Chemical Physics 398 (2012) 142–147

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Chemical Physics

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LIF diagnostics of hydroxyl radical in atmospheric pressure He-H2O dielectricbarrier discharges

G. Dilecce a,⇑, P.F. Ambrico a, M. Simek b, S. De Benedictis a

a Istituto di Metodologie Inorganiche e dei Plasmi – CNR – UOS di Bari, via Orabona, 4 – 70125 Bari, Italyb Institute of Plasma Physics, Czech Academy of Sciences, Za Slovankou, 3 – 18200 Praha, Czech Republic

a r t i c l e i n f o

Article history:Available online 21 March 2011

Keywords:Spectroscopic techniquesPlasma diagnosticsLIFOHDielectric barrier discharge

0301-0104/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.chemphys.2011.03.012

⇑ Corresponding author.E-mail address: giorgio.dilecce@ba.imip.cnr.it (G. D

a b s t r a c t

In this paper we present laser induced fluorescence (LIF) measurements of OH relative density in a He-H2Oatmospheric pressure dielectric barrier discharge, with an estimation of the absolute density based on thequantitative analysis of the LIF signal and on the decay of OH density in the post-discharge. The possibleinterference of H2O2 photo-dissociation is discussed and finally excluded. Densities of the order of1013 cm�3 have been estimated in mixtures with water vapour content ranging from 2.3 to 23 Torr partialpressure. LIF diagnostic characteristics and sensitivity in the OH case at atmospheric pressure are discussedin comparison with absorption techniques.

� 2011 Elsevier B.V. All rights reserved.

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1. Introduction

The recent research in plasma technologies based on atmo-spheric pressure (ATP) electric discharges has opened new prob-lems in the spectroscopic diagnostics of high-pressure, lowtemperature discharge media [1]. The detection/measurement ofradical species important in the plasma chemistry under investiga-tion cannot be in many cases obtained by simple spectroscopicanalysis of the discharge emissions, and must instead be addressedby means of absorption or laser induced fluorescence (LIF) spec-troscopy. Many of the foreseen applications of ATP discharges, likewaste gas treatment [2], plasma assisted combustion [3], plasmamedicine [4] and, more generally, plasmas in contact with liquids[5] or discharges in liquid media [6], envisage a discharge mediumwith some degree of humidity. The hydroxyl radical is thenthought to play an important role in the plasma chemistry of allthe mentioned applications. LIF diagnostics of the hydroxyl radicalis an old issue in atmospheric chemistry and combustion research.Pushed by the large interest into these two important fields, mostof the molecular data needed for the deployment of a correct andquantitative LIF application have been investigated and are nowavailable. The excitation-detection scheme makes use of theOH(A-X) system transitions:

OHðX2P;v ¼ 0Þ þ hmL ! OHðA2Rþ;v 0Þ ! OHðX2P;vÞ þ hmF ð1Þ

in which v0 = 0, 1 and v00 = 0, 1 accordingly. Two fundamental issuesrelevant to LIF application to OH measurement are the electronic

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ilecce).

quenching and vibrational relaxation of OH(A2R+,v = 0,1), and thepossible interferences of the probing laser on the local OH produc-tion. Quenching rate coefficients for the main air constituents andfor some hydrocarbons are now well known [7], together with theirdependence on gas temperature [8] and on the rotational level[9–11], and their route following the formation of Van der Waalscomplexes with the quencher species [12]. The second issue con-cerns the possibility that some extra OH molecule be generated,and subsequently probed, by the laser itself, through two photo-chemical mechanisms:

O3 þ hmL ! Oð1DÞ þ O2; ð2aÞOð1DÞ þ H2O! 2OH; ð2bÞ

H2O2 þ hmL ! 2OH: ð3Þ

The interference of these processes has been thoroughly inves-tigated in the frame of LIF measurement of tropospheric OH [13],and might be important in electrical discharges with water vapour,where both ozone and hydrogen peroxide can be produced andfound in large amounts. In the LIF measurements of OH densityin a pulsed corona discharge in humid air of [14], it was necessaryto subtract the contribution of process (2) from the total LIF signal.

