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Research ArticleKinetic Study of Atmospheric Pressure NitrogenPlasma Afterglow Using Quantitative Electron SpinResonance Spectroscopy

A Taacutelskyacute O Štec M Pazderka and V Kudrle

Department of Physical Electronics Masaryk University Kotlarska 2 61137 Brno Czech Republic

Correspondence should be addressed to V Kudrle kudrlescimunicz

Received 14 September 2016 Revised 18 November 2016 Accepted 28 November 2016 Published 19 March 2017

Academic Editor Niksa Krstulovic

Copyright copy 2017 A Talsky et alThis is an open access article distributed under the Creative CommonsAttribution License whichpermits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

Quantitative electron spin resonance spectroscopy is used to measure nitrogen atom density in atmospheric pressure dielectricbarrier discharge afterglow The experiment shows that oxygen injection into early afterglow increases the nitrogen dissociation incertain parts of the afterglowwhile it is decreased in the rest of the afterglow Numerical kineticmodelling supports and explains theexperimental data while the best fit provides some a priori unknown parameters such as initial concentrations and rate constants

1 Introduction

Nonequilibrium cold plasmas at atmospheric pressure andespecially dielectric barrier discharges are getting increasedattention of both basic and applied research They are widelyused in many industrial applications from ozone production[1] and lighting [2] to plasma surface modifications [3]Theirmain advantages over other competing technologies are theease-of-use economy and environmental considerations [4]Besides the already established applications there is rapidexpansion of atmospheric pressure plasmas into new areassuch as plasma-chemical synthesis of substances which aredifficult to attain by other techniques [5ndash7] plasma medicine[8] material disinfection and sterilisation [9] and use incosmetics [10] or in fashion industry [11]

Nitrogen plasmas are often used as a source of highdensity nitrogen atoms Molecular nitrogen is rather inertgas but atomic nitrogen is quite reactive which is madeuse of in plasma deposition of nitride films [12] or plasmanitridation [13] Although the low pressure plasmas arecurrently dominating this field there is strong incentivefor research and development of nitrogen plasma sourcesoperating at atmospheric pressure [14]

In order to develop new plasma-chemical technologiesit is important to understand the elementary processestaking place in both the active plasma and plasma afterglow

Nitrogen despite being a simple diatomicmolecule has quitecomplex plasma chemistry and kinetics especially in mix-tures with oxygen [15 16] As stated above the nitrogen atomsplay a significant kinetic role due to their high reactivityTheirconcentration is then one of the most important parametersto be experimentally determined Although there is broadrange of experimental techniques able to detect N atoms forexample optical emission spectroscopy only few of them aresuitable for absolute not relative measurements UV absorp-tion spectroscopy [17] NO titration [18] mass spectroscopy(MS) [19] laser induced fluorescence (LIF) [20] and catalyticprobes [21] for example are widely used Another challengeis an operation of such diagnostic technique in atmosphericpressure which for example greatly increases quenchingin LIF complicates pumping in MS or totally changes theplasma chemistry (NO titration)

In this paper the electron spinparamagnetic resonance(ESREPR) [30ndash32] method is used for N atom densitydetermination This method is very useful for detectionand identification of paramagnetic particles and especiallyradicals [33ndash35] Although the method is routinely used inchemistry its use in plasma physics is relatively rare Since thepioneering works [36ndash38] other mostly laser based methodsappeared which are now considered mainstream

Electron spin or paramagnetic resonance (ESREPR)is essentially microwave absorption spectroscopy [39] on

HindawiJournal of SpectroscopyVolume 2017 Article ID 5473874 10 pageshttpsdoiorg10115520175473874

2 Journal of Spectroscopy

N2 O2

Barrierdischarge

Rotameter

Valves

Gas cylinders

Digital

NMRmagnetometer

Microwavegenerator

Control

PC

Electromagnet

Digitalmanometer

Rotaryvacuumpump

coils

owmeter

Figure 1 Schematic drawing of the experimental set-up

Zeeman split [40] levelsThe transitions with very low energyseparation between the upper and the bottom levels (as isthe case inmicrowave spectroscopy) are very easily disturbedby the collisions with other particlesThe collisionalpressurebroadening of absorption line can be quite pronounced [4142] especially for gas phase atomic and molecular lines withvery low natural linewidths This makes most of the species(for extensive list see [43]) including for example atomicoxygen difficult to detect in atmospheric pressure plasmasHowever the particles in s-state such as nitrogen or hydrogenatoms in ground state do not exhibit pressure broadening andcan be detected by ESREPR even at atmospheric pressure

There are a number of works both theoretical andexperimental dealing with low pressure plasma diagnos-tics by ESREPR method [44ndash50] However due to above-mentioned difficulties of pressure broadening the diagnos-tics of atmospheric pressure plasmas by EPRESR is ratherunresearched topic as our previous paper [51] is to authorsrsquobest knowledge still the only one published

Although the nitrogen andoxygenplasmas are studied forvery long time there is still intensive research [52ndash54] goingon The discharges in N2-O2 mixtures have very complicatedplasma kinetics and some effects especially in mixtures withlow O2N2 ratio are not fully explained yet In our previouswork [55] it was reported that adding of small amountof oxygen into a low pressure nitrogen plasma afterglowcauses an increase of nitrogen atom density In this paper weextend the study of the influence of oxygen admixture on thenitrogen afterglow to atmospheric pressure

2 Experimental Apparatus and Methods

21 Experimental Set-Up The experimental apparatus isdepicted in Figure 1 Nitrogen plasma at atmospheric pres-sure was produced using industrial ozoniser LifeTech 50It is based on coaxial dielectric barrier discharge excited

by 25ndash35W power at 15 kHz frequency The stainless steelinner electrode has approx 2 cm diameter and is separatedby 07mm plasma gap and 25mm thick corundum (Al2O3)ceramics from the outer electrode which is realised by analuminium foil tightly wrapped around the ceramic tubeThe discharge tube is approx 15 cm long Although the metalelectrode in direct contact with plasma can significantlyreduce N atom density in the effluent due to increasedsurface recombinationreassociation this design [56] is wellestablished in industrial ozonisers

The discharge was operated in flowing (12 standard litresper minute) nitrogen coming from pressurised cylinder(Messer-Griesheim purity 99995) via pressure reductionvalve The volumetric flow rate of nitrogen was measuredby flowmeter with floating element (rotameter UPLS-R3)Downstream from the plasma source the oxygen is option-ally introducedThe oxygen comes from pressurised cylinder(Messer-Griesheim purity 99995) via Hastings mass flow-controller maintaining the oxygen flow at 75 sccm (standardcubic centimetre per minute) The tubing is made frompolyethylene and stainless steel

The oxygen is introduced into the afterglow tube justdownstream from the plasma generator using an inlet three-way valve This configuration permits presetting the oxygenflow on the flow-controller and rapidly switches on and offthe oxygen admixture The distance between the output portof the plasma generator and the inlet of oxygen is 6 cmThe afterglow tube is made of fused silica with 10mm outerdiameter and 8mm inner diameter This tube passes throughESREPR spectrometer resonator and ends in another outletthree-way valve In normal operation the plasma afterglowvents into open atmosphere and the afterglow tube length(approx 25 metres) together with high gas flow preventsany back-diffusion of air into the afterglow or plasma Insecond position of this outlet valve the whole apparatus canbe pumped down by the rotary vane oil vacuumpumpThis is

Journal of Spectroscopy 3

used for calibration by molecular oxygen at reduced pressureand for degassingcleaning as before every measurementseries the whole plasma system is evacuated to pressureabout 10 Pa to remove residual impurities After that thenitrogen flow is switched on and the outlet three-way valveis turned to normal position Then the discharge is run 1hour atmaximumpower to burn-in the plasma generator andremove the possible contamination off the electrode Onlyafter such procedure are the experiments started

