1. 3. Results and Discussion
QUASI-MONOCHROME LIGHT POLARIZATION STUDY IN BINARY ELECTROPOSITIVE- ELECTRONEGATIVE GAS MIXTURE
C. L. CIOBOTARU *1, I. GRUIA 2 and C. GAVRILA 3
1 National Institute for Lasers, Plasma and Radiation Physics, Magurele, , Romania
2Faculty of Physics, University of Bucharest, Magurele, , Romania
3 Technical University of Civil Engineering, Bucharest, 7000, Romania
Abstract:In electronegative-electropositive gas mixtures plasma, at a total pressure varying in the range of ten to hundred Torr, was reported the appearance of a quasi-mochromatization effect of the emitted radiation. This radiation could be the result of the generating mechanisms at molecular level, which is the case of the excimer radiation but also at atomic level. Thus, in the last case, in (Ne+1%Ar/Xe+H2) gas mixtures plasma in a dielectric barrier discharge (DBD), this effect consists in the reduction of the discharge emission spectrum practice at one single, strong spectral line, namely = nm. The present paper is concerned with the main characteristics comparative investigation of the principal reaction mechanisms involved in the quasi-monochromatization effect existence, namely the direct Harpoon reaction, in the case of the excimer radition, which appears as a result of the generation processes at molecular level and the resonant polar three-body reaction, as a result of the generation processes at atomic level. Also, the paper points out the role and the importance of the metastable electronegative atoms in the appearance of the monochromatization – effect at atomic level.
Introduction
The M-effect consists in the reduction of the discharge emission spectrum practice at one single line, namely λ = nm, in the specific case of (Ne+Ar/Xe+H2) gas mixtures . In this experiment was used a dielectric barrier discharge (DBD), at a total pressure of around 133 mbar, but the effect was observed also in D.C. and in RF discharges.
The emission spectrum of the pure neon plasma is presented in Fig. 1.a, comparative with the spectrum of (Ne+1%Ar + 40%H2) mixture dielectric barrier discharge which presents significant changes of its aspect, under similar experimental conditions - see Fig.1.b. Figure 2 presents the same monochromatisation effect of light in argon and hydrogen gas mixture in a D.C. discharge. It had been observed an emission spectrum nearly monochromatic, since when, at the Penning-type mixture, was added hydrogen with a maximum half partial pressure as against the total pressure of the gas mixture (after this value of hydrogen pressure the effect diminished drastically). The optimum H2 percentage into Ne is reached for the value of
As assumed in our previous papers, the monochromatization-effect of light is the result of the resonant polar three-body collision reaction of ionized, excited and metastables atoms, as a main creation mechanism. The general form of the reaction is:
P++ N- + Nmet → P*+ Nground state + (Nmet)* + ΔE , (ΔE≈0) (1)
where the used notations are the following: P and N are the symbols of the atoms of electropositive and respectively, electronegative gases in the mixture, P+ is the symbol for the positive ion, N- is the symbol of the negative ion, Nmet is the symbol for the metastable negative atom, (Nmet)* is the symbol of the excited electronegative atom standing in an upper state energy that the metastable level, P* is the electropositive atom in an excited state and ΔE is the reaction energy defect. This reaction is based on the existence of a third particle, in a convenient energetic state, namely the metastable atoms of the electronegative gas. Only if this condition is accomplished, the reaction becomes resonant, as in the case of H2+Ne :
Ne+ + H- + H*(n=2) → (Ne+H-) + H*(n=2) → Ne*(2p1) + H*(n=3) + H (n=1) + ΔE (2)
For pressures above the value of 13 mbar, the appearance of the trapping phenomenon of the resonance radiation allows the formation of electronegative quasi-metastable atoms standing in n = 2 level energy, with a life-time comparable with the one of the metastable energetic states. As it can be observed from the equation (2), the intermediary state of this reaction, namely the formation of a three-particles compound, corresponds to the binding of the negative-positive ions, close by the excited hydrogen atom on the resonance level 2p2 P 02 / 3 P (n=2). The cross-section of the resonant polar reaction three-body collision reaction is strongly affected by the attraction electric forces between the colliding particles, by the increase of the total pressure of the gas mixture and the plasma ionization degree. For the gas mixtures in which the electronegative gas has a strong electronic affinity, like the chlorine atoms, the generation reaction for the M-effect could be binary in the classical sense of the Landau-Zenner theory . The M-effect has a general, distinct character proved by his appearance not only in binary but also in multiple electronegative-electropositive gas mixtures, in D.C. or in A.C. discharges, as it is shown in Figs 3.