In this paper we present LIF measurements of OH relative den-sity in a He-H2O ATP dielectric barrier discharge, with an estima-tion of the absolute density based on the quantitative analysis ofthe LIF signal and on the decay of OH density in the post-discharge.The interest of such a relatively simple system is to provide abenchmark for LIF diagnostics on OH and data to compare to modelcalculations [15,16]. A great deal of work has been devoted to theinvestigation of a possible interference of process (3) (ozone is not

Fig. 1. LIF measurement outcomes in a double barrier discharge with 2.3 Torr H2Opartial pressure: (a) oscilloscope trace of the LIF pulse measured by the photomul-tiplier; (b) dispersed fluorescence spectrum measured by the ICCD, showing that12% of the fluorescence comes from level the (0,0) band. The dashed curve is thetrapezoidal spectral transmission function used for LIF detection by thephotomultiplier.

G. Dilecce et al. / Chemical Physics 398 (2012) 142–147 143

significant in He-H2O discharges) on the measurement followingtwo distinct approaches. First, measuring the OH LIF signal from(3) with a known concentration of H2O2 in a cell, and comparingit to the OH LIF signal from the discharge. Second, by taking advan-tage of the fact that OH produced by H2O2 photo-dissociation isrotationally hot [17]. If, even at ATP, the OH fragment preservessome memory of its nascent distribution, the observation of asomewhat supra-thermal rotational distribution can be taken asa marker of the photo-dissociation origin of the observed OH. Pro-cess (2b) too generates rotationally hot OH [18], making thereforeit possible to use this same concept for the ozone photolysis case.

2. Experimental apparatus

2.1. Discharge

The parallel plates dielectric barrier discharge is used here withtwo configurations: double barrier and single barrier. The doublebarrier version consists of two 12 � 20 mm2 copper electrodes,two 0.7 mm thick alumina plates (Coorsetek fine grain, 96% purity)that cover the electrodes, and four spacers that fix the dischargegap at 5 mm. In the single barrier version a 12 � 20 mm2 hole ismade in one of the alumina plates, so that one copper electrodesis directly into contact with the discharge volume, and the effectivedischarge gap is 5.7 mm. The discharge vessel is made of a quartztube equipped with vacuum fittings at each end. Gas inlet, pump-ing and laser IN and OUT quartz windows are connected to thesefittings. Optical observation is done through the quartz tube. PureHe (99.995% purity) is fed into the vessel through two separatedlines each controlled by an MKS flow meter/controller. One ofthe two lines goes through a bubbler filled with either water or a50% hydrogen peroxide solution. The partial pressure of the vapourin the gas is then given by

Pi ¼ Pvapi � U1

U1 þU2; ð4Þ

where U1 is the He flux through the bubbler, U2 is the He fluxthrough the other gas line and Pvap

i is the vapour pressure at295 K, i = w for water and h for H2O2. The vapour pressures are cal-culated by the modified Raoult’s law

Pvapw ¼ cwvwP0

w and Pvaph ¼ chvhP0

h ð5Þ

with P0i = vapour pressure of the pure component, vi the mole frac-

tion of the liquid phase, and the activity coefficients ci calculatedaccording to [19].

Gas pumping is achieved by a 30 m3/h rotary pump, that servesboth for evacuating the vessel and for maintainning the 760 Torrpressure under gas flow by pumping through an adjustable needlevalve. The HV supply is composed of a low voltage sinusoidal gen-erator, a power amplifier (Industrial Test Equipment Powertron1000 A), and a high voltage transformer. The applied voltage ispulsed, i.e. the low sinusoidal voltage is fully modulated by asquare wave. The voltage is composed of a packet of TON duration,with sinusoidal waveform at frequency f, followed by a TOFF dura-tion period of null voltage. The TTL pulse that modulates the sinu-soidal voltage is used as a trigger source for the synchronization tothe discharge waveform of all the time-resolved measurements.

2.2. Laser induced fluorescence

We have adopted the excitation-detection scheme (1) withv0 = 1, i.e. laser absorption at the (1,0) band, exploring the wave-length range (2814–2820) Å that includes the low N range of theR1 branch of the OH(X2P,v = 0) rotational manifold, and theQ1(1) (2819.13 Å) and Q21(1) (2819.16 Å) lines that are simulta-

