The reaction kinetics in nitrogen flowing afterglow was studied by optical emission spectroscopy. The DC flowing post-discharge in pure nitrogen was created in a quartz tube at the total gas pressure of 1000 Pa. The optical emission spectra were measured along the flow tube. It was found that N atoms are the most important particles in the late nitrogen afterglow. In order to explain the decrease of N atom concentration, it was also necessary to include the surface recombination of N atoms to the model

Krcma, F.

Mazankova, Věra

Trunec, D.

English

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

Mazankova V. et al.: Study of Nitrogen Atom Recombination by Optical Emission Spectroscopy   Plasma Physics and Technology 2015, 2, 1, 50-53 Study of Nitrogen Atom Recombination by Optical Emission Spectroscopy  Mazánkova V.1, Trunec D.2, Krčma F.1  1Faculty of Chemistry, Brno University of Technology, Purkyňova 118, Brno 612 00, Czech Republic, mazankova@fch.vutbr.cz, krcma@fch.vutbr.cz 2 Masaryk University, Kotlářská 2, 611 37 Brno, Czech Republic, trunec@physics.muni.cz The reaction kinetics in nitrogen flowing afterglow was studied by optical emission spectroscopy. The DC flowing post-discharge in pure nitrogen was created in a quartz tube at the total gas pressure of 1000 Pa. The optical emission spectra were measured along the flow tube. It was found that N atoms are the most important particles in the late nitrogen afterglow. In order to explain the decrease of N atom concentration, it was also necessary to include the surface recombination of N atoms to the model.  Keywords: optical emission spectroscopy, nitrogen afterglow, surface recombination coefficient1 INTRODUCTION In flowing nitrogen afterglows (post-discharges) at low pressure, a large number of nitrogen atoms is created in the discharge or in the short-lived (pink) afterglow during colli-sions of nitrogen molecules with electrons, vibrationally excited nitrogen molecules and metastable states. The nitrogen atoms are lost by volume and wall recombination [1]. The probability of volume recombination (a three-body collision) tends to be lower than the wall recombination probability γ. The probability γ is the ratio between the number of atoms ef-fectively recombining at the surface over the number of atom-wall collisions [2]. Nitrogen late afterglow is characterized by a visible emission in the yellow part of the spectrum, associated with the recombination of the ni-trogen atoms in the gas phase.  Nitrogen post-discharges are widely used for various industrial applications such as nitrid-ing [3], plasma surface modification [4] or plasma sterilization [5]. Besides laboratory and technological plasmas, the nitrogen post-discharge is studied also in connection with the kinetics of the upper Earth atmosphere (corona borealis [6]) and the processes occur-ring in nitrogen post-discharges are also taken into account in some extraterrestrial systems, for example in the Titan atmosphere [7]. This paper presents a method for obtaining the atomic nitrogen recombination probability (γ). It is based on the measurement of 11-7 transi-tion intensity in first positive nitrogen spectral system (FPS). 2 EXPERIMENTAL SET-UP The flowing configuration of nitrogen DC dis-charge was used for the experimental study. This experimental set-up was already used for our previous study with nitrogen [8, 9]. A simplified schematic drawing of the experi-mental set-up is given in Fig. 1. The active discharge was created in a quartz discharge tube with the inner diameter of 12 mm at the constant total gas pressure of 1000 Pa and the discharge power of 130 W. Hollow molyb-denum electrodes were placed in the side arms (at the interelectrode distance of 120 mm) of the main discharge tube to minimize their sputtering and also to minimize the influence of the light emitted in the electrode