The distance from the inlet of oxygen admixture to themeasuring ESREPR resonator can be changed by shiftingthe whole plasma system To ease this the plasma systemincluding afterglow tube is placed on nonmagnetic railsparallel to ESREPR resonator axis The furthest possibleposition is limited by the rail length to 90 cm However inthe present study the maximum extension was not used asthe ESREPR signal in this extreme position was alreadydifficult to measure The minimum distance in which themeasure could be carried out is about 20 cm Although itis mechanically possible to place the discharge closer tothe resonator at shorter distances the magnetic field of theESREPR spectrometer electromagnet affected the plasmagenerator

The ESREPR spectrometer JEOL JES-PE is speciallyadapted for use in plasma physics It is classical continuouswave spectrometer [34] using klystron source operating at Xband and with standard sensitivity around 1010 paramagneticparticles in the interaction volume of TE011 resonator Theoutput voltage of the analogue spectrometer is measured bydigital voltmeter (Metra M1T390 5 digits) and transmittedto a personal computer via GPIB The ESREPR signalis analysed and postprocessed using custom software Itautomatically identifies the spectral lines and calculates theirpeak-to-peak height peak-to-peak width 119908 and area underthe absorption line 119868N (as ESREPR lines are typically [34]recorded in the form of derivation double integration isneeded)

22 The Principle of Measurement and Calibration ProcedureThe ESREPR phenomenon is based on resonant absorptionof microwave photons by transition between Zeeman split[40] energy levels Typically the resonance is achieved byvariable magnetic field (which sets the energy level splitting)while the microwave frequency (ie photon energy) is fixedThe area under absorption spectral line is proportional [38]to the concentration of absorbing paramagnetic particlesTheproportionality constant depends on spectrometer settingsand the transition probability (Einstein coefficient) Aftera calibration of the spectrometer by a known sample itis possible to measure the concentration of paramagneticparticles absolutely [57]

In this work the spectrometer calibration [44] is carriedout using molecular oxygen O2 The measuring resonator isfilled with gaseousmolecular oxygen at known concentration(calculated from pressure and temperature) its intensivespectral line C (the traditional naming found in manypapers eg [38 41 43 44]) is recorded and correspondingdouble integral 119868C is calculated Using the same settings ofthe spectrometer the ESREPR line of atomic nitrogen is

00

50000

100000

150000

200000

Inte

gral

10080604020Pressure of O2 (Pa)

Figure 2 Calibration of EPR spectrometer using the line C ofmolecular oxygen Linear dependency of absorption line integral119868C (ie double integral of measured signal) on oxygen pressurevalidates the calibration process and its slope provides a constant[O2]119868C needed for the calibration

recorded and its double integral 119868N is calculated Knowingthe Einstein coefficients of both species the concentration ofatomic nitrogen is given by simple relation

[N] = 588 sdot 10minus3119868N119868C times [O2]

The numeric constant in this formula is the ratio of Einsteincoefficient of molecular oxygen line C and that of onecomponent of atomic nitrogen triplet [43]

As the oxygen calibration is carried out in the flow-regime due to HagenndashPoiseuille law [58] there exist pressuregradients along the tubemaking the pressure in the ESREPRresonator slightly different from the one indicated by pressuremeter The vacuum conductivity of the tube depends [59]on pressure too Moreover as at low pressures the partialpressure of O2 could be smaller than the total pressureindicated due to small vacuum leaks finite base pressure ofthe pump used or degassing from walls it is better to recordO2 line for several indicated pressures than to rely on singlevalue only The result of such calibration measurement isshown in Figure 2

The ESREPR lines of atomic nitrogen are extremelynarrow and therefore sensitive to any broadening [41] Theground state of nitrogen N(4S32) has zero orbital magneticmomentum and its paramagnetism is given only by the elec-tron spin In that case there should be no collision inducedbroadening its spectral linewidth should be independent ofpressure [60] In work [61] this was studied in the pressurerange from 50 to 600 Pa and in temperature range from80 to 300K It was found that with decreasing temperaturethe apparent linewidth 119908 = 6120583T remained constant whilethe integral and thus the concentration [N] decreased Thelinewidth was thus independent of N atom density whichsuggested that the role of spin-spin relaxation at these Nconcentrations (around 1013ndash1014 cmminus3) is negligible


00

2

4

6

8

10

2 4 6 8 10 12 14 16O2 influx (sccm)

Spec

tral

line

wid

th (1

0minus6

T)

Figure 3 Linewidth of N atom ground state (4S32) EPR line as afunction of oxygen admixture

In present work a similar measurement of N(4S32)linewidth was carried out in pure nitrogen afterglow butat atmospheric pressure Practically the same value 119908 =64 120583T was observed The linewidth (and spectral line shape)being constant the concentration of N atoms should bedirectly proportional to the absorption line height and byconsequence also to the peak-to-peak height of the measured(ie absorption derivative) line It can be advantageous touse this peak-to-peak height over the spectral line integral asthe latter exhibits much higher statistical variations due to anincreased influence of a noise overlaying the line shoulders

However a presence of other paramagnetic particles canaffect even the s-states The experimental result nitrogenspectral linewidth as a function of molecular oxygen admix-ture is shown in Figure 3 The changing nitrogen linewidthprevents the use of simple line height as concentrationindicator and the integral must be used instead This effectis consistent with [60]

3 Results and Discussions

31 Experimental Results Electron paramagneticspin reso-nancewas used tomeasure the concentration of atomic nitro-gen N(4S32) in atmospheric pressure discharge afterglowThis density [N] was measured along fused silica tube in purenitrogen afterglow and with 625 ppm of oxygen injected intothe early afterglow see Figure 4

The most important result is the fact that the twocurves apparently intersect It means that a small admixtureof oxygen can cause both an increase and a decrease inconcentration of nitrogen atoms depending on which partof the afterglow is observed This nontrivial behaviour isa typical demonstration of complexities inherent in N2-O2kinetics

In flowing kinetic studies if the gas velocity is known thespatial distribution of measured species can be transferred totheir temporal evolution Sometimes the same gas velocity inthe whole cross-section (ie plug flow) is assumed withoutany further consideration However there can be a significant

05

10

15

20

25

30

Distance from oxygen inlet (cm)

Crossing

70 806040 50302010

N2 with O2 admixturePure N2

Con

cent

ratio

n of

N(4

S)(1013

cmminus3)

Figure 4 Spatial distribution of the nitrogen atom concentration(in ground state) along the afterglow tube for two cases with andwithout 625 ppm oxygen admixture

radial gradient of gas velocity due to a shear flow anda boundary layer While a laminar flow in circular tubeproduces well-known parabolic velocity profile in a well-developed turbulent flow the shear zone is thin comparedto the tube diameter so the plug flow is appropriate Thecharacter of flow in a tube can be deduced from the Reynoldsnumber [62]

119877 = 119906119889]

where 119906 is mean flow velocity 119889 is the inner tube diameterand ] is cinematic viscosity For nitrogen at standard pressureand temperature ] = 150 sdot 10minus5m2 sminus1 according to [63]which gives 119877 = 2100 for our experimental conditions Thisvalue falls in critical Reynolds number range 119877 = 1800ndash2300where a transition between the laminar and the turbulent flowhappens [64] One can expect that the inhomogeneity causedby the lateral oxygen inlet together with other imperfectionsintroduces some additional turbulence Based on this reason-ing there is enough turbulence to consider the plug flow as avalid approximation in this paper too

In that case the radially uniform flow velocity is approx4ms Using this value the measured dependence of thenitrogen atom density on position in the afterglow (Figure 4)was recalculated to the dependence on time see Figure 5

It is experimentally impossible to measure the concen-tration near the oxygen inlet (ie at times close to 119905 = 0 s)as the T-piece (needed for the inlet) cannot pass throughthe ESREPR measuring resonator opening and the closedistance between the plasma generator and the ESREPRelectromagnet would magnetically influence the plasma gen-erator and the plasma itself However if one excludes a back-diffusion it is evident that with or without oxygen admixturethe value at 119905 = 0 s must be the same that is both curvesmust start at the same initial value depicted in Figure 5 by redcircle While this common initial value is a priori unknown