neously involved when we tune the laser around 2819.14 Å foranalytical purposes. The experimental apparatus for LIF is the sameas that used in [20], and its scheme can be found in that paper. ANd-YAG pumped tunable pulsed dye laser is used. The necessarywavelength is obtained by means of R560 dye and a BBO secondharmonic generator. The temporal pulse width is about 10 ns, theenergy of the beam entering the discharge volume is about250 lJ, and the nominal bandwidth is 0.4 cm�1, i.e. about 0.032 Åin the wavelength range considered. The laser wavelength is mea-sured with 5 � 10�6 relative accuracy by a High Finesse WS5 wave-meter. The laser beam is sent into the discharge gap parallel to thedirection of the gas flow. The fluorescence is collected perpendicu-larly to the laser beam, by a 1:1 telescope made of two 2 inches,30 cm fl quartz lenses. Spectral selection is achieved by a500 mm focal length SPEX 500M monochromator, equipped witha 600 gr./mm gratings blazed at 500 nm. The maximum slit widthis 3 mm. The spectrally selected light is measured either by a Ham-amatsu R2949 photomultiplier and measured by a HP54545B dig-itizing oscilloscope, either by an Andor DH5H7-18F-01 gatedintensified CCD detector (ICCD). The laser firing and the oscillo-scope are triggered by the discharge TTL modulation pulse for timeresolved measurements inside and outside the discharge packet.

The outcomes of LIF signal measurements are shown in Fig. 1. Thetemporal evolution of the LIF pulse measured by the photomultiplieris reported in Fig. 1(a). After the laser pulse the LIF fluorescence de-cays as a single exponential. A fit of this decay gives the total (radi-ative + collisional) quenching rate of the laser excited level. Thedispersed fluorescence spectrum is shown in Fig. 1(b), together witha simulation calculated by LIFBASE [21]. The spectrum shows a wellshaped Boltzmann distribution at gas temperature, in spite of thefact that the laser excites only the rotational level N = 1. Rotationalenergy transfer (RET) rate coefficients are 6.1 � 10�11 cm3 s�1 forHe and 9.1 � 10�10 cm3 s�1 for H2O [22], so that the RET rate in thecondition of Fig. 1 is about 1.5 � 109 s�1. Some hundreds of RET col-lisions occur within the lifetime of the OH(A,v=0,1) levels with am-ple possibility for a complete rotational relaxation towards gastemperature. About 12% of the fluorescence comes from the (0,0)band, i.e. from level v = 0 of OH(A) population by vibrational relaxa-tion from level v = 1. Both rotational and vibrational distributions ofOH(A) fluorescence remain almost the same as that shown inFig. 1(b) for all the conditions of H2O partial pressure. The LIF pulsemeasurements by the photomultiplier are done with 0.1 mm input

144 G. Dilecce et al. / Chemical Physics 398 (2012) 142–147

and 3 mm output slit width, that give a trapezoidal spectral trans-mission function of the monochromator, with 96 Å nominal widthof the flat part, that is enough to include almost completely both(1,1) and (0,0) bands (see Fig. 1). The measured time behaviour ofFig. 1(a) is then mainly determined by the kinetics of OH(A,v = 1)level.

In order to check the validity of formulae (4) and (5) for the cal-culation of vapour partial pressures in our experimental layout, wehave done the following test. In the discharge vessel without run-ning the discharge. i.e. in a cell configuration, we have fed the cellwith variable U1 bubbling through the 50% hydrogen peroxidesolution, constant U1 + U2 = 1000 sccm and pressure P = 760 Torr.LIF pulses due to OH from photo-dissociated H2O2 are recorded,from which the total quenching rate of OH(A,v = 1), KQ is measuredas a function of U1. The results are reported in the Stern–Volmerplot of Fig. 2, in which U1 is converted into water vapour densityby means of formulas (4) and (5) and used as abscissa. As a firstapproximation we neglect the collision quenching of OH(A) byhydrogen peroxide. The relevant rate coefficient is not known,but it is known that H2O is the fastest quencher among all thoseexplored in the literature, including big molecules like isobutaneor n-butane [7]. It appears then reasonable to assume that thequenching rate constant by H2O2 cannot be much larger than thatby H2O. The vapour pressure of H2O2 is much less, about 1/18 at295 K, than that of water. The slope of this plot is then mostly givenby the rate coefficient of collision quenching by H2O. Its value is(7.31 ± 0.14) � 10�10 cm3 s�1, and compares reasonably well withthe literature value of (6.6 ± 0.4) � 10�10 cm3 s�1 given in [23],with a difference that might be partly attributed to the neglect ofthe quenching by hydrogen peroxide, or to an underestimationlower than 10% of the water vapour content calculated by formulae(4) and (5).

