INFLUENCE OF MAGNETIC FIELD ON MONOCHROME VISIBLE LIGHT IN ELECTROPOSITIVE ELECTRONEGATIVE GAS MIXTURES DISCHARGES PLASMA

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1 THE PUBLISHING HOUSE PROCEEDINGS OF THE ROMANIAN ACADEMY, Series A, OF THE ROMANIAN ACADEMY Volume 7, Number /, pp. 3 3 INFLUENCE OF MAGNETIC FIELD ON MONOCHROME VISIBLE LIGHT IN ELECTROPOSITIVE ELECTRONEGATIVE GAS MIXTURES DISCHARGES PLASMA Luminita Catalina CIOBOTARU, Ion GRUIA University of Bucharest, Faculty of Physics, P.O. Box MG-, RO-775, Bucharest-Magurele, Romania Corresponding author: Ion GRUIA, gruia_ion@yahoo.com Abstract. The present paper deals with the comparative study of the degree of monochromatization of visible light, an effect that appears in electronegative-electropositive gas mixtures plasma, measured through a dimensionless parameter, called the M-parameter. The measurements were performed using as electropositive gases Ar, He, and Ne and as electronegative ones H, O, and Cl, in different combinations and percentages. We have used the following types of plasma: dielectric barrier discharge (DBD), very low radio frequency discharge (7.5 5 khz) and continuum discharge, in different geometries of the experimental devices. The total pressures of the gas mixtures covered the range between and torr (. mbar). The biggest value of M-parameter was reached in (Ne+H ) gas mixture DBD plasma, being of about. Applying a constant magnetic field on (Ne+H ) gas mixture RF discharge plasma we have found that the quality of the M-effect was improved due to the fact that the wall of the tube discharge was negatively charged. Key words: monochromatization light, M-effect, electropositive-electronegative gas mixture.. INTRODUCTION.. Definition of the M-parameter The monochromatization effect of visible light (in short, the M-effect) consists in the intensity decrease of a great number of lines from the noble gas spectrum in which appears, as dominant, one or a few very intensive spectral lines, when a little quantity of electronegative gas was added. Originally, the experimental measurements were performed in (Ne+Ar/Xe+H ) Penning-type gas mixtures plasma, using a dielectric barrier discharge (DBD) at moderate to high total gas pressures. The emission spectra of (Ne+%Ar) and (Ne+%Ar+5% H ) gas mixtures plasma, for a total pressure gas mixture of torr, were reported in []. It was observed the appearance of a quasi-monochromatic emission spectrum when, at the Penning gas mixture, was added hydrogen having the half partial pressure as against the total pressure of the gas mixture. The primary reddish colour of discharge changed then in an yellow one while the whole visible emission spectrum was virtually reduced to one intensive spectral line with λ = 55.3 nm of the pure neon. In all the measurements performed, even when used different types of gas mixtures (the noble gas remaining the neon) and different values of the electronegative gases percents (not bigger than 5%), it was obtained the same effect. The only change of the recorded emission spectrum consisted in the relative intensities amendment of the different neon emission spectral lines. In order to facilitate the comparison of the results obtained in different electronegative-electropositive gas mixtures, we have introduced the M ( λ, λ ) parameter, defined as following: i j I( λi ) M ( λi, λ j) =, I( λ ) j () Where I (λ i ) and I (λ j ) are the top values of the noble gas spectral lines relative intensities from the recorded spectrum, corresponding to the wavelengths I (λ i ) (called the dominant spectral line) and I (λ j ) (called the standard spectral line), respectively. This one can be chosen at random between any of the lines of the noble

