STUDY OF THE CARBON ATOMS PRODUCTION IN METHANOL/ ETHANOL-NITROGEN FLOWING POST-DISCHARGE PLASMA

Transcription

1 STUDY OF THE CARBON ATOMS PRODUCTION IN METHANOL/ ETHANOL-NITROGEN FLOWING POST-DISCHARGE PLASMA L.C. CIOBOTARU 1,2, I. GRUIA 2,* 1 National Institute R&D of Lasers, Plasma and Radiation Physics, Magurele, Romania 2 Faculty of Physics, University of Bucharest, Romania * Corresponding author: gruia_ion@yahoo.com Received March 12, 2015 Optical emission spectroscopy (OES) in a flowing afterglow of a methanol/ethanol ( %) N 2 D.C. plasma was the used method in order to measure carbon atom concentration. From the intensities of the nitrogen first positive system and the CN radical violet bands, and by means of a kinetics mechanism, the C atom concentration was found to be in the range cm -3 for nitrogen-methanol and cm -3 for nitrogen-ethanol mixtures respectively, at pressures between mbar. The calculated carbon atoms density was established to be one order of magnitude bigger in the case of the ethanol-nitrogen mixture than in the one of methanol-nitrogen. Key words: afterglow nitrogen plasma, Methanol/ethanol, CN radical violet radiation, Nitrogen first positive system, NO titration method. 1. INTRODUCTION The nitrogen mixtures post-discharge plasma was subject of a great industrial interest in different areas, such as nitrating process, enhancement of polymer printability, adhesion proprieties and remote plasma enhanced chemical vapours deposition of thin nitride films etc. Thanks to the large life-time of the dissociated nitrogen atoms (about 10 s), a homogenous concentration of active species can be obtained in a larger volume discharge of about 5 cubic meters. Another interesting quality of nitrogen mixture post-discharge plasma is represented by the nonequilibrium thermodynamic character which results in high efficiency of initiation and sustaining plasma-chemical reactions involved in metal surface treatment (Penning ionization, energy transfer, molecular dissociation etc.) [1 3]. In our previous papers [4 6] notable changes in the active species population were reported to appear in nitrogen D.C. flowing discharges when small amounts of hydrogen/methane were added. For instance, increases of the N 2 (C) population up to 6 times were recorded for (N 2 x H 2 ) gas mixtures, (1) with x ranging between ( )%. Subsequently, the conversion of methane into more useful chemicals (C 2 hydrocarbons, methanol, acetylene, etc.) is still subject of great industrial interest. Rom. Journ. Phys., Vol. 60, Nos. 9 10, P , Bucharest, 2015

2 2 Study of the carbon atoms production in methanol/ethanol nitrogen flowing post-discharge plasma 1537 To initiate the process of conversion it is necessary to break the strong C-H bond. There are several ways for this aim such as: pyrolysis, direct oxidation, catalytic oxidative coupling and plasma assisted processes. The appearance of carbon atoms in nitrogen-methane flowing after-glow plasma represents another important aspect not only for obtaining new chemicals but also for the formation of carbon films. This is a complex process in which are involved multiple species like neutral and charged hydrocarbon radicals, especially CH x + (x = 2, 3, 4) positive ions. While in the active discharge the main mechanism for the production of free C atoms are reactions of the type CH x +e C +..., (2) in the after-glow discharge zone the influence of the electronic collisions on the initial dissociation of methane becomes less significant due to both their decreasing concentration and energy of electrons. This observation is valid for charged species too. It was reported the fact that the distance of methane introduction into nitrogen afterglow from the end of active discharge has an important influence on plasma chemical reactions leading to the preparation of carbon films [7 13]. The present paper deals with a calculation method of carbon atoms concentration in a flowing after-glow of a methanol (CH 4 O) /ethanol (C 2 H 7 O)/ ( %) N 2 D.C. plasma by using the ratio between the intensity of the nitrogen first positive and the CN radical violet spectral bands, respectively. 2. EXPERIMENTAL DEVICE A schematic view of the experimental set-up is presented in Figure 1. The discharge was ignited in a Pyrex tube with 22 mm inner diameter and 24 mm outer diameter, between two side-armed identical hollow Ni-Cr cylinder electrodes of 10 mm diameter, spaced at 400 mm distance. The first electrode, in the sense of flowing gas, was the anode. Spectral purity gases (99.98 %) were used. Gas flows were regulated and measured in STP conditions by MKS mass flow-meters, with a measurement scale of maximum 1500 sccm for nitrogen and 10 sccm for hydrocarbon vapours. The pressure was measured at the end of the discharge tube using a thermal gauge. The tube was connected to a fore-pump in order to maintain the right pressure and flow rate during the experiment. The after-glow discharge tube had the same diameter as the discharge tube and the total length of 500 mm. In this zone was introduced, by a thin metallic tube, a hydrocarbon vapours flow (methanol or ethanol). The glass vial containing the ethanol/methanol liquid was kept to a constant temperature inside of an electrical heated oven. The vapours pressures for methanol/ethanol liquid are rather high, namely 100 torr at 21 o C for methanol and 35 o C for ethanol respectively, as indicate the cited sources (CRC Handbook of Chemistry and Physics 44 th Ed. for methanol and Lange's Handbook