3. Results

We will present in the following results relevant to measure-ments in cell configuration and in three discharge conditions.Parameters common to the three discharge cases are: f = 10 kHz,TON = 10 ms TOFF = 90 ms, P = 760 Torr. The discharge cases are: (I)double barrier, Pw = 2.3 Torr, total flux 1000 sccm; (II) single bar-rier, Pw = 2.3 Torr, total flux 1000 sccm; (III) single barrier,Pw = 23 Torr, total flux 100 sccm. The corresponding discharge volt-age and current are reported in Fig. 3. The single barrier configura-tion has a much larger current than the double barrier one, andboth show a diffuse character when the water vapour content is

Fig. 2. Stern–Volmer plot of the OH(A,v = 1) total quenching rate, KQ, as a functionof the water vapour density calculated from U1 by formulas (4) and (5).

low. At larger water vapour partial pressure the discharge turnsto a filamentary mode, as shown in case (III). The discharge power,calculated as the energy/cycle times the frequency f, is 2.57 W,6.35 W and 7.5 W for cases (I), (II) and (III) respectively. The timeevolution of the LIF signal along the TON – TOFF cycle is shown inFig. 4. ILIF is the integral of the LIF pulse divided by the quantumyield -(radiative rate)/(total quenching rate)-, while on the rightaxis an absolute density scale is given according to the estimationmade in the next paragraph.

3.1. Interference of H2O2 photo-dissociation

We have excluded any interference in the measurement fromprocess (3) following two routes. First we have measured ILIF inthe cell with U1 = 100 sccm bubbling in the 50% hydrogen peroxidesolution, U2 = 0 and discharge-off. In such a condition, the partialpressure of H2O2 in the cell is Ph = 0.59 Torr, corresponding to adensity of 1.91 � 1016 cm�3. This is a very large density, abouttwo orders of magnitudes larger than that calculated in [16] for adischarge in a gas mixture similar to that of conditions (I) and(II). With this cell condition ILIF is about 200 times lower thanILIF at condition (I). This is quite a reassuring number, but it doesnot guarantee that the contribution of process (3) remains negligi-ble also in the deep post-packet, where ILIF has decreased by threeorders of magnitude, and in all the possible discharge conditions.This analysis, furthermore, relies on a model calculation of hydro-gen peroxide content. Since we want to set-up a measurement thatis independent of any previous estimation, we have looked at therotational excitation of OH(X2P) by measuring LIF excitation spec-tra. Some of these measurements are reported in Fig. 5. In spite ofthe high pressure, and of the fast RET relaxation, the spectrum ofthe OH generated by photo-dissociation still shows a memory ofa hot rotational distribution, with a basic temperature of 430 Kand a clear overpopulation of levels N = 6–9 above the Boltzmanndistribution. In the cell measurement, in fact, the laser, whoseduration is 10 ns, is probing OH molecules created few ns beforeby photo-dissociation, without then leaving sufficient time for a

Fig. 3. Discharge (applied) voltage and current in the three discharge cases: (I)double barrier, Pw = 2.3 Torr, total flux 1000 sccm; (II) single barrier, Pw = 2.3 Torr,total flux 1000 sccm; (III) single barrier, Pw = 23 Torr, total flux 100 sccm. In cases (I)and (II) the current is periodic, and its average over 30 measurements is shown. Incase (III) the current is not periodic, and a single shot measurement is reported.

Fig. 4. Time evolution of the LIF signal along the TON – TOFF cycle. ILIF is the integralof the LIF pulse divided by the quantum yield, while on the right axis an absolutedensity scale is given according to the estimations detailed in the text.

Fig. 5. LIF excitation spectra in: (a) cell conditions, Ph = 0.59 Torr, Pw = 2.3 Torr,P = 760 Torr; (b) discharge with Pw = 2.3 Torr, P = 760 Torr. Simulations calculatedby LIFBASE with Lorentzian laser line, 0.05 Å half-width, and line broadening givenby Doppler and pressure broadening, the latter calculated according to the data of[24].

Fig. 6. Calculation of OH density from LIF measurements: (a) overlap of the laserline with the broadened Q1(1) and Q21(1) lines; (b) fit of the LIF model to themeasured LIF pulses in conditions (I) and (III).

G. Dilecce et al. / Chemical Physics 398 (2012) 142–147 145

complete rotational relaxation. Note that we have done similar LIFspectra at lower total pressure finding, as expected, larger rota-tional excitation. Any hot rotational distribution memory is insteadtotally absent from the discharge spectrum, which exhibits a clearBoltzmann distribution at 320 K. We have measured LIF excitationspectra in all the conditions of this paper up to the deep post-pack-et, always finding Boltzmann rotational distributions at about320 K, showing that the OH probed by LIF is always that producedin the discharge, that has sufficient time to relax its rotational dis-tribution towards equilibrium with the gas temperature.

