing NO and further NO2.AcknowledgementsThe research was supported by Czech Ministry of Educa-tion, Youth and Sports, project No. COST CZ LD15010and within the collaboration of the COST Action CM1401.References[1] S. Saloum, M. Naddaf, and B. Alkhaled. Diagnosticsof N2-Ar plasma mixture excited in a 13.56 MHz hollowcathode discharge system: application to remote plasmatreatment of polyamide surface. J. Phys. D: Appl.Phys., 41(4):045205, 2008.[2] M. Mafra, T. Belmonte, F. Poncin-Epaillard,A. Maliska, and U. Cvelbar. Treatment ofhexatriacontane by Ar-O2 remote plasma: Formation ofthe active species. Plasma Process. Polym.,6(S):S198–S203, 2009.[3] S. Lerouge, M. R. Wertheimer, and L’H. Yahia. Plasmasterilization: a review of parameters, mechanisms, andlimitations. Plasma Polym., 6(3):175–188, 2001.[4] J. F. Loiseau, P. Pignolet, and B. Held. Numericalsimulation of Ar-N2 excitation transfer in flowingafterglow. J. Phys. D: Appl. Phys., 25(5):745–750, 1992.[5] M. N. Shneider, A. Baltuška, and A. M. Zheltikov.Population inversion of molecular nitrogen in an Ar: N2mixture by selective resonance-enhanced multiphotonionization. J. Appl. Phys., 110(8):083112, 2011.[6] K. Kutasi, P. Sá, and V. Guerra. O2 dissociation inAr-O2 surface-wave microwave discharges. J. Phys. D:Appl. Phys., 45(19):195205, 2012.[7] K. Kutasi, R. Zaplotnik, G. Primc, and M. Mozetic.Controlling the oxygen species density distributions inthe flowing afterglow of O2/Ar-O2 surface-wavemicrowave discharges. J. Phys. D: Appl. Phys.,47(2):025203, 2014.[8] V. Mazánková, D. Trunec, and F. Krčma. Study ofargon flowing afterglow with nitrogen injection. J.Chem. Phys., 139(3):164311, 2013.[9] V. Mazánková, D. Trunec, and F. Krčma. Study ofnitrogen flowing afterglow with mercury vapor injection.J. Chem. Phys., 141(3):154307, 2014.[10] M. Schulze, A. Yanguas-Gil, A. von Keudell, andAwakowicz P. A robust method to measure metastableand resonant state densities from emission spectra inargon and argon-diluted low pressure plasmas. J. Phys.D: Appl. Phys., 41(3):065206, 2008.[11] D. Trunec, P. Spanel, and D. Smith. The influence ofelectron-electron collisions on electron thermalization inHe and Ar afterglow plasmas. Chem. 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Study of Argon Afterglow with the Air Addition

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Study of Argon Afterglow with the Air Addition

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