Table 1 Main plasma-chemical reactions in the pure nitrogen afterglow

Reaction number Reaction Rate coefficient Reference(1) N(4S) + N(4S) + N2 rarr N2(A) + N2 1198961 = 105 sdot 10

minus13 cm3 sminus1 [22](2) N(4S) + wallrarr N2 + wall 1198962 = see text(3) N2(A) + N(4S)rarr N2(X) + N(2P) 1198963 = 5 sdot 10

minus11 cm3 sminus1 [23](4) N(2P) + N2 rarr N(2D) + N2 1198964 = 2 sdot 10

minus18 cm3 sminus1 [24](5) N(2P) + Nrarr N(2D) + N 1198965 = 18 sdot 10

minus12 cm3 sminus1 [25](6) N(2D) + N2 rarr N(4S) + N2 1198966 = 6 sdot 10

minus15 cm3 sminus1 [26](7) N2(A) + N2(A)rarr N2(C) + N2 1198967 = 2 sdot 10

minus12 cm3 sminus1 [27]

00000

05

10

15

20

25

30

35

40

Time (s)

015010005

N2 + O2 experimentPure N2 experiment

Reasonable extrapolation for pure N2

Reasonable extrapolation for N2 + O2

Con

cent

ratio

n of

N(4

S)(1013

cmminus3)

Figure 5 Data from the previous figure recalculated from distanceto timeThe rough estimation of initial [N] value at 119905 = 0 s is showntoo The sketch of a reasonable time evolution is shown by dashedlines

it effectively sets the probable shapes of both curves (withand without oxygen admixture) in the unmeasurable regionThese are shown in Figure 5 using the dashed lines One cansee that the case without oxygen admixture should be simplygoverned by losses while the case with oxygen admixtureinitially exhibits some process producing the nitrogen atomsand only later the losses dominate Higher losses of N withO2 present cause the steeper descent of this curve and so bothcurves intersect

32 Kinetic Modelling of Plasma Afterglow in Pure NitrogenThe model is based on collisional processes between theprincipal species in the nitrogen afterglow N2 moleculeselectronically excited molecules N2(A) atoms N(S) N(P)and N(D) and it takes into account also the wall pro-cesses The model is focused on N(4S) and so the speciesN2(Xv) despite being another important energy carrier isnot included as it cannot directly cause N2 dissociationand it does not exhibit very strong quenching by nitrogenatoms (which N2(A) does) An overview of the main plasma-chemical processes based on [28] is presented in Table 1

The kinetic equations describing the processes in Table 1were transformed to a set of differential equations and solved

numerically using the Eulermethod As thismethodmight beunstable for rapidly changing functions it is necessary to usesufficiently small time-step This convergence was verified inthe presented model Some parameters such as 1198962 and initialconcentrations of N(4S) andN2(A) are a priori unknown andmust be determined by fitting to the experimental data Theconcentration ofmolecular nitrogen [N2] = 277sdot10

19 cmminus3 atstandard pressure and temperature was used as one of initialvalues

The reaction (1) describes the volume recombination andthe reaction (2) the wall recombination of atomic nitrogenin ground state N(4S) The model includes the reactions ofthe N(2P) N(2D) and N2(A) metastables too However sig-nificant simplifications are possible In the experiment onlythe N(4S) concentration is actually measured Moreover themetastable N atoms generally end in ground state (reactions(4)ndash(6)) So the reaction (3) effectively produces one N(4S)atom and is equivalent to N2(A) deexcitation

Further simplification stems from the mutual ratio of[N2(A)] and [N(4S)] Although these values are a prioriunknown they can be roughly estimated A rough estimationof the initial concentration [N2(A)]0 of 10

11 cmminus3 can betaken from papers [65ndash67] which used sufficiently similarconditions to the present ones (atmospheric pressure nitro-gen afterglow operated in similar tube diameter and similarinput power) As the visual extrapolation of pure nitrogencurve (red hollowdiamonds) in Figure 5 gives the initial valueof [N(4S)]0 around 1013 cmminus3 one may safely assume that[N(4S)]0 gt [N2(A)]0

Using these simplifications the ldquofullrdquo model of Table 1is effectively reduced to the set of kinetic equations (1) (2)(3) and (7)The relative difference 120575 = ([N(4S)]|reduced model minus[N(4S)]|full model)[N(

4S)]|reduced model between the reducedand full models is shown in Figure 6 for different initial ratiosof [N(4S)] and [N2(A)]

In the first few milliseconds there is a big differencebetween the full and the reduced models due to the reaction(3) But in later reaction times in which the real experimentis carried out (as discussed above due to experimentalconstraints it is not possible to measure at afterglow positionsbefore 006 s) there is nearly negligible difference betweenthe full and the reduced models Taking into account theexperimental error around 10 and an assumption aboutinitial values 1011 cmminus3 lt [N2(A)]0 lt [N(

4S)]0 one can safelyuse the reduced model only


000minus08

minus06

minus04

minus02

00

02

Time (s)025020015010005

Relat

ive d

ier

ence

120575

[N(4S)]t=0[N2(A)]t=0 = 1

[N(4S)]t=0[N2(A)]t=0 = 2

[N(4S)]t=0[N2(A)]t=0 = 3

[N(4S)]t=0[N2(A)]t=0 = 5

[N(4S)]t=0[N2(A)]t=0 = 10

Figure 6 Relative difference 120575 between full (see (1)ndash(7)) andreduced (see (1) (2) (3) and (7)) models for several initial ratiosbetween [N(4S)] and [N2(A)]

10

15

20

25

30

35

Reduced modelExperiment

000Time (s)

025020015010005

Con

cent

ratio

n of

N(4

S)(1013

cmminus3)

Figure 7 Fit of the kinetic model of the pure nitrogen after-glow given by reduced reactions from Table 1 to the experimentaldata giving the previously unknown initial value [N(4S)]0 = 33 sdot1013 cmminus3 and the value of 1198962

The least squares fit of experimental data by this reducedmodel is shown in Figure 7 and gives the values of previouslyunknown parameters [N(4S)]0 = 33 sdot 10

13 cmminus3 and 1198962 =31 sminus1 To the rate constant 1198962 exists corresponding coefficient(probability) of surface recombinationreassociation 120574 =124 sdot 10minus6 which is in good agreement with [68] where theyget 120574lit = 135 sdot 10

minus6 Following the reasoning of the previousparagraph one may assume that these values do not stronglydepend (see Figure 6) on the initial concentration of N2(A)(for sufficiently small [N2(A)]0)

33 Kinetic Modelling of Nitrogen Afterglow with OxygenAdmixture There are many published sets of kinetic pro-cesses for N2-O2 mixtures with varying degree of detail [1628 53ndash56 69ndash72] Taking into account the time scales andtypical values of concentrations these extensive sets can besubstantially reduced and simplified Essentially it is possibleto extend the model of pure nitrogen afterglow (see reactions(1)ndash(7)) by includingN2(Xv) O2 O NONO2 andN2O (seeTable 2)

The inclusion of vibrationally excited nitrogen moleculesis necessary as these have even greater importance [73] inN2 + O2 plasma afterglow In a nitrogen pink afterglowthe vibrationally excited molecules carry a significant partof energy in the afterglow [74 75] This energy is thenresponsible for the formation of N2(A) N2(a

1015840) and finally forthe ionisation of N2 molecules Although the pink afterglowwas not observed [76] in atmospheric pressure plasma onemay assume [73 77] the concentration of N2(V = 12) greaterthan 5 sdot 1013 cmminus3 at afterglow position of 15ms

The concentration of molecular oxygen [O2] = 156 sdot1016 cmminus3 is calculated according to the ideal gas law from theexperimental conditions (625 ppmofO2 inN2 at atmosphericpressure and room temperature)

The reaction set of Table 2 can be further reduced asfollows Kinetic reaction (22) is negligible to the reaction (12)because 11989622 is about two orders of magnitude smaller than11989612 For the same reason reaction (15) can be neglected withrespect to reaction (14) Furthermore (26) according to thenumerical calculations has not significant influence on theconcentration of N(4S) so it can be omitted too