3 1538 L.C. Ciobotaru, I. Gruia 3 of Chemistry 10-th Ed. for ethanol). However, in order to avoid a possible phenomenon of condensation, the hydrocarbons vapours admission/detection parts of the device were heated with a warm air flow at a temperature of about 35 o C. Spectral data were obtained within the glow and after-glow discharge zones, and they were recorded by means of a classic spectral analysis system consisting of: Varian-Techtronic spectrophotometer (S) equipped with a grating of 1200 grooves/mm and λ = ( nm) measurement range, a Hamamatsu R585-type photomultiplier (PM), converting the optical signal into electrical one, and a (x/y) recorder (R). Alternatively, it was used an OMA-multichannel detection device connected to a computer, which allows the spectra recording in a period of time lower than 2s. A quartz optical fibre (OF) was placed half way from the glowdischarge zone, for the first set of measurements of the radiation emitted by the active species present in this zone (position 1) and in late post-discharge zone at a distance of 300 mm from the cathode, for the second set of measurements of the radiation emitted by N 2 (B) and CN active species (position 2). The spectral resolution of the system was of δλ < 0.1 nm. A D.C. power supply able to give up to 5 kv /0.2 A and adjustable power up to 1 kw was used. Fig. 1 Experimental device (glow and afterglow discharge zone), (D1, D2 flowmeters: V1, V2, V3 valves; S spectrophotometer; P photomultiplier; R recorder; OF optical fiber; G thermal gauge, A anode, K cathode, OMA Optical Multichannel Analyzer). The experimental conditions for the (nitrogen hydrocarbon vapours) DC flowing discharge study were the following: total pressure of the gas mixture: mbar, nitrogen flow rate: sccm, D.C. discharge current intensity: ma, hydrocarbon flow rate: 0 10 sccm, hydrocarbon vapours concentration in nitrogen: %, full afterglow period: up to 10-1 s, the lateafterglow discharge temperature: 300 K, breakdown gas electrical power: 360 W.