3.2. OH density estimation from LIF outcomes

In a simple two levels system description of the LIF process, thepopulation of states OH(X,v = 0,N = 1), PX, and OH(A,v = 1,N = 1), isdescribed by the following two coupled rate equations, where thestatistical weights are equal for both initial and final state, andtherefore the Einstein coefficients BX´A = BA´X = B:

dPA

dt¼ BELðtÞðPX � PAÞ � Q APA;

dPX

dt¼ �BELðtÞðPX � PAÞ: ð6Þ

ELðtÞ is the laser energy density, that is a function of time, given byW(t)/cS with c the velocity of light, S the laser beam section and W(t)the laser power. We have calculated W(t) using a measured lasertime profile normalized such that the time integrated W(t) is equalto the measured laser pulse energy. The Einstein coefficient B, takenfrom the LIFBASE database for the Q1(1) rotational line, must bemultiplied by the overlap integral of the laser line and theQ1(1) + Q21(1) lines, as shown in Fig. 6(a). The rotational line broad-ening is given by the Doppler plus pressure broadening. This latteris calculated according to the data of [24]. The laser line width is de-duced from the fit of the excitation spectra of Fig. 5. QA is the total,radiative plus collisional, quenching rate, that is measured from theLIF pulse. PX(t = 0) is the initial population of OH(X,v = 0,N = 1), thatis a fraction F of the total OH(X,v = 0) population. Such rotationalpopulation factor is F ¼ 0:2353 at Trot = 320 K. This simple modeldoes not take into account RET collisions that can modify the pop-ulations during the interaction of the laser with the two levels sys-tem, and neglects possible electronic collision/radiative routes backfrom A-state to X-state. All these phenomena contribute to the refillof the X-state depleted by laser absorption, and their neglect is asource of overestimation of [OH].

The signal measured by the oscilloscope is then given by:

SðtÞ ¼ PAðtÞAX

4pTgeGR: ð7Þ

A is the emission coefficient of the observed transition. Asshown in Fig. 1(b)) the population of N = 1 is quickly redistributedto form a Boltzmann rotational distribution, and even 12% of thetotal population is driven into level v = 0 by vibrational relaxation.We use this observation to calculate an effective emission coeffi-cient as A = 0.88A(1,1) + 0.12A(0,0) = 9.37 � 105 s�1 using the emis-sion coefficients of the two bands, and not that of singlerotational lines.

X is the solid angle subtended by the collection optics, i.e. by a2 inches lens at a focal length of 300 mm.

T = 0.51 is the transmission factor of the fluorescence opticalpath, given by the grating efficiency at 3100 Å and a nominal 4%loss of the six transmitting/reflective surfaces of the path.

g = 0.25 is the quantum efficiency of the photo-cathode,G = 2.5 � 107 the gain of the photomultiplier and e, the electroncharge, converts the number of photoelectrons per second into a

Fig. 7. Post-packet decay of LIF signal in conditions (I), (a) and (III), (b), withcalculations by model (10). The dashed blue curve is the solution of Eq. (10) withthe quadratic (self-destruction) term only, with an initial OH density such as tomatch the initial decay rate. The continuous red curve is a calculation by thecomplete loss terms that reasonably matches the experimental data. Modelparameters are: (a) [OH] = 6 � 1012 cm�3, Kl = 52 s�1, b = 10 s�1, C = 50 s�1; (b)[OH] = 1.9 � 1013 cm�3, Kl = 890 s�1, b = 145 s�1, C = 20 s�1. (For interpretation ofthe references to colour in this figure legend, the reader is referred to the webversion of this article.)

146 G. Dilecce et al. / Chemical Physics 398 (2012) 142–147

current. Typical values from the Hamamatsu R2949 data sheet areused for these numbers.

R = 50 X is the input resistance of the digitizing oscilloscope.Finally this calculations gives an absolute number of fluorescingmolecules that must be divided by the sampled volume to getthe required density. Such volume is calculated for a laser beamwith circular section of 2 mm diameter, multiplied by the0.1 mm width of the monochromator input slit that we have usedfor the measurements. With these numbers we have fitted themeasured LIF pulses with the two parameters [OH] and QA. Twosuch fits are shown in Fig. 6(b), for the (I) and (III) experimentalconditions, at the end of the discharge packets, with calculateddensities of 1 � 1013 cm�3 and 2.7 � 1013 cm�3 respectively.

In addition to the approximations of model (6), any factor of for-mula (7) is a source of errors, so that these numbers must be takenas order of magnitude estimations. In the next paragraph we willcheck the consistency of these estimations with the observeddecays of OH density.