Most of the equations describe the loss of ground statemetastable atom N(4S) Equation (13) describes productionof N(4S) but its contribution is not significant in comparisonwith the losses due to reactions (8) (9) and (10) This is incontrast with the experiment where an increase in N(4S)concentration is observed when oxygen is added Using thelarger equation set of [28] does not help either To explain the[N(4S)] increase (23) is needed where vibrationally excitedmolecule in the electronic ground state N2(Xv) produces Natom and nitric oxidemolecule by reacting with oxygen atom[69 77]

The heterogeneous reactions in the model include thewall deexcitation of N2(Xv) see (24) and N2(A) see (25)

As usual the kinetic model must be supplemented bythe initial concentrations The initial value of [N(4S)]0 wastaken from the pure nitrogen model Due to the adsorptionof oxygen on the walls of the afterglow tube [78ndash81] asmaller value of thewall recombination coefficient 1198962 (changefrom 31 sminus1 to 05 sminus1) was assumed Atomic oxygen wallrecombination coefficient 11989616 was considered to be same as1198962 for simplicity Rate constants for wall deexcitation (24) and(25) were taken to be the same for both species that is 11989624 =11989625 with estimated value in range of 1ndash10 sminus1 to correspond to[82] The precise value of 11989624 and 11989625 is calculated by fittingthe model to the experimental data

The final simplified set of equations considered in thereduced model of N2-O2 afterglow consists of (1)ndash(3)(7)ndash(14) (16)ndash(21) and (23)ndash(25) The remaining unknowns


Table 2 Additional set of kinetic reactions for nitrogen afterglow with oxygen admixture mostly based on [28]

Reaction number Reaction Rate coefficient Reference(8) N + O2 rarr NO + O 1198968 = 103 sdot 10

minus16 cm3 sminus1 [22](9) N + O + N2 rarr NO + N2 1198969 = 274 sdot 10

minus13 cm3 sminus1 [22](10) N + NOrarr N2 + O 11989610 = 182 sdot 10

minus13 cm3 sminus1 [22](11) O + NO + N2 rarr NO2 + N2 11989611 = 954 sdot 10

minus13 cm3 sminus1 [22](12) N2(A) + O2 rarr N2(X) + O + O 11989612 = 254 sdot 10

minus12 cm3 sminus1 [23](13) N2(A) + Orarr NO + N(2D)rarr NO + N(4S) 11989613 = 7 sdot 10

minus12 cm3 sminus1 [29](14) O + O + N2 rarr O2 + N2 11989614 = 791 sdot 10

minus14 cm3 sminus1 [28](15) O + O + O2 rarr 2O2 11989615 = 105 sdot 10

minus16 cm3 sminus1 [28](16) O + wallrarr O2 + wall 11989616 = see text(17) N + NO2 rarr N2 + O2 11989617 = 7 sdot 10

minus13 cm3 sminus1 [28](18) N + NO2 rarr N2 + O + O 11989618 = 91 sdot 10

minus13 cm3 sminus1 [28](19) N + NO2 rarr N2O + O 11989619 = 3 sdot 10

minus12 cm3 sminus1 [28](20) N + NO2 rarr 2NO 11989620 = 23 sdot 10

minus12 cm3 sminus1 [28](21) O + NO2 rarr NO + O2 11989621 = 254 sdot 10

minus12 cm3 sminus1 [28](22) N2(A) + O2 rarr N2O + O 11989622 = 78 sdot 10

minus14 cm3 sminus1 [23](23) N2(Xv) + Orarr NO + N 11989623 = see text(24) N2(Xv) + wallrarr N2 + wall 11989624 = see text(25) N2(A) + wallrarr N2 + wall 11989625 = see text(26) NO2 + NO2 + N2 rarr N2O4 + N2 11989626 = 564 sdot 10


0 2 4 6 8 10

Con

cent

ratio

n of

X (c

mminus3)

X = N(4S)

X = NOX = O

X = N2(A)

X = N2(X v)X = NO2

Time (10minus4 s)

1014

1013

1012

1011

1010

109

108

(a)

Con

cent

ratio

n of

X (c

mminus3)

X = N(4S)

X = NOX = O

X = N2(A)

X = N2(X v)X = NO2

000Time (s)

025020015010005

1015

1014

1013

1012

1011

1010

(b)

Figure 8The temporal evolution of main species considered in the reduced kinetic model of N2-O2 afterglow For clarity the data are shownin two time scales

are the initial concentrations of N2(A) N2(Xv) and the rateconstants 11989623 11989624 and 11989625 These parameters were deter-mined from the least squares fit of model to the experimentaldata

Figure 8 shows the calculated temporal evolution ofconcentrations of main species considered in the presentmodel in short (a) and long (b) time scales The dominantspecies (besides the parent N2 and O2) are the nitrogen andoxygen atoms Interestingly despite the very unfavourable

ratio of [O2][N2] = 625 ppm there are more oxygen atomsthan nitrogen atoms in the late afterglow Electronically andvibrationally excited nitrogen molecules disappear quicklyBoth oxides of nitrogen nitrogen dioxide NO2 and nitricoxide NO remain after 01 s constant [NO] being approx 10times higher than [NO2]

Full comparison of the model and the experiment isshown in Figure 9 for both cases with andwithout the oxygenadmixture Values of a priori unknown parameters obtained


00

05

10

15

20

25

30

35

40

Con

cent

ratio

n of

N(4

S)(1013

cmminus3)


N2 + O2 modelPure N2 model

000Time (s)

025020015010005

Figure 9 The parameters from the fit are used to calculatethe extrapolation to shorter afterglow times of kinetic models ofatmospheric pressure nitrogen afterglow with and without oxygenadmixture

by the fit of N2-O2 model are [N2(A)]0 = 34 sdot 1012 cmminus3

[N2(X v)]0 = 45 sdot 1014 cmminus3 11989623 = 16 sdot 10

minus11 cm3 sminus1 and11989624 = 11989625 = 87 s

minus1 The value of 11989623 obtained from the fitis in very good agreement with [16] where this constant wasestimated to be at least 10minus11 cm3 s

The agreement between the models and the experimentaldata in Figure 9 is very good Moreover the model coversalso the times shorter than 50ms which are inaccessibleby the experiment The effect of initially increased [N] justafter oxygen admixture which was predicted in Figure 5 istherefore verified and explained by reaction of vibrationallyexcited N2(Xv) with atomic oxygen

4 Conclusions

Quantitative electron spinparamagnetic resonance spec-troscopy was used to measure the concentration of nitrogenatoms in flowing atmospheric pressure dielectric barrier dis-charge afterglow with typical [N] values around 2 sdot 1013 cmminus3The evolution of this concentration along the afterglow tubewas shown to be significantly affected by the relatively smallamount (625 ppm) of oxygen added into the early afterglowThe nitrogen dissociation is increased just after the oxygeninlet and it is decreased in later parts of the afterglow

Numerical kinetic model explains this behaviour by abalance of production and loss terms both of which areaffected by the presence of oxygen Main reaction producingN atoms is the collision of N2(Xv) with oxygen atoms Byfitting the model to the experimental data it was possi-ble to estimate several a priori unknown and not directlymeasurable quantities such as the initial concentrations ofN(4S) N2(A) and N2(Xv) wall recombination coefficient ofN atoms and rate coefficient of some reactions

Competing Interests

The authors declare that they have no competing interests

Acknowledgments

This research has been supported by the Project LO1411 (NPUI) funded by Ministry of Education Youth and Sports ofCzech Republic

References

[1] B Eliasson M Hirth and U Kogelschatz ldquoOzone synthesisfrom oxygen in dielectric barrier dischargesrdquo Journal of PhysicsD Applied Physics vol 20 no 11 pp 1421ndash1437 1987