2. Experimental set-up
A photo - view and a schematic diagram of the experimental set- up are presented in Fig. 4 and Fig. 5, respectively. In order to allow the passage of the UV radiation, the discharge was produced in a quartz tube with 16 mm inner diameter and 20 mm outer diameter respectively, between two identical wolfram-thorium cylinder electrodes of 12 mm diameter, spaced at 6 mm distance. In front of the discharge tube was placed a reflection mirror in order to minimize the lost of emitted radiation. The experimental discharge device can be pumped down up to a pressure of 1.33 x10– 4 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 2 kV corresponding to a maximum electrical current intensity of 150 mA, two optional frequencies of 25 kHz and 50 kHz, respectively and a filling factor of about 10-20%. The optical emission spectra of the plasma discharges were registered using an OMA (Optical Analyzer Multichannel) with a spectral range of nm, 0.5 s time of integration and a resolution of 1.5 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.
3. Results and Discussion
The measurements of the polarization degree were performed in hydrogen-neon gas mixtures for the dominant spectral line with λ = nm, at two values of total pressures namely 19 Torr (25% H2) and 45 Torr (8.5% H2), respectively. Each set of measurements was done for two frequencies, at 25 kHz and 50 kHz respectively. The discharge electrical current intensity was varying with a rate of 2.5 mA within the range of 6 to 24 mA. Based on the values indicated in the Tables 1 and 2 were realized the graphs which are presenting the dependence of the polarization degree of the 585.3nm spectral line on the discharge electrical current intensity (Figs. 6 and 7).
As it was noticed before, the maximum of the monochromatization - effect is reached for a hydrogen percentage value into neon of Figure 6, which was realized for a percentage of hydrogen of 25, bigger than in the Fig. 6, shows an interesting sinusoidal shape of the nm spectral line polarization degree dependence on the electric current intensity. This shape is almost identical, even slightly shifted, for both frequencies of 25 kHz and 50 kHz, respectively. The same periodical aspect is kept in the shape from the Fig. 7 but it is less marked. As we have noticed before, the monochromatization effect is based mainly on the polar resonant three-body reaction between the atoms of the electronegative and electropositive atoms and the metastables of the electronegative gas, which implies the existence of a sharp energetic condition (aprox.0.01eV) to be fulfilled. The sinusoidal behavior of the polarization degree on the intensity of the electrical discharge current suggests a possible causal relation with the energy amount dissipated in discharge from the electrical power supply.
4. Conclusions
It was emphasized the fact that in hydrogen-neon mixture plasma at a total pressure of tens Torr and a hydrogen percentage value bigger than 20 appears a process of light polarization associated with the monochromatization effect. This process was studied in correlation with the electrical discharge current intensity and seems to have a periodical character. Further studies are required for different experimental conditions like: electronegative gas percentages, gas mixtures total pressures, electronegative-electropositive gas mixtures, in order to explain this behavior and the possible connection between the (quasi)mochromatisation effect of light in electronegative-electropositive gas mixtures and the subsequent polarization phenomenon.
Fig. 1.a Emission spectrum of dielectric barrier discharge in pure Neon gas.
Fig. 1.b Emission spectrum of dielectric barrier discharge in (Ne+1%Ar) + 40%H2 gas mixture
Fig.2. Emission spectrum of a DC discharge in (Ar+40.6 %H2 ) at ptot=197 Torr (near cathode).
Fig.6 The dependence of polarization degree (Oy) on the discharge electrical current intensity (Ox)
ptot=19 Torr, 25%H2
Fig.7 The dependence of polarization degree (Oy) on the discharge electrical current intensity (Ox)
ptot=45 Torr, 8.5%H2
Fig. 3: Emission spectrum of the M-effect in (Ne-Ar-Xe +50% H2) mixture DBD plasma at 80torr total pressure
Table 2: Experimental data for ptot = 45Ttorr
(8.3% H2) Electrical Discharge Current [mA] Polarization Degree for ν=25Khz
Polarization Degree for ν=50Khz
6 0, ,
7,5 0, ,
10 0, ,
12,5 0, ,
15 0, ,
17,5 0, ,
20 0, ,
22,5 0, ,
23,75 0, ,
Table 1: Experimental data for ptot = 19 Torr (25% H2) Electrical Discharge Current [mA]
Polarization Degree for ν=25Khz
Polarization Degree for ν=50Khz
6 0, ,
7,5 0, ,
10 0, ,
12,5 0, ,
15 0, ,
17,5 0, ,
20 0, ,
22,5 0, ,
25 0, ,
Fig.4 Photo-view of the experimental set-up
Fig.5. Schematic diagram of the experimental device : PF- polarization filter, R-recorder (computer), L-optical lens, A - anode, K- cathode, OMA -Optical Multichannel Analyzer.