4. Discussion

4.1. Decay analysis

The main production mechanisms of OH in a He-H2O dischargeare electron collision and attachment dissociation, ion-electronand ion-ion recombination [25]. In the post-packet interval, then,significant OH production mechanisms should not be present,and the decays shown in Fig. 4 are determined by destruction pro-cesses only. Among the many OH loss mechanisms, the followingtwo involve reaction of OH with itself:

OH þ OH! H2Oþ O; ð8ÞOH þ OH þM ! H2O2 þM ð9Þ

with rate constants k8 = 1.48 � 10�12 cm3 s�1 and k9 = 3.7 �10�31[He] cm3 s�1 listed in [26]. The latter value is a low pressurelimit value and cannot be used as it is for the calculation of k9 atatmospheric pressure. The source of this number is the measure-ment of [27], in which the total second order rate constant k8 + k9

was measured by saturated LIF, and then the contribution of reac-tion (9) deduced by analysis of the falloff curve (k8 + k9 as a functionof pressure) through a complex formula (formula (22) of [27]). Thevalue reported in [26] is a parameter of such formula. For our pur-poses we use the value k8 + k9 ’ 5.2 � 10�12 cm3 s�1, taken from thedirect measurements at 1 bar reported in Fig. 5 of [27]. The secondorder reactions (8) and (9) give a loss rate that depends on [OH]2,and can then provide useful information on [OH] itself. We haverationalized the experimental decays by a simplified model withthe only aim of extracting information on [OH] by an estimationof the contribution of the ‘‘self-destruction’’ processes (8) and (9).The decays can be described by the following equation:

d½OH�dt

¼ �2ðk8 þ k9Þ½OH�2 �X

i

ki½Xi� þ C

!½OH� ð10Þ

in which, in addition to the quadratic self-destruction term, there isa linear term splitted into two parts. The first one contains a lossrate by reaction with Xi species. Xi maybe O, H, O2, O3, H2O2 [26],and their density is in general time-dependent. A complete set-upof this time-dependent loss rate requires then a detailed kineticmodel describing the time evolution of these species. We have sim-plified it in the following form:X

i

ki½Xi� ¼ Kle�bt; ð11Þ

that is equivalent to attributing it to one dominant specie whosedensity decays exponentially in the post-packet. The second part of

the linear term contains a constant loss rate C that describes diffusion,provided OH is some way lost at the surfaces, and reactions with spe-cies whose density does not vary with time. A more complete model isadvisable, but it is out of our reach at present. Some conclusion can beanyway drawn. First we observe that, as shown in Fig. 7, a descriptionof the decay by the self-destruction term only is completely unrealis-tic. The density required to attribute to the quadratic term the initialdecay rate is 1.9� 1013 cm�3 for case (I) and 1.43 � 1014 cm�3 for case(III). The OH density must then be considerably lower than these num-bers, and in both cases a large contribution to the decay must comefrom the linear and constant terms of Eq. (10). Looking in more detailat case (I) decay, we observe that the estimated value of[OH] = 1� 1013 cm�3 at the end of the packet is still too large for agood match of model calculations with the experimental decay, andthat a better agreement is found with [OH]=6� 1012 cm�3. Note thatthis value must be taken as a an average value for a good fit: a variationof [OH] by ±2� 1012 cm�3 still gives a reasonable match with the mea-sured decay, so that also in this case we must speak of an estimate ofOH density. It is anyway in a fairly good agreement with LIF estima-tions, even if it is impossible to say if the LIF overestimation must beattributed to the neglect of RET collisions in model (6) or to errors inthe parameters of formula (7). [OH] values lower than 3 � 1012 cm�3

would hardly be compatible with LIF estimations. We then take the va-lue (6 ± 3)� 1012 cm�3 as the best estimate we are able to achieve for[OH] at the end of the discharge packet in condition (I). Scaling accord-ingly [OH] in condition (III), we find a value that is compatible with thedecay as shown in Fig. 7(b). This ‘‘calibration’’ is used in Fig. 4. We pointout that no certain physical meaning can be attributed to the othermodel parameters, that are used in this analysis only to separate rea-sonably the quadratic term from the other loss mechanism.