[2] J P Boeuf ldquoPlasmadisplay panels physics recent developmentsand key issuesrdquo Journal of Physics D Applied Physics vol 36 no6 pp R53ndashR79 2003

[3] M Strobel C S Lyons and K Mittal Eds Plasma SurfaceModification of Polymers Relevance to Adhesion VSP 1994

[4] UKogelschatz ldquoDielectric-barrier discharges their history dis-charge physics and industrial applicationsrdquo Plasma Chemistryand Plasma Processing vol 23 no 1 pp 1ndash46 2003

[5] L M Zhou B Xue U Kogelschatz and B Eliasson ldquoNonequi-librium plasma reforming of greenhouse gases to synthesis gasrdquoEnergy amp Fuels vol 12 no 6 pp 1191ndash1199 1998

[6] L Zajıckova M Elias O Jasek et al ldquoAtmospheric pressuremicrowave torch for synthesis of carbon nanotubesrdquo PlasmaPhysics and Controlled Fusion vol 47 no 12 pp B655ndashB6662005

[7] P Synek O Jasek L Zajıckova B David V Kudrle and NPizurova ldquoPlasmachemical synthesis of maghemite nanopar-ticles in atmospheric pressure microwave torchrdquo MaterialsLetters vol 65 no 6 pp 982ndash984 2011

[8] G Fridman G Friedman A Gutsol A B Shekhter V NVasilets and A Fridman ldquoApplied plasma medicinerdquo PlasmaProcesses and Polymers vol 5 no 6 pp 503ndash533 2008

[9] G Fridman AD BrooksM Balasubramanian et al ldquoCompar-ison of direct and indirect effects of non-thermal atmospheric-pressure plasma on bacteriardquo Plasma Processes and Polymersvol 4 no 4 pp 370ndash375 2007

[10] J Heinlin G Morfill M Landthaler et al ldquoPlasma medicinepossible applications in dermatologyrdquo JDDG Journal derDeutschenDermatologischen Gesellschaft vol 8 no 12 pp 968ndash976 2010

[11] V Stepanova P Slavıcek M Stupavska J Jurmanova and MCernak ldquoSurface chemical changes of atmospheric pressureplasma treated rabbit fibres important for felting processrdquoApplied Surface Science vol 355 pp 1037ndash1043 2015

[12] W A P Claassen W G J N Valkenburg M F C Willemsenand W M Wijgert ldquoInfluence of deposition temperaturegas pressure gas phase composition and RF frequency oncomposition and mechanical stress of plasma silicon nitridelayersrdquo Journal of the Electrochemical Society vol 132 no 4 pp893ndash898 1985

[13] G G Tibbetts ldquoRole of nitrogen atoms in lsquoionminusnitridingrsquordquoJournal of Applied Physics vol 45 no 11 pp 5072ndash5073 1974

[14] A Fridman Plasma Chemistry Cambridge University Press2008

[15] I Stefanovic N K Bibinov A A Deryugin I P VinogradovA P Napartovich and K Wiesemann ldquoKinetics of ozone and


nitric oxides in dielectric barrier discharges in O2NO119909 andN2O2NO119909 mixturesrdquo Plasma Sources Science and Technologyvol 10 no 3 p 406 2001

[16] V Guerra and J Loureiro ldquoNon-equilibrium coupled kineticsin stationary N2-O2 dischargesrdquo Journal of Physics D AppliedPhysics vol 28 no 9 pp 1903ndash1918 1995

[17] S Tada S TakashimaM ItoMHori TGoto andY SakamotoldquoMeasurement and control of absolute nitrogen atom densityin an electron-beam-excited plasma using vacuum ultravioletabsorption spectroscopyrdquo Journal of Applied Physics vol 88 no4 pp 1756ndash1759 2000

[18] P Vasina V Kudrle A Talsky P Botos M Mrazkova andM Mesko ldquoSimultaneous measurement of N and O densitiesin plasma afterglow by means of NO titrationrdquo Plasma SourcesScience and Technology vol 13 no 4 pp 668ndash674 2004

[19] S Agarwal B Hoex M C M Van de Sanden D Maroudasand E S Aydil ldquoAbsolute densities of N and excited N2 in a N2plasmardquo Applied Physics Letters vol 83 no 24 pp 4918ndash49202003

[20] J Amorim G Baravian and J Jolly ldquoLaser-induced resonancefluorescence as a diagnostic technique in non-thermal equilib-rium plasmasrdquo Journal of Physics D Applied Physics vol 33 no9 pp R51ndashR65 2000

[21] F Gaboriau U Cvelbar M Mozetic A Erradi and B RouffetldquoComparison of TALIF and catalytic probes for the determina-tion of nitrogen atom density in a nitrogen plasma afterglowrdquoJournal of Physics D Applied Physics vol 42 no 5 Article ID055204 2009

[22] O E Krivonosova S A Losev V P Nalivaiko Y K Mukoseevand O P Shatalov Plasma Chemistry vol 14 of edited by B MSmirnov Energoatomizdat Moscow Russia 1987

[23] M P Iannuzi J B Jeffries and F Kaufman ldquoProduct Channelsof theN2 (A

3Σ+119906) +O2 interactionrdquoChemical Physics Letters vol

87 no 6 pp 570ndash574 1982[24] R J Donovan andDHusain ldquoRecent advances in the chemistry

of electronically excited atomsrdquo Chemical Reviews vol 70 no4 pp 489ndash516 1970

[25] D I Slovetskii Mechanisms of Chemical Reactions in Nonequi-librium Plasma Mir Moscow Russia 1980 (Russian)

[26] M H Bortner and T Bauer Defense Nuclear Agency ReactionRate Handbook DNA 1948H US GPO Washington DC USA1971

[27] V P Silakov Mechanism of Supporting the Long-Lived Plasmain Molecular Nitrogen at High Pressure 010-90M MoscowEngineering Physical Institute 1990

[28] I A Kossyi A Y Kostinsky A A Matveyev and V P SilakovldquoKinetic scheme of the non-equilibrium discharge in nitrogen-oxygen mixturesrdquo Plasma Sources Science and Technology vol1 no 3 1992

[29] L G Piper ldquoThe excitation of O(1S) in the electronic energytransfer between N2(A3S+ u) and Ordquo The Journal of ChemicalPhysics vol 77 pp 2373ndash2377 1982

[30] E K Zavoisky ldquoParamagnetic relaxation of liquid solutions forperpendicular fieldsrdquo Journal of Physics-USSR vol 9 pp 211ndash216 1945

[31] E K Zavoisky ldquoSpin-magnetic resonance in paramagneticsrdquoJournal of Physics-USSR vol 9 pp 211ndash245 1945

[32] E K Zavoisky ldquoSpinmagnetic resonance in the decimeter-waveregionrdquo Journal of Physics-USSR vol 10 pp 197ndash198 1946

[33] A Carrington and A D McLachlan Introduction to MagneticResonance with Applications to Chemistry and Chemical PhysicsHarper and Row New York NY USA 1967

[34] C P Poole Electron Spin Resonance John Wiley amp Sons NewYork NY USA 2nd edition 1983

[35] J A Well and J R Bolton Electron Paramagnetic ResonanceJohn Wiley amp Sons New York NY USA 2007

[36] A Abragam and J H Van Vleck ldquoTheory of the microwaveZeeman effect in atomic oxygenrdquo Physical Review vol 92 no6 pp 1448ndash1455 1953

[37] M Tinkham and M W P Strandberg ldquoTheory of the finestructure of the molecular oxygen ground staterdquo PhysicalReview vol 97 no 4 pp 937ndash950 1955

[38] S Krongelb and M W P Strandberg ldquoUse of paramagnetic-resonance techniques in the study of atomic oxygen recombi-nationsrdquoThe Journal of Chemical Physics vol 31 no 5 pp 1196ndash1210 1959

[39] T K Ishii Handbook of Microwave Technology ApplicationsAcademic Press New York NY USA 1995