2 Luminita Catalina CIOBOTARU, Ion GRUIA 35 gas emission spectrum. In some circumstances, namely in different gas mixtures studies, given the ratio of the M parameter calculated for the mixture over that in the pure gas, we have used a new parameter, called the : the higher the value of α, the stronger the M-effect. The parameter is obviously dimensionless because it was devised as a ratio of two spectral lines relative intensities belonging to the noble gas emission spectrum. In our comparative study this parameter will be considered as a quantitative measure of the M-effect intensity... The main generation mechanism of the M-effect The theoretical model of the generation mechanisms of this effect was assumed as a result of a large number of experimental observations, both in ac/dc discharges []. Basically, the monochromatization effect of light is a process that appears at atomic level, in contrast to excimer radiation, which is due to molecular generation processes being the result of the three-body collision polar resonant reaction between neutral, ionized and excited atoms of electropositive/electronegative gas. As in all resonant-type reactions, this particularity implies the fulfilment of an energetic condition, ideally zero energy defects (in the case of M- effect ΔE. ev). The general form of the reaction was established as follows: * ( ) E ( E ) + met * ground state met P N N P N N, Δ Δ () where P and N are the symbols for the atoms of electropositive and electronegative gases respectively, P + is the symbol for the positive ion, N is the symbol for the negative ion, N met is the symbol for the metastable negative met * * atom, (N ) is the symbol for the excited electronegative atom standing in a higher energy state, P is the electropositive atom in an excited state, and Δ E is the reaction energy defect. In the most significant case of the neon and hydrogen gas mixture, the reaction () can be thus customized as: + * * * * * Ne + H + H ( n= ) (Ne H ) + H ( n= ) Ne ( p) + H ( n= 3) + H( n= ) ±Δ E. (3) We must emphasize the fact that a three-body resonant reaction is an atypical one, having only a small probability of accomplishment. Nevertheless, there are some specific features of gas mixture and dischargestype (RF, DBD and dc discharge negative light zone), which facilitate the increase of the resonant process probability, as follows: The existence of negative and positive ions that brings a high probability of collisions due to the electrostatic attraction forces of opposite sign particles; The low energy electrons specific for the afterglow phase of RF discharge that leads to the formation of a great number of negative ions through the attachment process of these electrons to the neutral atoms of the electronegative gas. The big number of negative ions that exists in the negative light zone of a luminescent discharge; The moderate to high values of the gas mixtures total pressures that increase the rate of three-body interactions; The existence of the trapping phenomenon of the resonance radiation that allows the formation of electronegative quasi-metastable atoms in energy level, having a comparable long life-time with the one of the metastable energy states. Obviously, this characteristic favours their participation in the three-body reaction. We must notice once again the essential role of the negative ions in the appearance of the M-effect.. RESULTS AND DISCUSSION.. The ac discharge The ac discharge device used in the present study was largely presented in our previous paper [3]. The discharge tube represents an application of the dielectric barrier discharge namely a plasma discharge panel as shown in []. The ac discharge was produced in electronegative-electropositive gas mixtures between two thin, parallel and linear aluminium electrodes of 5 mm width and mm length. Both electrodes were covered with μm glass dielectric layer. The thin film conductors were obtained in vacuum, by deposition on two glass plates, via an applied mask. According to the Paschen law, in relation with the inter-electrodes distance value, in our case d mm, the working pressures range was varying from 3 to 3 torr. The minimum breakdown voltage for a gas mixture total pressure up to torr was around the value of kv with the rectangular pulse shape. For the