4.2. Detection limit

In condition (III), i.e. saturated water vapour partial pressure,with an OH(A) quenching rate of about 4.9 � 108 s�1, and an inputslit width of 0.1 mm, the minimum detectable LIF signal corresponds

G. Dilecce et al. / Chemical Physics 398 (2012) 142–147 147

to a density of about 1010 cm�3. With the largest possible input slitwidth of 3 mm, this value goes down to 3 � 108 cm�3. The worstcondition we can imagine is that of air with saturated water vapour.The OH(A) quenching raises by a factor three, according to the data of[28], raising the limit to 109 cm�3, but this limit must be furtherraised for two reasons. First, in nitrogen strong Second Positive Sys-tem emissions overlap in the fluorescence observation spectral re-gion; second, the inter-electrode spacing must be reduced to atleast 2 mm, limiting the solid angle for fluorescence collection. Withthese limitations we estimate that densities as low as(0.5 � 1) � 1010 cm�3 should be observable in humid air. This isquite a good value, compared to absorption techniques. In [29], apulsed classical absorption technique, achieved a smallest detect-able concentration of 5 � 1013 cm�3 over an absorption length of20 cm. Near IR (�1.5 lm) cw – Cavity Ring-Down Spectroscopy(cw-CRDS), applied to a He-H2O DBD in [30] achieved a detectionlimit of the order of 3 � 1011 cm�3 over a 10 cm absorption length.CRDS applied on the (0,0) band of the 3064 Å system was applied in[31] to an Ar atmospheric pressure inductively coupled dischargeover a 1.8 cm absorption length. In that case a large OH densitywas found, in the range (1.7 � 8.5) � 1014 cm�3 such that, to avoidsaturation effects the authors used the weak S21(1) line for absorp-tion. No mention was done of the detection limit, that, with lowerOH density could be enhanced by choosing a stronger absorptionline.

We conclude that LIF detection limit at ATP is good compared tothat of the most sensitive cavity absorption techniques, with theadvantages of a much greater spatial and time resolution. It is alsoclear that it is anyway advisable to sustain LIF measurements witha calibration technique.

4.3. Comparison with literature data

The results of Fig. 4 show an OH density that does not dependstrongly on the discharge current, and has a more marked depen-dence on the water content. The only literature data that can be di-rectly compared to ours are those of [30]. In that case, a parallelplate DBD discharge in He-H2O mixture with water partial pres-sure of about 5.1 Torr was investigated by cw-CRDS. At 10 mm in-ter-electrode gap, with 6 kV peak voltage and f = 70 kHz an OHdensity of 2.1 � 1013 cm�3. In our discharge we can interpolate at5.1 Torr of H2O density of about 1 � 1013 cm�3, at 4 kV, 5 mm in-ter-electrode gap and f = 10 kHz, finding then a good agreementwith the measurements of [30]. Our values appear to be insteadmuch lower than those calculated by a global model in [16], inwhich densities in excess of 1014 cm�3 were calculated for waterpartial pressure similar to our case (I) value.

The only measurements of post-discharge decay of OH are thoseof [29], made by the pulsed absorption technique, in Ar(Air)-H2Omixtures. Given the low sensitivity of the technique, the dynamicrange of the decay was low, and the measured densities quite high,with a maximum of 1015 cm�3, and a corresponding decay rate of1.9 � 103 s�1 at saturated water vapour with Ar mixture. There isa discrepancy between these two values, since the measured decayrate is too low to account for such a large density. The rate constantk8 + k9 with M = Ar is not known. If it is similar to that for He or N2

(see Fig. 5 of [27]), the initial decay with a density of 1015 cm�3

should be of the order of 104 s�1, i.e. about one order of magnitudelarger than the measured one.

5. Conclusions

We have discussed in this study the application of laser inducedfluorescence to the measurement of OH in a He-H2O dielectric bar-rier discharge. No appreciable interference from laser photo-disso-ciation processes of discharge by-products has been revealed, andthe measurements have shown a very good sensitivity. This has al-lowed a recovery of the OH density decay in the deep post-dis-charge, up to almost 100 ms, with a more than three orders ofmagnitude dynamic range. The combination of LIF signal quantita-tive analysis and rationalization of the first part of the density de-cay has allowed an estimate of the OH density that is in reasonableagreement with literature data. The OH density depends quiteweakly on the discharge current an somewhat more strongly onthe water vapour content. Its maximum value, at saturated vapourpressure has been found to be about 1013 cm�3.

Acknowledgements

M. Simek’s stay at IMIP-CNR laboratory has been supported byCNR-AVCR cooperative agreement 2010-2012.

References

[1] G. Dilecce, P.F. Ambrico, S.D. Benedictis, Pure Appl. Chem. 82 (2010) 1201–1207.