[40] P Zeeman ldquoOver den invloed fleener magnetisatie op denaard van het door een stof uitgezonden lichtrdquo Verslagen DerKoninklijke Akademie vanWetenschappen te Amsterdam vol 5p 181 1896

[41] A A Westenberg and N Dehaas ldquoObservations on ESR linew-idths and concentration measurements of gasminusphase radicalsrdquoThe Journal of Chemical Physics vol 51 no 12 pp 5215ndash52251969

[42] T J Cook and T A Miller ldquoGas-phase EPR linewidths andintermolecular potentials I Theoryrdquo The Journal of ChemicalPhysics vol 59 no 3 pp 1342ndash1351 1973

[43] A A Westenberg ldquoUse of ESR for the quantitative determina-tion of gas phase atom and radical concentrationsrdquo Progress inReaction Kinetics vol 7 pp 23ndash82 1973

[44] A AWestenberg andN DeHaas ldquoQuantitative measurementsof gas phase O andNAtom concentrations by ESRrdquoThe Journalof Chemical Physics vol 40 no 10 pp 3087ndash3098 1964

[45] T J Cook B R Zegarski W H Breckenridge and T A MillerldquoGas phase EPR of vibrationally excited O2rdquo The Journal ofChemical Physics vol 58 no 4 pp 1548ndash1552 1973

[46] W H Breckenridge and T A Miller ldquoDetection of metastable3P argon atoms by gas phase EPRrdquoChemical Physics Letters vol12 no 3 pp 437ndash442 1972

[47] V Dolezal M Mrazkova P Dvorak A Talsky and V KudrleldquoHydrogen line broadening in afterglow observed by means ofEPRrdquo Acta Physica Slovaca vol 55 no 5 pp 435ndash439 2005

[48] V Kudrle A Talsky A Kudlac V Krapek and J Janca ldquoInflu-ence of admixtures on production rate of atomic nitrogenrdquoCzechoslovak Journal of Physics vol 50 no 3 pp 305ndash308 2000

[49] P Dvorak V Dolezal M Mrazkova V Kudrle A Talskyand J Janca ldquoEPR measurements in hydrogen post-dischargerdquoCzechoslovak Journal of Physics vol 54 no 3 pp C539ndashC5432004

[50] V Kudrle P Vasina A Talsky and J Janca ldquoMeasurement ofconcentration of N atoms in afterglowrdquo Czechoslovak Journal ofPhysics vol 52 pp 589ndash595 2002

[51] V Kudrle P Vasina A Talsky M Mrazkova O Stec and JJanca ldquoPlasma diagnostics using electron paramagnetic reso-nancerdquo Journal of Physics D Applied Physics vol 43 no 12Article ID 124020 2010

[52] C D Pintassilgo V Guerra O Guaitella and A RousseauldquoStudy of gas heating mechanisms in millisecond pulsed dis-charges and afterglows in air at low pressuresrdquo Plasma SourcesScience and Technology vol 23 no 2 Article ID 025006 2014


[53] C D Pintassilgo O Guaitella and A Rousseau ldquoHeavy specieskinetics in low-pressure dc pulsed discharges in airrdquo PlasmaSources Science and Technology vol 18 no 2 Article ID 0250052009

[54] A Ricard S G Oh and V Guerra ldquoLine-ratio determinationof atomic oxygen and metastable absolute densities in an RFnitrogen late afterglowrdquo Plasma Sources Science and Technologyvol 22 no 3 Article ID 035009 2013

[55] M Mrazkova P Vasina V Kudrle A Talsky C D Pintassilgoand V Guerra ldquoOn the oxygen addition into nitrogen post-dischargesrdquo Journal of Physics D Applied Physics vol 42 no 7Article ID 075202 2009

[56] A A Westenberg ldquoIntensity relations for determininggasminusphase OH Cl Br I and freeminuselectron concentrations byquantitative ESRrdquo The Journal of Chemical Physics vol 43 no5 pp 1544ndash1549 1965

[57] W Siemens ldquoUeber die elektrostatische induction und dieverzogerung des stroms in flaschendrahtenrdquo Annalen derPhysik vol 178 no 9 pp 66ndash122 1857

[58] S P Sutera and R Skalak ldquoThe history of Poiseuillersquos lawrdquoAnnual Review of Fluid Mechanics vol 25 no 1 pp 1ndash20 1993

[59] A Roth Vacuum Technology Elsevier 2012[60] J M Gershenzon A B Nalbandyan and V B Rozenshtein

Magnetic Resonance in Gases Publishing House of Academy ofSciences of Armenian Soviet Socialist Republic 1987

[61] A Talsky and M Kunovsky ldquoDie dissoziation des stickstoffenin der mikrowellen entladungrdquo Scripta Facultatis ScientiarumNaturalium Universitatis Purkynianae Brunensis Physica vol18 no 6 article 229 1988

[62] O Reynolds ldquoAn experimental investigation of the circum-stances which determine whether the motion of water shallbe direct or sinuous and of the law of resistance in parallelchannelsrdquo Philosophical Transactions of the Royal Society ofLondon vol 174 no 0 pp 935ndash982 1883

[63] J Kestin andW Leidenfrost ldquoAn absolute determination of theviscosity of eleven gases over a range of pressuresrdquo Physica vol25 no 7ndash12 pp 1033ndash1062 1959

[64] M Ohmi and M Iguchi ldquoCritical Reynolds number in anoscillating pipe flowrdquo Bulletin of JSME vol 25 no 200 pp 165ndash172 1982

[65] A-M Pointu A Ricard E Odic and M Ganciu ldquoNitrogenatmospheric pressure post discharges for surface biologicaldecontamination inside small diameter tubesrdquo Plasma Processesand Polymers vol 5 no 6 pp 559ndash568 2008

[66] A M Pointu E Mintusov and P Fromy ldquoStudy of anatmospheric pressure flowing afterglow in N2NO mixture andits application to the measurement of N2(A) concentrationrdquoPlasma Sources Science and Technology vol 19 no 1 Article ID015018 2009

[67] A M Pointu and G D Stancu ldquoN2(A) as the source of excitedspecies of N2 N andO in a flowing afterglow of N2NOmixtureat atmospheric pressurerdquo Plasma Sources Science and Technol-ogy vol 20 no 2 Article ID 025005 2011

[68] V Mazankova D Trunec and F Krcma ldquoStudy of nitrogenflowing afterglow with mercury vapor injectionrdquo Journal ofChemical Physics vol 141 no 15 Article ID 154307 2014

[69] B F Gordiets C M Ferreira V L Guerra et al ldquoKineticmodel of a low-pressure N2-O2 flowing glow dischargerdquo IEEETransactions on Plasma Science vol 23 no 4 pp 750ndash768 1995

[70] G Cartry L Magne and G Cernogora ldquoExperimental studyand modelling of a low-pressure N2-O2 time afterglowrdquo Journalof Physics D Applied Physics vol 32 no 15 pp 1894ndash1907 1999

[71] C D Pintassilgo J Loureiro and V Guerra ldquoModelling of aN2ndashO2 flowing afterglow for plasma sterilizationrdquo Journal ofPhysics D Applied Physics vol 38 no 3 2005

[72] W Van Gaens and A Bogaerts ldquoKinetic modelling for anatmospheric pressure argon plasma jet in humid airrdquo Journalof Physics D Applied Physics vol 46 no 27 Article ID 2752012013

[73] D Blois P Supiot M Barj et al ldquoThe microwave sourcersquosinfluence on the vibrational energy carried by in a nitrogenafterglowrdquo Journal of Physics D Applied Physics vol 31 no 19article 2521 1998

[74] J Loureiro P A Sa and V Guerra ldquoRole of long-livedN2(X1Σg+v) molecules and N2(A3Σu+) and N2(arsquo1Σu-) statesin the light emissions of an N2 afterglowrdquo Journal of Physics DApplied Physics vol 34 no 12 pp 1769ndash1778 2001

[75] N Sadeghi C Foissac and P Supiot ldquoKinetics of N2(A3sum+

119906)

molecules and ionization mechanisms in the afterglow of aflowing N2 microwave dischargerdquo Journal of Physics D AppliedPhysics vol 34 no 12 pp 1779ndash1788 2001