3 3 Influence of magnetic field on monochrome visible light in electropositive-electronegative gas mixtures discharges plasma 3 DBD - type discharges, the range of variation for the common working frequencies is between 5 khz. In our experimental conditions the optimum breakdown discharge frequency was settled at the value of 7.5 khz. The measurement method was the following: first were recorded the emission spectra of the noble gas plasma and afterwards the emission spectra for electropositive + p[%] electronegative gas mixture plasma at the same total pressure value of 3 torr (where p% is the notation for the percent of electronegative gas used in the gaseous mixture). The percents of electronegative gas were added in a growing rate of 5% in the range varying from 5% to 3%. The emission spectra were recorded with a Varian Techtronic-type spectrometer with the following characteristics: grating of grooves/mm, 5 mm wide slit and 3 nm measurement spectral range. The gaseous mixtures used in the present experiment were: (Ar+H ), (He+O ) and (He+Cl ). For each gaseous mixture plasma we plotted the following graphs: α = f (λ) for the electropositive pure gas, p tot =constant α = f (λ) for the studied gaseous mixture, p%=constant, p tot =constant α = f (p%), for the chosen dominant spectral lines, p tot =constant. Based on these studies (see Figs. ) we established the optimum of the added electronegative gas percents for which the value of the M parameter, defined via eq. (), reaches the maximum value. In (Ar+H ) gas mixture we considered λ =.3 nm as the standard spectral line and λ = 9.5 nm/73. nm as dominant line. The total pressure of the gas mixture was of 3 torr. For (Ar+H ) gas mixture we have found several dominant spectral lines, namely 5. nm, 7.9 nm, 9.5 nm, 7.7 nm, 77. nm, and 73. nm, but because all these lines indicated a shift to the infrared zone of the argon emission spectrum when different hydrogen percents were added, finally have been chosen only two lines, namely λ = 9.5 nm/ 73. nm and λ =.3 nm. 9.5 nm pure Ar p tot =3 torr Ar+5% O p tot =3 torr α = nm α =.9.3nm α = 9.5 nm α = nm α = Fig. The curve α = f(λ) of pure Argon emission spectrum. Fig. The curve α = f(λ) of the (Ar+5%H ) 9.5 nm α = 9 Ar+.7% H p to t =3 torr 73. nm α =5. 5 Ar+% H p tot =3 torr nm α = nm α = Fig. 3 The curve α = f(λ) of the (Ar+.7%H ) Fig. The curve α = f(λ) of the (Ar+%H ) α = 9 p =.7 Ar + H M = I 9.5nm /I.3nm p to t = 3 torr α = 5. p =.7 Ar+ H M= I 73.nm /I.3nm p tot = 3torr %(H ) %(H ) Fig. 5 The dependence of value on the hydrogen percent for the wavelength λ = 9.5 nm. Fig. The dependence of value on the hydrogen percent for the wavelength λ = 73. nm.

4 Luminita Catalina CIOBOTARU, Ion GRUIA 37 Figures 5 and show that the maximum values for the M parameter are reached for the wavelength λ = 9.5 nm at p[%]h =.7 and for λ = 73. nm at p[%]h = 5, respectively. In (He+O ) gas mixture at a total pressure of 3.5 torr, we have considered as standard spectral line λ =5. nm and as a dominant one λ = 335. nm. In fact, more spectral lines of the helium emission spectrum changed their relative intensities when hydrogen was added, being spectacularly enhanced, so that it appeared a shift of the spectrum to the UV zone nm α = pure He p tot =3.5 torr nm α =. He+% O p tot =3.5 torr nm α = nm α = λ(nm) Fig. 7 The curve α = f(λ) of pure helium emission spectrum. Fig. The curve α = f(λ) of the (He+%O ) nm α =. He+% O p tot =3.5 torr nm α =. 5. nm α =.9 He+3% O p tot =3.5 torr nm α = Fig. 9 The curve α = f(λ) of the (He+%O ) Fig. The curve α = f(λ) of the (He+3%O ) nm α =. He+35% O p tot =3.5 torr He+ O. p tot =3.5 torr. α =I 335.nm /I 5.nm. p %=3 α = nm α = λ(nm) O % Fig. The curve α = f(λ) of the (He+35%O ) Fig. The dependence of on the O percent for the wavelength λ = 335. nm. Figure shows the fact that the maximum value of the M parameter was reached for the spectral line λ= 335. nm at the added oxygen percent of p = 3%. In (He+Cl ) gas mixture plasma it can be observed, similar to the case of (He+O ), an appearance of a global shift of the emission spectrum shape in the sense of an enhancement of the UV zone spectral lines to the detriment of the ones situated in the middle part of the spectrum toward the infrared zone. The dominant lines were more than one, as follows: λ =.3 nm/5. nm/. nm/335. nm/35. nm. We have chosen as a dominant spectral line λ =. nm and as the standard line λ = 5. nm. The total gas mixture was of 3 torr (Fig.3 Fig.).

