Time and Space Resolved LIF-Spectroscopy of O- and N-Atoms in aDielectric Barrier Discharge

M. Spaan, Ch. Lukas, V. Schulz-von der Gathen, H.F. Dobele

Institutflt Laser- und plasmaphysik, UnivaSit GH Essen, 45117 EssenG-Y

Introduction

The Dielectric Barrier discharge (DBD) is finding application for a variety of plasmachemicalprocesses like ozone production, the generation of UV and VUV light and for surfacetreatment. It is also considered for cleaning of automobile exhaust gases. The motivation forthis application results from the strongly non-thermal character of the discharge which allowsan efficient energy coupling to the electron component whereas the residual gas in which thesubsequent chemical reactions take place is cold and in general at atmospheric pressure. Thedischarge in the reactor consists of a multitude of tiny filaments that are distributed in theinterelectrode gap and ignite independently during the phase of steep changes of the electricfield.

Our activities aim at measurements by methods of laser spectroscopy of concentrations ofreactive species of importance to the decomposition of the toxic molecules. In this context theplasma generated atomic radicals N and 0 are of central concern. Our objective is thereforethe space and time resolved relative concentration measurements of these atoms in theimmediate vicinity of single narrow discharge filaments in order to allow the comparison withsimulation calculations (performed by other groups) so that an improved understanding of theDBD will result.

Experimental

Our model reactor is operated with steeply rising voltages (15 kW90 ns). The consequence isthat all discharge filaments in a half cycle appear within a well defined time interval. Thisignition interval has a typical duration of 10 ns. An important side effect is that these steepvoltage ramps lead to a larger number of filaments in the discharge volume and allow, at thesame time, the application of laser based spectroscopy with high temporal resolution. Thehome-made semiconductor based HV power supply allows repetition frequencies up to50 kHz. The measurements reported here were taken at a repetition frequency of 20 kHz.

Atomic nitrogen is detected by TALIF whereby N atoms are excited by two photons ofwavelength h = 206 nm from their ground state into the 3p(3/2) level. The subsequentfluorescence occurs at X = 743 nm. Reduction of the laser energy to 20 - 100 pJ (duration8 ns, focal spot diameter 50 pm) exclude perturbations by photodissociation caused by thediagnosing laser beam. Two dimensional relative distributions of the N radical were measuredat a single discharge filament which develops on a stainless steel pin. The interelectrode gapis 1.5 mm in these measurements.

figure 1: Radial proy?kes of the N-density fur differentheights above the pin electrode in pure IV2

F&we 2: N-density profiles for different times aflerstreamer ignition in a mixture off 0% 02 in Nz

Fig. 1 shows five radial profilesobtained at different heights abovethe pin (position x=0, y=O) at 2 psafter ignition for the case of an anodicdielectric in pure nitrogen at apressure of 950 mbar. The radius ofthe discharge channel - inferred Tomthe distribution of atomic nitrogen -amounts to 200 pm. Besides a slightwidening of the channel a strongincrease of the N density withapproximation to the dielectric isobserved. The nature of this intensesource of radicals which existspossibly also in the intervals betweenthe ignitions is unclear so far.

Spatial resolution - 50 pm - isobtained by displacement of the entirereactor. The temporal resolution ofthe TALIF measurements is limitedby the duration of the ignitionintervals within the voltage rampsduring which the filaments ignite.With the present mode of excitationof the DBD the time resolution isapprox. 10 ns. The addition of oxygento a nitrogen discharge reduces theintensity of the TALIF signal toapprox. l/50 of the initial amplitude.Possible causes are “cooling” of theEEDF and attachment of 02. Despitethe strong quenching N radicals canstill be detected in oxygen containingdischarges by TALIF. Fig. 2 showsthe temporal decay of the nitrogendensity after the extinction of thedischarge channel. The measurementswere taken at a height of 0.1 mmabove the metal tip. It is obvious that

the N atoms disappear on a time scale of 20 ps and that diffusion which leads to a broadeningof the channel plays only a minor role.

The time dependence of the N concentration was investigated for various gas mixtures bychanging the delay between the laser shot and the plasma ignition. Fig. 3 shows the relativedensity distributions of the N radicals for pure nitrogen and for a mixture of 10 % oxygen innitrogen. The two curves are normalised to the relative maximum in both cases. The presenceof O2 also results in a reduction of the N lifetime by almost an order of magnitude.

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Hgure 3: Temporal behavior of the Ndensity in digervnt gas mixtures

Atomic oxygen is also of great importance for the ongoing chemical reactions after theplasma phase. It was detected by TALIF in a similar manner. The 0 atoms are excited by 2 x226 nm; the fluorescence light is emitted at h = 844 nm. Fig. 4 shows a comparison of radialprofiles obtained for comparable conditions. The measurements refer to a plane 0.4 mm abovemetal electrode at 1 ps t&x ignition of the filament. We attribute the clearly broader radialprofile in case of oxygen to the dissociation energy of molecular oxygen which is 4.7 eVlower than the corresponding value for Nz.

l.O- -NEld- 0

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0 2 -

0.0 . I . I . I . I ’ I ’ I . I . 1 . 1 .-1.0 XI.8 -0.6 -0.4 42 0.0 0 2 0.4 0.6 0.8 *

radiale Position [mm]0

BIgwe 4: Comparison of the radial ptvjles of N- and O-atoms

In addition to the measurements reported here LIF spectroscopy was also applied to measureNO y-band transitions which yield further information on the gas temperature and NOconcentration. Emission spectroscopy of N2- and 0 provides information on the longitudinalfilament structure. The gas composition inside the plasma region and at the reactor outlet ismonitored by FTIR spectroscopy. These measurements will be described in detail elsewhere.(Funding by the “Bundesministerium f3r Bildung, Wissenschafk, Forschung und Technologie”is acknowledged, Project Number 13 N 7195)