[76] A-M Pointu A Ricard B Dodet E Odic J Larbre and MGanciu ldquoProduction of active species in N2-O2 flowing post-discharges at atmospheric pressure for sterilizationrdquo Journal ofPhysics D Applied Physics vol 38 no 12 pp 1905ndash1909 2005

[77] M Capitelli C M Ferreira B F Gordiets and A I OsipovPlasma Kinetics in Atmospheric Gases vol 31 Springer Scienceamp Business Media 2013

[78] V Zvonicek V Guerra J Loureiro A Talsky and M TouzeauldquoSurface and volume kinetics of O(3P) atoms in a low-pressureO2-N2microwave dischargerdquo inProceedings of the 23rd Interna-tional Conference on Phenomena in Ionized Gases (ICPIG rsquo97)vol 4 p 160 Toulouse France July 1997

[79] G Cartry L Magne G Cernogora M Touzeau and M VialleldquoEffect of wall treatment on oxygen atoms recombinationrdquo inProceedings of the 23rd International Conference on Phenomenain Ionized Gases (ICPIG rsquo97) vol 2 pp 70ndash71 Toulouse FranceJuly 1997

[80] G Cartry LMagne andG Cernogora ldquoAtomic oxygen recom-bination on fused silica modelling and comparison to low-temperature experiments (300 K)rdquo Journal of Physics D AppliedPhysics vol 33 no 11 2000

[81] K M Evenson and D S Burch ldquoAtomicminusnitrogen recombina-tionrdquo The Journal of Chemical Physics vol 45 no 7 pp 2450ndash2460 1966

[82] G Black H Wise S Schechter and R L Sharpless ldquoMeasure-ments of vibrationally excited molecules by Raman scatteringII Surface deactivation of vibrationally excited N2rdquoThe Journalof Chemical Physics vol 60 no 9 pp 3526ndash3536 1974

Submit your manuscripts athttpswwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Inorganic ChemistryInternational Journal of

Hindawi Publishing Corporation httpwwwhindawicom Volume 2014

International Journal ofPhotoenergy


Carbohydrate Chemistry

International Journal of


Journal of

Chemistry


Advances in

Physical Chemistry

Hindawi Publishing Corporationhttpwwwhindawicom

Analytical Methods in Chemistry

Journal of

Volume 2014

Bioinorganic Chemistry and ApplicationsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

SpectroscopyInternational Journal of


The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Medicinal ChemistryInternational Journal of


Chromatography Research International


Applied ChemistryJournal of



Theoretical ChemistryJournal of


Journal of

Spectroscopy

Analytical ChemistryInternational Journal of


Journal of


Quantum Chemistry


Organic Chemistry International

ElectrochemistryInternational Journal of



CatalystsJournal of


N2 O2

Barrierdischarge

Rotameter

Valves

Gas cylinders

Digital

NMRmagnetometer

Microwavegenerator

Control

PC

Electromagnet

Digitalmanometer

Rotaryvacuumpump

coils

owmeter

Figure 1 Schematic drawing of the experimental set-up

Zeeman split [40] levelsThe transitions with very low energyseparation between the upper and the bottom levels (as isthe case inmicrowave spectroscopy) are very easily disturbedby the collisions with other particlesThe collisionalpressurebroadening of absorption line can be quite pronounced [4142] especially for gas phase atomic and molecular lines withvery low natural linewidths This makes most of the species(for extensive list see [43]) including for example atomicoxygen difficult to detect in atmospheric pressure plasmasHowever the particles in s-state such as nitrogen or hydrogenatoms in ground state do not exhibit pressure broadening andcan be detected by ESREPR even at atmospheric pressure

There are a number of works both theoretical andexperimental dealing with low pressure plasma diagnos-tics by ESREPR method [44ndash50] However due to above-mentioned difficulties of pressure broadening the diagnos-tics of atmospheric pressure plasmas by EPRESR is ratherunresearched topic as our previous paper [51] is to authorsrsquobest knowledge still the only one published

Although the nitrogen andoxygenplasmas are studied forvery long time there is still intensive research [52ndash54] goingon The discharges in N2-O2 mixtures have very complicatedplasma kinetics and some effects especially in mixtures withlow O2N2 ratio are not fully explained yet In our previouswork [55] it was reported that adding of small amountof oxygen into a low pressure nitrogen plasma afterglowcauses an increase of nitrogen atom density In this paper weextend the study of the influence of oxygen admixture on thenitrogen afterglow to atmospheric pressure

2 Experimental Apparatus and Methods

21 Experimental Set-Up The experimental apparatus isdepicted in Figure 1 Nitrogen plasma at atmospheric pres-sure was produced using industrial ozoniser LifeTech 50It is based on coaxial dielectric barrier discharge excited

by 25ndash35W power at 15 kHz frequency The stainless steelinner electrode has approx 2 cm diameter and is separatedby 07mm plasma gap and 25mm thick corundum (Al2O3)ceramics from the outer electrode which is realised by analuminium foil tightly wrapped around the ceramic tubeThe discharge tube is approx 15 cm long Although the metalelectrode in direct contact with plasma can significantlyreduce N atom density in the effluent due to increasedsurface recombinationreassociation this design [56] is wellestablished in industrial ozonisers

The discharge was operated in flowing (12 standard litresper minute) nitrogen coming from pressurised cylinder(Messer-Griesheim purity 99995) via pressure reductionvalve The volumetric flow rate of nitrogen was measuredby flowmeter with floating element (rotameter UPLS-R3)Downstream from the plasma source the oxygen is option-ally introducedThe oxygen comes from pressurised cylinder(Messer-Griesheim purity 99995) via Hastings mass flow-controller maintaining the oxygen flow at 75 sccm (standardcubic centimetre per minute) The tubing is made frompolyethylene and stainless steel

The oxygen is introduced into the afterglow tube justdownstream from the plasma generator using an inlet three-way valve This configuration permits presetting the oxygenflow on the flow-controller and rapidly switches on and offthe oxygen admixture The distance between the output portof the plasma generator and the inlet of oxygen is 6 cmThe afterglow tube is made of fused silica with 10mm outerdiameter and 8mm inner diameter This tube passes throughESREPR spectrometer resonator and ends in another outletthree-way valve In normal operation the plasma afterglowvents into open atmosphere and the afterglow tube length(approx 25 metres) together with high gas flow preventsany back-diffusion of air into the afterglow or plasma Insecond position of this outlet valve the whole apparatus canbe pumped down by the rotary vane oil vacuumpumpThis is







00

50000

100000

150000

200000

Inte

gral









00

2

4

6

8

10

2 4 6 8 10 12 14 16O2 influx (sccm)

Spec

tral

line

wid

th (1

0minus6

T)








05

10

15

20

25

30


Crossing

70 806040 50302010


Con

cent

ratio

n of

N(4

S)(1013

cmminus3)



119877 = 119906119889]













00000

05

10

15

20

25

30

35

40

Time (s)

015010005




Con

cent

ratio

n of

N(4

S)(1013

cmminus3)















000minus08

minus06

minus04

minus02

00

02

Time (s)025020015010005

Relat

ive d

ier

ence

120575

[N(4S)]t=0[N2(A)]t=0 = 1

[N(4S)]t=0[N2(A)]t=0 = 2

[N(4S)]t=0[N2(A)]t=0 = 3

[N(4S)]t=0[N2(A)]t=0 = 5

[N(4S)]t=0[N2(A)]t=0 = 10


10

15

20

25

30

35


000Time (s)

025020015010005

Con

cent

ratio

n of

N(4

S)(1013

cmminus3)
































0 2 4 6 8 10

Con

cent

ratio

n of

X (c

mminus3)

X = N(4S)

X = NOX = O

X = N2(A)

X = N2(X v)X = NO2

Time (10minus4 s)

1014

1013

1012

1011

1010

109

108

(a)