5 3 Influence of magnetic field on monochrome visible light in electropositive-electronegative gas mixtures discharges plasma He+% Cl p tot =3 torr 5. nm α =.7..5 nm α =.3... He+% Cl P tot =3 torr 5. nm α = nm α = λ(nm) Fig. 3 The curve α = f(λ) of the (He+%Cl ) Fig. The curve α = f(λ) of the (He+%Cl )....5 nm α =.37 He+3% Cl P tot =3 torr... He+ Cl p tot = 3 torr α =I.5 nm /I 5. nm p % Cl =3 α = nm α = p % Cl Fig. 5 The curve α = f(λ) of the (He+3%Cl ) Fig. The dependence of on the O percent for the wavelength λ =.5nm... The dc discharge The experimental setup used for measurements in dc discharges consists in a Pyrex glass made discharge tube of 3 mm diameter, having a central part of quartz, in order to allow the passage of the UV radiation [5]. The length of this part was of mm and the total length of the entire tube of mm. The two electrodes are made of Φ.5 mm wolfram-thorium rod. The top of the electrodes ( mm) is sharp while the rest is covered with a layer of glass in order to limit the existence of parasite discharges out of the interelectrodes space. The distance between the two electrodes was of mm. The discharge tube was connected to the vacuum pumping unit and can be filled with various spectral purity gas mixtures at the established pressures. An Optical Multichannel Analyzer (OMA), with the spectral range 5 9 nm and.5 nm resolution, was used in order to record the spectra of the emitted light. The range of the total pressures of the gas mixtures was the same as in the case of the ac discharges, namely 3 to 3 torr. For this range, the discharges breakdown electrical voltage increased linearly, reaching a maximum value of.3 kv. A limitation electrical resistance of kω was used in experiment. In (He+O ) gas mixture dc discharge plasma it was emphasized the existence of a global shift of the emission spectrum shape in the sense of an enhancement of the IR zone spectral lines to the detriment of the other lines of the emission spectrum. We have chosen as a dominant spectral line λ = 77.5 nm and as the standard line λ = 335. nm. The total gas mixture was of 97. torr (Figs. 7 ). Figure shows that the maximum value of the M parameter was reached for the spectral line λ = 77.5 nm at an added oxygen percent of p = 3%. The experimental data are summarized in Table. 7 5 pure He p tot =97. torr α =. He+5% O p tot =97. torr.5 3. α = 77.5 nm α = nm α = Fig. 7 The curve α = f (λ) of pure helium emission spectrum. Fig. The curve α = f (λ) of the (He+3%O )

6 Luminita Catalina CIOBOTARU, Ion GRUIA 39 He+3% O p tot =97. torr 77.5 nm α = He+ 3% O p tot =97. torr 77.5 nm α = α =.57 α = Fig. 9 The curve α = f (λ) of the (He+3%O ) Fig. The curve α = f (λ) of the (He+3%O ) 77.5 nm He+3% O α =.5 9 p tot =97. torr 7 He+ O α =I 77.5 nm /I p tot= =97. torr p % =3. α = α = p % O Fig. The curve α = f (λ) of the (He+3%O ) Fig. The dependence of on the O percent for the wavelength λ = 77.5 nm. Table Experimental data for M-effect in ac discharges (DBD and RF), and in dc discharges (negative light zone) Ar + p H Type of gas mixture p (%) λ (nm) λ (nm) (dimensionless).7/ /.7/ / /.7.3. He +p O He + p Cl Total gas mixture pressure (torr) Ne+ p H He +p O Experimental data for M-effect in ac discharges (DBD and RF) in dc discharges (negative light zone) As it can be observed from the data, the M-effect was more substantial in (Ne+% H ) gas mixture plasma, for the total pressure in the range of 3 torr, in ac discharge by comparison with the dc discharge..3. The influence of a constant magnetic field on the M-effect In order to study the influence of a constant magnetic field on the appearance and magnitude of the M-effect, we have chosen the (Ne+.5% H ) gas mixture RF discharge plasma, at a total pressure of torr, where the experimental conditions are at the optimum value in order to obtain the M-effect. The magnetic field was applied perpendicularly to the drift velocity of the electrons, namely on an imaginary line