regions. The nitrogen gas was of 99.9999 % purity and it was further cleaned by Oxiclear and LN2 traps. The reactor system was pumped contin-uously by a rotary oil pump separated from the discharge tube by another LN2 trap. The gas flow of 800 sccm was automatically controlled by the Bronkhorst mass flow controller. The total gas pressure in the discharge tube was measured by a capacitance gauge connected to the end of the discharge tube. The gas temper-ature was 300 K. The bulk flow velocity vg of nitrogen in the tube was calculated from the continuity equa-tion and the state equation for ideal gas. The calculated velocity vg was 12 ms-1. The flow analysis was performed in the same way as in previous studies using flowing afterglow [10, 11]. Mazankova V. et al.: Study of Nitrogen Atom Recombination by Optical Emission Spectroscopy   51    Fig. 1: Scheme of the experimental setup. 1- catalyzer Oxiclear; 2- mass flow controller; 3- quartz dis-charge tube; 4-capacitance gauge; 5-quartz optical fiber; 6-monochromator Jobin Yvon Triax 550; 7-CCD; 8-PC  The distance d needed to develop a full para-bolic velocity profile is given by d = 0.277 a R, where a is the flow tube radius (0.6 cm) in cm and R is the Reynolds number. For our ex-perimental conditions, the Reynolds number R is equal to 184, which gives d = 31 cm. Fur-thermore, our experimental setup introduced perturbations into the flow pattern at the side arms and at the nitrogen inlet. So, the flow will be in transition between plug and parabol-ic flows and in this case the bulk flow velocity can be used for the calculation of the decay time. This approach was found to be correct in previous studies [8-10]. In present experiment the line intensities (which are proportional to the exited particle concentrations) were meas-ured across the diameter of the flow tube. Bolden et al. [12] have experimentally demon-strated that this is equivalent to monitoring the concentration at the center of the tube for fully developed parabolic flow conditions. The optical spectra were measured by Jobin Yvon monochromator TRIAX 550 with CCD detector. The 300 gr/mm grating was used for overview spectra in the range from 300 nm to 600 nm. The emitted light was led to the en-trance slit of the monochromator by the mul-timode quartz optical fiber movable along the discharge tube. 3 RESULTS The optical spectra were measured as a  function of position along the flow tube in the distances 30 – 52 cm from the active discharge. An example of the typical recorded post-discharge spectrum is shown in Fig. 2. The nitrogen late afterglow emission is dominated by the  N2(B, v=11) → N2(A, v=7)             (1) band of the first positive system at 580 nm [13], see also Fig. 2. This band is correlated with the three-body recombination process with the rate constant kvol = 4.4×10-33 cm6 s−1 taken from [14] N + N +N2 → N2(B, v=11) + N2.           (2) As the N2(B, v=11) state can de-excite either radiatively to the A state with a global frequency νrad or collisionally by quenching with the nitrogen molecules (kq), the emitted intensity can be written as   ]N[]N[N][)]11 (B,N[2qrad22vol2711kkvI      (3)  where the values νrad = 1.7×105 s−1 and kq = 5×10−11 cm3 s−1 were taken from [15]. So it appears that the evolution of the N atom con-centration can be followed monitoring I11−7. For pressures higher than 133 Pa is kq[N2] >> νrad and equation (3) reduces to  Mazankova V. et al.: Study of Nitrogen Atom Recombination by Optical Emission Spectroscopy   52  711 N][  I .              (4) The N atom concentration determined from equation (4) is shown in Fig. 3 as a function of decay time. These N atom concentrations were multiplied by a constant so that they can be coincident at t = 0 s (distance 30 cm from ac-tive discharge) with absolute N atom concen-tration determined in previous study by NO titration [16].  