Con

cent

ratio

n of

X (c

mminus3)

X = N(4S)

X = NOX = O

X = N2(A)

X = N2(X v)X = NO2

000Time (s)

025020015010005

1015

1014

1013

1012

1011

1010

(b)







00

05

10

15

20

25

30

35

40

Con

cent

ratio

n of

N(4

S)(1013

cmminus3)



000Time (s)

025020015010005







4 Conclusions



Competing Interests


Acknowledgments


References

















































































119906)


















Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of







00

50000

100000

150000

200000

Inte

gral









00

2

4

6

8

10

2 4 6 8 10 12 14 16O2 influx (sccm)

Spec

tral

line

wid

th (1

0minus6

T)








05

10

15

20

25

30


Crossing

70 806040 50302010


Con

cent

ratio

n of

N(4

S)(1013

cmminus3)



119877 = 119906119889]













00000

05

10

15

20

25

30

35

40

Time (s)

015010005




Con

cent

ratio

n of

N(4

S)(1013

cmminus3)















000minus08

minus06

minus04

minus02

00

02

Time (s)025020015010005

Relat

ive d

ier

ence

120575

[N(4S)]t=0[N2(A)]t=0 = 1

[N(4S)]t=0[N2(A)]t=0 = 2

[N(4S)]t=0[N2(A)]t=0 = 3

[N(4S)]t=0[N2(A)]t=0 = 5

[N(4S)]t=0[N2(A)]t=0 = 10


10

15

20

25

30

35


000Time (s)

025020015010005

Con

cent

ratio

n of

N(4

S)(1013

cmminus3)
































0 2 4 6 8 10

Con

cent

ratio

n of

X (c

mminus3)

X = N(4S)

X = NOX = O

X = N2(A)

X = N2(X v)X = NO2

Time (10minus4 s)

1014

1013

1012

1011

1010

109

108

(a)

Con

cent

ratio

n of

X (c

mminus3)

X = N(4S)

X = NOX = O

X = N2(A)

X = N2(X v)X = NO2

000Time (s)

025020015010005

1015

1014

1013

1012

1011

1010

(b)







00

05

10

15

20

25

30

35

40

Con

cent

ratio

n of

N(4

S)(1013

cmminus3)



000Time (s)

025020015010005







4 Conclusions



Competing Interests


Acknowledgments


References

















































































119906)


















Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


00

2

4

6

8

10

2 4 6 8 10 12 14 16O2 influx (sccm)

Spec

tral

line

wid

th (1

0minus6

T)








05

10

15

20

25

30


Crossing

70 806040 50302010


Con

cent

ratio

n of

N(4

S)(1013

cmminus3)



119877 = 119906119889]













00000

05

10

15

20

25

30

35

40

Time (s)

015010005




Con

cent

ratio

n of

N(4

S)(1013

cmminus3)















000minus08

minus06

minus04

minus02

00

02

Time (s)025020015010005

Relat

ive d

ier

ence

120575

[N(4S)]t=0[N2(A)]t=0 = 1

[N(4S)]t=0[N2(A)]t=0 = 2

[N(4S)]t=0[N2(A)]t=0 = 3

[N(4S)]t=0[N2(A)]t=0 = 5

[N(4S)]t=0[N2(A)]t=0 = 10


10

15

20

25

30

35


000Time (s)

025020015010005

Con

cent

ratio

n of

N(4

S)(1013

cmminus3)
































0 2 4 6 8 10

Con

cent

ratio

n of

X (c

mminus3)

X = N(4S)

X = NOX = O

X = N2(A)

X = N2(X v)X = NO2

Time (10minus4 s)

1014

1013

1012

1011

1010

109

108

(a)

Con

cent

ratio

n of

X (c

mminus3)

X = N(4S)

X = NOX = O

X = N2(A)

X = N2(X v)X = NO2

000Time (s)

025020015010005

1015

1014

1013

1012

1011

1010

(b)







00

05

10

15

20

25

30

35

40

Con

cent

ratio

n of

N(4

S)(1013

cmminus3)



000Time (s)

025020015010005







4 Conclusions



Competing Interests


Acknowledgments


References

















































































119906)


















Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of










00000

05

10

15

20

25

30

35

40

Time (s)

015010005




Con

cent

ratio

n of

N(4

S)(1013

cmminus3)















000minus08

minus06

minus04

minus02

00

02

Time (s)025020015010005

Relat

ive d

ier

ence

120575

[N(4S)]t=0[N2(A)]t=0 = 1

[N(4S)]t=0[N2(A)]t=0 = 2

[N(4S)]t=0[N2(A)]t=0 = 3

[N(4S)]t=0[N2(A)]t=0 = 5

[N(4S)]t=0[N2(A)]t=0 = 10


10

15

20

25

30

35


000Time (s)

025020015010005

Con

cent

ratio

n of

N(4

S)(1013

cmminus3)
































0 2 4 6 8 10

Con

cent

ratio

n of

X (c

mminus3)

X = N(4S)

X = NOX = O

X = N2(A)

X = N2(X v)X = NO2

Time (10minus4 s)

1014

1013

1012

1011

1010

109

108

(a)

Con

cent

ratio

n of

X (c

mminus3)

X = N(4S)

X = NOX = O

X = N2(A)

X = N2(X v)X = NO2

000Time (s)

025020015010005

1015

1014

1013

1012

1011

1010

(b)







00

05

10

15

20

25

30

35

40

Con

cent

ratio

n of

N(4

S)(1013

cmminus3)



000Time (s)

025020015010005







4 Conclusions



Competing Interests


Acknowledgments


References

















































































119906)


















Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


000minus08

minus06

minus04

minus02

00

02

Time (s)025020015010005

Relat

ive d

ier

ence

120575

[N(4S)]t=0[N2(A)]t=0 = 1

[N(4S)]t=0[N2(A)]t=0 = 2

[N(4S)]t=0[N2(A)]t=0 = 3

[N(4S)]t=0[N2(A)]t=0 = 5

[N(4S)]t=0[N2(A)]t=0 = 10


10

15

20

25

30

35


000Time (s)

025020015010005

Con

cent

ratio

n of

N(4

S)(1013

cmminus3)
































0 2 4 6 8 10

Con

cent

ratio

n of

X (c

mminus3)

X = N(4S)

X = NOX = O

X = N2(A)

X = N2(X v)X = NO2

Time (10minus4 s)

1014

1013

1012

1011

1010

109

108

(a)

Con

cent

ratio

n of

X (c

mminus3)

X = N(4S)

X = NOX = O

X = N2(A)

X = N2(X v)X = NO2

000Time (s)

025020015010005

1015

1014

1013

1012

1011

1010

(b)







00

05

10

15

20

25

30

35

40

Con

cent

ratio

n of

N(4

S)(1013

cmminus3)



000Time (s)

025020015010005







4 Conclusions



Competing Interests


Acknowledgments


References

















































































119906)


















Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of



















0 2 4 6 8 10

Con

cent

ratio

n of

X (c

mminus3)

X = N(4S)

X = NOX = O

X = N2(A)

X = N2(X v)X = NO2

Time (10minus4 s)

1014

1013

1012

1011

1010

109

108

(a)

Con

cent

ratio

n of

X (c

mminus3)

X = N(4S)

X = NOX = O

X = N2(A)

X = N2(X v)X = NO2

000Time (s)

025020015010005

1015

1014

1013

1012

1011

1010

(b)







00

05

10

15

20

25

30

35

40

Con

cent

ratio

n of

N(4

S)(1013

cmminus3)



000Time (s)

025020015010005







4 Conclusions



Competing Interests


Acknowledgments


References

















































































119906)


















Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


00

05

10

15

20

25

30

35

40

Con

cent

ratio

n of

N(4

S)(1013

cmminus3)



000Time (s)

025020015010005







4 Conclusions



Competing Interests


Acknowledgments


References

















































































119906)


















Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


































































119906)


















Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of










Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of

kinetic study of atmospheric pressure nitrogen plasma afterglow … · researcharticle kinetic...

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