7 3 Influence of magnetic field on monochrome visible light in electropositive-electronegative gas mixtures discharges plasma 7 connecting the two electrodes. The experimental setup is presented in Fig. 3. In order to allow the passage of the UV radiation, the discharge was produced in a quartz tube with 5 mm inner diameter and mm outer diameter, respectively, between two identical wolfram-thorium cylinder electrodes of mm diameter, spaced at mm distance. In front of the discharge tube was placed a reflection mirror in order to minimize the emitted radiation loss. The experimental discharge device can be pumped down up to a pressure of about mbar and then filled with various gas mixtures of spectral purity. The RF electrical power supply used in the experiment had the following characteristics: maximum output electrical tension of kv corresponding to a maximum electrical current intensity of 5 ma, alternative voltage frequency of 5 khz and a filling factor of about %. The optical emission spectra of the plasma discharges were registered using an OMA with a spectral range between to 9 nm,.5 s time of integration and a resolution of.5 nm, after the passage of the emitted radiation through a polarization filter and a focusing lens system. The registered data are processed by means of a computer. The results are plotted in Fig. and the corresponding data for the M parameter are given in Table. M=I 55.nm /I,3nm for P in constant magnetic field for P 9 in constant magnetic field for P without magnetic field for P 9 without magnetic field I discharge (m A) Fig. 3 Schematic diagram of the experimental device (PF polarization filter, C computer, L lens, A anode, K cathode, OMA Optical Multichannel Analyzer). Fig. The dependence of the M parameter on the discharge electric current for the two components of light I and I 9 in constant magnetic field, and without magnetic field. i discharge (ma) Table Values of M parameter for B =, and for B constant (Diaphragm.5,.5%H +57.5%Ne, p tot = torr) for B = (without magnetic field) for constant B for λ=55. nm M==I for λ=.3 nm for λ=55. nm for λ=.3 nm 55 /I M==I 55 /I M==I 55 /I I (a.u.) I 9 (a.u.) for I I (a.u.) I 9 (a.u.) for I 9 I (a.u.) I 9 (a.u.) for I I (a.u.) I 9 (a.u.) M==I 55 /I for I Here I and I 9 represent the intensity of light measured in the oscillation plane and in a perpendicular plane, respectively, and i is the intensity of the electric current in discharge. As it can be observed, the values of the M-parameter are bigger as compared with the situation without applying the magnetic field. In our opinion, this is because the applied magnetic field leads to the appearance of the Lorentz force that moves the free electrons from the plasma. These electrons charged alternatively the wall of the discharge tube with a negative charge. In contrast to the high frequencies discharges (with magnitude order of hundreds of GHz), where the spatial charge generated in volume discharge has no time to be redistributed together with the modification of the electric field direction for this reason the processes from the cathode do not play an essential role the low frequencies discharges are formed by series of distinct electric current pulses (the best of this situation is represented by the dielectric barrier discharge, in pulsed mode) []. As the applied voltage on electrodes was in the domain of very low radio frequency (up to 3 khz), the wavelengths of the electromagnetic field are very large, namely of the order of Km, which means much more than the geometrical dimensions of the discharge tube used in experiment. As reported in [7] for gas pressures of to torr, in ac discharge plasma, the time between two successive electronic collisions encountered by the neon atoms is about 3 - s. In a half cycle of