[2] J. Van Durme, J. Dewulf, C. Leys, H. Van Langehowe, Appl. Catal. B: Environ. 78(2008) 324–333.

[3] S.M. Starikovskaia, J. Phys. D: Appl. Phys. 39 (2006) R265–R299.[4] Special issue ‘‘Plasma Medicine’’, M. Laroussi, A. Fridman Guest Editors, Plasma

Process. Polym. 7 (3–4), 2010, pp. 187–352.[5] P. Bruggeman, J. Liu, J. Degroote, M.G. Kong, J. Vierendeels, C. Leys, J. Phys. D:

Appl. Phys 41 (2008) 215201.[6] P. Sunka, V. Babicky, M. Clupek, P. Lukes, M. Simek, J. Schmidt, M. Cernak,

Plasma Sources Sci. Technol. 8 (1999) 258–265.[7] I.J. Wysong, J.B. Jeffries, D.R. Crosley, J. Chem. Phys. 92 (1990) 5218–5222.[8] P.H. Paul, J. Phys. Chem. 99 (1995) 8472–8476.[9] R.A. Copeland, M.J. Dyer, D.R. Crosley, J. Chem. Phys. 82 (1985) 4022–4032.

[10] R.J. Cattolica, T.G. Mataga, Chem. Phys. Lett. 182 (1991) 623–631.[11] B.L. Hemming, D.R. Crosley, J.E. Harrington, V. Sick, J. Chem. Phys. 115 (2001)

3099–3104.[12] M.I. Lester, R.A. Loomis, R.L. Schwartz, J. Phys. Chem. A 101 (1997) 9195–9206.[13] G.P. Smith, D.R. Crosley, J. Geophys. Res. 95 (90) (1990) 427–442.[14] R. Ono, T. Oda, J. Appl. Phys. 93 (10) (2003) 5876–5882.[15] I.A. Soloshenko, V.V. Tsiolko, S.S. Pogulay, A.G. Kalyuzhnaya, V.Y. Bazhenov, A.I.

Shchedrin, Plasma Sources Sci. Technol. 18 (2009) 045019.[16] D.X. Liu, P. Bruggeman, F. Iza, M.Z. Rong, M.G. Kong, Plasma Sources Sci.

Technol. 19 (2010) 025018.[17] K.-H. Gericke, S. Klee, F.J. Comes, J. Chem. Phys. 85 (8) (1986) 4463–4479.[18] K.-H. Gericke, F.-T. Comes, Chem. Phys. 65 (1982) 113–121.[19] W.C. Schumb, C.N. Satterfield, R.L. Wentworth, Hydrogen peroxide, A.C.S.

Monograph No. 128, Reinhold Publishing Corporation, New York, 1955.[20] G. Dilecce, P.F. Ambrico, S. De Benedictis, Plasma Sources Sci. Technol. 16

(2007) 511–522.[21] J. Luque, D. Crosley, Int. rep. p99-009,Technical report, SRI, 1999.[22] R. Kienle, A. Jorg, K. Kohse-Hoinghaus, Appl. Phys. B 56 (1993) 249–258.[23] R.A. Copeland, M.L. Wise, D.R. Crosley, J. Phys. Chem. 92 (1988) 5710.[24] V.L. Kasyutich, Eur. Phys. J. D 33 (2005) 29–33.[25] P. Bruggeman, D.C. Schram, Plasma Sources Sci. Technol. 19 (2010) 045025.[26] R. Atkinson, D.L. Baulch, R.A. Cox, J.N. Crowley, R.F. Hampson, R.G. Hynes, M.E.

Jenkin, M.J. Rossi, J. Troe, Atmos. Chem. Phys. 4 (2004) 1461–1738.[27] R. Forster, M. Frost, D. Fulle, H.F. Hamann, H. Hippler, A. Schlepegrell, J. Troe, J.

Chem. Phys. 103 (1995) 2949–2957.[28] L.R. Williams, D.R. Crosley, J. Chem. Phys. 104 (1996) 6507–6514.[29] C. Hibert, I. Gaurand, O. Motret, J.M. Pouvesle, J. Appl. Phys. 85 (1999) 7070–

7075.[30] Z.W. Liu, X.F. Yang, A.M. Zhu, G.L. Zhao, Y. Xu, Eur. Phys. J. D 48 (2008) 365–

373.[31] C. Wang, F.J. Mazzotti, S.P. Koirala, C.B. Winstead, G.P. Miller, Appl. Spectrosc

58 (2004) 734–740.