MAPPING OF ATOMIC NITROGEN IN SINGLE FILAMENTS OF A BARRIER DISCHARGE MEASURED BY TWO PHOTON FLUORESCENCE SPECTROSCOPY (TALIF)

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1 MAPPING OF ATOMIC NITROGEN IN SINGLE FILAMENTS OF A BARRIER DISCHARGE MEASURED BY TWO PHOTON FLUORESCENCE SPECTROSCOPY (TALIF) C. LUKAS, M. SPAAN, V. SCHULZ VON DER GATHEN, H. F. DÖBELE Institut für Laser und Plasmaphysik, Universität GH Essen, Germany Fluorescence measurements with two photon excitation of atomic nitrogen with radiation of wavelength λ = 26.7 nm were performed at a dielectric barrier discharge reactor. Density distributions in single filaments were measured with a spatial resolution of ~ 5 µm and a time resolution of ~ 5 ns. Various combinations of discharge parameters and gas mixtures were investigated. 1 Introduction Dielectric Barrier Discharges (DBD) are finding broad interest for plasma chemical applications, because they can be operated up to atmospheric pressure and are strongly non thermal. DBD reactors are in use for ozone generation, 1 for the treatment of surfaces and for the generation of UV and VUV radiation. 2 In recent years, dielectric barrier discharge reactors are also being studied and applied for plasma chemical decomposition of toxic media. The possibility to operate the discharge at atmospheric pressure is of crucial importance for this application. The displacement current through the dielectric leads to the formation of a multitude of single discharge channels with diameters of a few hundred microns and with lifetimes of the order of several ten nanoseconds. These discharge channels are statistically distributed within the discharge gap of typically several millimeters width. The non thermal character of this type of discharge leads to an electron temperature of the order of 1 ev, 3 whereas the neutral gas is approximately at room temperature. The energy is therefore mainly coupled into the electronic component which leads to an efficient production of radicals which initiate chemical reactions and can lead to the decomposition of admitted substances. Several investigations using DBD reactors are devoted to the problem of NO reduction 4 from Diesel engine exhaust. N and O radicals play a keyrole in this respect, and it is hoped that on the basis of measured relative concentrations a working understanding of both the oxidizing and reductive pathways can be obtained. There are several arguments to apply a steeply rising voltage pulse train to the plasma reactor. A fast rising pulse leads to a more homogeneous distribution of filaments within the discharge gap. Another argument which is also important from the diagnostics point of view is the fact that with fast rising pulses all filaments ignite within a time interval of only some

2 ten nanoseconds duration so that the discharge phase and the post reaction phase are well separated. This allows time resolved measurements at single filaments during this first ignition phase. The temporal behavior of the measured concentrations will be compared with the results of simulation calculations by other groups which are presently under work. We are performing in parallel with the LIF measurements in situ FTIR analysis of the main constituents present in the exhaust gas and hope to obtain a coherent picture of the reaction paths by combining the results of these two different diagnostic methods. 2 Experiment TALIF is a well established method to obtain space and time resolved infomations on atomic concentrations. We have shown in an earlier contribution 5 that for the two photon excitation of atomic nitrogen radiation with wavelength λ = 26.7 nm (ground state > 3p 4 S 3/2 ) is well suited. The laser system is shown schematically in figure 1. Figure 1: Setup of the TALIF experiment The subsequent fluorescence radiation of wavelength λ = 744 nm is detected with a gated PMT. Discharge filaments are generated with space and time fluctuations in a normal DBD reactor with flat electrodes. In order to realize a series of locally stable filaments a special reactor with a series of metallic tips was applied in this case. The filaments are now localized in space so that space resolved measurements can be performed by displacing the entire reactor with respect to the probing laser beam.

3 Dielectric (DE+) Figure 2: Pinreactor for space and time resolved measurments at a selected filament The steeply rising discharge voltage (15 kv in 9 ns) leads to the ignition of all filaments within a time interval of 5 ns. Varying the delay of the laser pulse leads to the possibility to measure radicals densities with time resolution. The FTIR spectroscopy performed in parallel yields quantitative information on NO, N 2 O, NO 2, CO, CO 2, HNO 3 and on C x H y. The molecules can be probed along three different paths within the reactor and, in addition, behind the reactor. 2 Results A two dimensional distribution of atomic nitrogen in a discharge filament is displayed in figure 3. The measurements indicate a diameter of the discharge filament of approximately 2 µm. A puzzling result is the high concentration of atomic nitrogen close to the dielectric. N LIF Signal [a.u.] ,8,4 Radial Position [mm] Pin (M ),,4,8,6,4,2 1,,8 Axial Position [mm] Figure 3: Two dimensional distribution of atomic nitrogen in a single filament. The measurements are taken 1µs after ignition; voltage amplitude: 6 kv; polarity of the dielectric: positive; gap width: 1.3 mm

4 Figure 4: Positions of measurements This observation has prompted a time resolved measurement series of atomic nitrogen in a pure nitrogen discharge at various selected positions within and close to the discharge filament (see figure 4). Figure 5 shows a time resolved measurement. The reduction of the fluorescence signal as a function of the delay between the ignition and the firing of the laser allows to determine the lifetime of the nitrogen radicals in the filament. The various curves corresponding to the positions indicated in figure 4 have been normalized with respect to their maximum value in the center. A clearly slower decay close to the dielectric is obvious. Admixture of oxygen leads to drastic changes: Already 5% of admixed oxygen leads to a reduction of about one order of magnitude. A further increase of the oxygen ratio does not entail any changes of the lifetime of atomic nitrogen. normalized LIF Signal [a.u.] Time after Ignition [µs] Figure 5: Lifetime of atomic nitrogen for various positions in and close to the filament.

5 normalized N LIF Signal [a.u.] % O 2 5% O 2 1% O 2 15% O Time after Ignition [µs] Figure 6: Lifetime of atomic nitrogen as a function of Oxygen admixture in N 2 / O 2 discharges 3 Concluding remarks Our measurements of relative distributions of atomic nitrogen in a single filament of a DBD will be continued for more realistic gas compositions and will be correlated with FTIR measurements as mentioned. The measurement of absolute radical densities is precluded for the time being, by the preponderant influence of collisional quenching of the upper state population after optical excitation. There is hope that this situation can be improved with a more complete data basis regarding quenching coefficients for the major constituents present in the discharge. Acknowledgment Funding by the Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie is gratefully acknowledged. (project number 13 N 7195) References 1 U. Kogelschatz, in: Process Technologies for Water Treatment, S. Stucki, ed., Plenum, New York 1988 (p. 87) 2 B. Eliasson and U. Kogelschatz, Appl. Phys. B 46 (1988) B. Eliasson and U. Kogelschatz, IEEE Transactions On Plasma Science 19 (1991) B. M. Penetrante, M. C. Hsiao, B. T. Merrit, G. E. Vogtlin, P. H. Wallman, M. Neiger, O. Wolf, T. Hammer and S. Broer, Appl. Phys. Lett. 68, 3719 (1996) 5 M. Spaan, H. F. Döbele, Bulletin of the American Physical Society, 43, No. 5, (1998)