8 Luminita Catalina CIOBOTARU, Ion GRUIA 3 the supply alternative voltage it results that the number of the collisions is about.. A part of these collision processes are influenced by the existence of Lorentz forces, producing the negative charge of the wall. The negative potential of the wall promotes the volume process of the negative ions formations, which play a basic role in the appearance of the M-effect, and reduces the possibility of the neutralization process of the negative ions on the discharge tube wall, which is not negligible because of the small dimensions of the discharge tube of 5 mm and the moderate total pressure of torr []. There is a difference even between the M parameter values calculated for I and I 9 due to the fact that the quasi-monochromatization of the visible light effect implies the existence of a certain degree of polarization too [9]. The situation, in the peculiar case of the dielectric barrier discharge, is even more interesting because the existence of the polarization effect of the electric charges on dielectric walls is a process leading to the existence of a polarization electric voltage of an opposite sign against the electric voltage of the supply source. A significant increase of the negative ions number in the phase of afterglow DBD, as a result of the addition of the hydrogen, has an important influence on diminishing the value of the polarization tension, with the subsequent increase of the electric voltage in discharge. In this way, the existence of the magnetic constant field applied on the discharge could produce important change in value of the polarization voltage, hence in the experimental conditions of the M-effect generation. 3. CONCLUSIONS In this paper it was considered the monocromatization effect of the visible light (the so called M-effect), which was measured using the M-parameter (or α -parameter), in different types of discharge, namely the DBD, RF, and dc discharges. The M-effect appears to be the most intensive in ac discharges by comparison with the dc discharges, having the biggest values of the M parameters. The study of the application of a constant magnetic field on a (.5% H +57.5%Ne) gas mixture RF discharge plasma, which was directed perpendicularly on an imaginary line connecting the two electrodes of the tube discharge, emphasized the fact that the intensity of the M-effect was improved by the existence of the magnetic field. The main explanation proposed is that the negative potential of the wall increased the life-time of the negative ions in discharge volume. Further studies of the M-effect will be required in order to evaluate more accurately the influence of the application of a constant (or variable) magnetic field on the DBD and RF discharges. ACKNOWLEDGMENTS This paper is dedicated to Professor Geavit MUSA, Corresponding Member of the Romanian Academy, who put forward the M-effect. His brilliant scientific researches formed a basis not only for important results in Low Temperature Plasma Physics but also for the walk of life for many of his former collaborators. REFERENCES. G. MUSA, L.C. CIOBOTARU, Tentative explanation of selective population of the p level of neon atoms in the case of M - effect in neon +hydrogen and neon+oxygen gas mixtures, Journal of Optoelectronics and Advanced Materials,,, pp ,.. L.C. CIOBOTARU, Considerations on the Penning reactions role for the Monochromatization -effect in noble gases-hydrogen mixtures, Optoelectronics and Advanced Materials Rapid Communications, 5, 3, pp ,. 3. G. MUSA, L. C. CIOBOTARU, BARBU IONUT, The M-effect in A.C./D.C. discharges in He+O / Cl gas mixtures, Journal of Optoelectronics and Advanced Materials,, 3, pp. 9 97,.. A. BALTOG, G. MUSA, Polar recombination as the main process explaining the M-effect, Contributions to Plasma Physics, 3, pp. 3, L.C. CIOBOTARU, Monochromatization effect ac/dc discharges comparative behavior, Romanian Reports in Physics,,, pp. 3,.. N.A. KAPTOV, Electrical phenomena in gases and vacuum, Edit. Tehnică, Bucharest, G. MUSA, A. POPESCU, A. BALTOG, C.P. LUNGU, Monochromatization of the radiation of discharges in multiple gas mixtures, Romanian Reports in Physics, 5, 3, pp. 7, G. MUSA, N. NICULESCU, A. POPESCU, V. COVLEA, A. CORMOS, A. POP, The influence of the poloidal field on the glow-discharge, Proceedings of XVII th International Conference on Phenomena in Ionized Gases (ICPIG), Budapest, Hungary, 95, pp L.C. CIOBOTARU, I. GRUIA, Quasi-Monochrome Light Polarization Study in Binary Electropositive-Electronegative Gas Mixture, Romanian Journal of Physics,, 3, pp. 57 5,. Received February 9,