Fig. 2: Overview spectrum of nitrogen post-discharge     Fig. 3: Time dependence of nitrogen atom concentration. Squares – concentrations determined from Eq. (4); triangles – concentrations determined by NO titration [16]; full line – calculation from the model with γ = 2.16 × 10-6 ; dashed line – calculation from the model with γ = 1.35 × 10-6The kinetic model of nitrogen late afterglow developed previously [9] was used to fit measured data using least square method. The value γ = (2.16 ± 0.07) × 10-6 was obtained Mazankova V. et al.: Study of Nitrogen Atom Recombination by Optical Emission Spectroscopy   53 from this method.  However, the N atom con-centrations determined from measurement of  FPS intensity decreases faster than the N atom concentration determined by NO titration and also faster than calculated N atom concentra-tion from the model in previous study [9] with the value γ = 1.35 × 10-6. This discrepancy could be caused by incorrect determination of 11-7 transition intensity in FPS due to overlap of this band with neighbouring vibrational bands. 4 CONCLUSION The recombination of N atoms in nitrogen late afterglow was studied in flowing afterglow experiment at pressure of 1000 Pa. The optical emission spectra were measured at different positions along the quartz flow tube (and hence at different times of afterglow). The spectra were dominated by 11-7 band of the nitrogen first positive system. This band is correlated with three body (volume) recombi-nation of atomic nitrogen and thus it is possi-ble to determine the relative concentration of N atoms from band intensity. Then the wall recombination probability γ for N atoms was calculated from the decrease of N atom con-centration along the flow tube and the value γ = (2.16 ± 0.07) × 10-6 was obtained. Acknowledgements  The present work was supported by the project "R&D center for low-cost plasma and nanotech-nology surface modifications" (CZ.1.05/2.1.00/03.0086) funding by the European Regional Development Fund and by the Czech Science Foundation, contract No. 13-24635S.  REFERENCES  [1] Guerra V, Sa P A, Loureiro, J Eur. Phys. J. - Appl. Phys. 28 (2004) 125. [2] Rouffet B, Gaboriau F, Sarrette J P, J. Phys. D: Appl. Phys. 43 (2010) 185203. [3] Bockel S, Belmonte T, Michel H, Ablitzer D, Surf. Coat. Technol. 97 (1997) 618. [4] Junkar I, Vesel A, Cvelbar U, Mozetic M, Strnad S, Vaccum 84 (2009) 83. [5] Kutasi K, Pintassilgo C D, Coelho P J, Loureiro J, J. Phys. D: Appl. Phys. 39 (2006) 3978. [6] Ashrafi M, Lanchester B S, Lummerzheim D, Ivchenko N, Jokiaho O, Ann. Geophys. 27 (2009) 2545. [7] Horvath G, Krcma F, Polachova L, Klohnova K,  Mason N J, Zahoran M, Matejcik S, Eur. J. Phys. D: Appl. Phys. 53 (2011) 11001. [8] Mazankova V, Trunec D, Krcma F, J. Chem. Phys. 139 (2013) 164311. [9] Mazankova V, Trunec D, Krcma F, J. Chem. Phys. 141 (2014) 154307. [10] Kolts J H, Setser D W, J. Chem. Phys. 68 (1978) 4848. [11] Piper L G, Velazco J E, Setser D W, J. Chem. Phys. 59 (1973) 3323. [12] Bolden R C, Hemsworth R S, Shaw  M J,  Twiddy N D, J. Phys. B 3 (1970) 45. [13] Noxon J F, J. Chem. Phys. 36 (1962) 926. [14] Krivonosova O E, Losev S A, Nalivaiko V P, Mukoseev Y K, Shatalov O P, Reviews of Plasma Chemistry vol. 1 (Ed. Smirnov B M). New York: Consultants Bureau, 1991, 9. [15] Gordiets B, Ricard A, Plasma Sources Sci. Technol. 2 (1993) 158. [16] Mazankova V,  Polachova L, Krcma F, Horvath G, Mason N J, In: Proceedings, 19th Symposium on Physics of Switching Arc, Brno, (Ed. Aubrecht V and Bartlova M). Brno University of Technology, 2011, 283.  

Study of Nitrogen Atom Recombination by Optical Emission Spectroscopy

https://ojs.cvut.cz/ojs/index.php/PPT/article/download/7837/6149

Study of Nitrogen Atom Recombination by Optical Emission Spectroscopy

Abstract

Similar works

Full text

Available Versions

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