4 4 Study of the carbon atoms production in methanol/ethanol nitrogen flowing post-discharge plasma RESULTS AND DISCUSSIONS 3.1. THE MAIN ACTIVE SPECIES AND REACTION PRODUCTS IN ACTIVE DISCHARGE There were performed researches for the identification of the active species, reaction products and reaction mechanisms in nitrogen-hydrocarbon plasma discharge and post-discharge [14 17]. Measurements were performed in an electric constant current intensity mode, for different values. Basically, the processes which appear in nitrogen-hydrocarbon glow and after-glow discharge are characteristic for the following three distinct steps: a. The active discharge zone: the gas mixture formed by the nitrogen and hydrocarbon vapours enters inside the zone where the molecules are dissociated, ionized and excited ( 10-2 s), mainly via electronic collisions; b. The early post discharge zone: the recombination between electrons and ions ( s); c. The late post-discharge zone: the recombination of atoms and the molecular collisions become the dominant processes ( s). This is the zone of interest in present experiment. The main nitrogen active species usually present in pure nitrogen plasma discharge are the following: the radiative species N 2 (B), N 2 (C) which belong to the N 2 neutral molecule; the radiative species N 2 + (B) which belongs to the N 2 + molecular ion; the vibrational excited molecular nitrogen N 2 (X); the metastable N 2 (A); the atoms of nitrogen. The active species N 2 (X), N 2 (A) and N atoms could not be distinguished because their spectral emissions do not appear in the ( ) nm spectrum range. The N 2 + (B) molecular ion radiative species was also not observed because of the fact that the optical fibre was located half way from the glow-discharge zone, while the N 2 + molecular ion is usually present only in very proximity of the cathode. The effective radiative species identified in a pure molecular nitrogen discharge, in our experimental conditions, were N 2 (B) and N 2 (C), which emit the typical nitrogen spectral systems 1+ and 2+, respectively. The emission spectrum for the pure nitrogen plasma discharge is presented in Figure 2. The main mechanism of creating the N 2 (B) species in the discharge zone is represented by the so called pooling reaction : with the reaction rate k = cm 3 s -1 [18]. N 2 (A) + N 2 (A) N 2 (B) + N 2 (X), (3)

5 1540 L.C. Ciobotaru, I. Gruia 5 This process is followed by the radiative transition process: N 2 (B 3 Π g, v ) N 2 (A 3 Σ u +, v) + hν(1 + ), (4) with an emission probability of A 4 = [18]. Concerning the N 2 (C) active species population it was mainly created (there is also the pooling reaction mechanism) through the agency of two consecutive reactions: the electronic excitation of the nitrogen molecular energy ground-state with the reaction rate k = cm 3 s -1 [18]. the radiative transition e + N 2 (X 1 Σ g + ) N 2 (C 3 Π u ) + e (5) N 2 (C 3 Π u ) N 2 (B 3 Π g ) + hν(2 + ) (6) which has the radiative life time τ = (45.4 ± ± 0.5) 10-9 s for pressures varying in the ( ) mbar range [19]. At equilibrium, the N 2 (C) population, which is proportional to the intensity of the nitrogen second positive system, is given by relation: N 2 (C) = k e C n e [N 2 (X)] / A, (7) where k e C is the excitation coefficient by electronic collisions n e, the electronic concentration and A, the Einstein probability of a spontaneous transition between the excited states C and B of the nitrogen molecule. In nitrogen-hydrocarbon gas mixture discharges, decomposition of the hydrocarbon molecules takes place by breaking the C-H strong chemical bond. The most important plasma-chemical reactions occurring in this process are the dissociation by electronic/ excited molecular nitrogen collisions and the reactions of radicals with nitrogen atoms. The main reaction products obtained in these reactions are the CH x radicals and the NH/CN species. The emission band of the NH species, situated around λ = 336 nm, was not observed due to the great intensity of the nitrogen spectral line λ = nm which belongs to the 2 + nitrogen spectral system. The reactions forming the free carbon atoms, directly or in more steps, are possible. Also, there is probable the process of direct formation of CN, CN 2 or HCN molecules, in ground or excited states, which can interact with other particles in the afterglow. The carbon atoms could also appear in discharge as a consequence of the direct dissociation reactions of methanol/ ethanol molecules: CH 3 OH C+H 2 O+H 2 (8) C 2 H 6 OH C+ H 2 O+CH x. (9)

6 6 Study of the carbon atoms production in methanol/ethanol nitrogen flowing post-discharge plasma THE MAIN ACTIVE SPECIES AND REACTION PRODUCTS IN POST-DISCHARGE As a general observation, in the post-discharge zone there is no electric field and therefore the electronic collisions are no longer the most important creating mechanism of the active species. In these circumstances, the main active species identified in pure nitrogen post-discharge plasma containing hydrocarbon traces, identified by emission spectroscopy, are: nitrogen first positive, nitrogen second positive, nitrogen first negative CN red and CN violet systems. The intensities dependence of these spectral systems is related to several variables such as the decay time, discharge power, total pressure gas, hydrocarbon concentration, etc. In post discharge there are three significant zones: in the first one there were registered a decrease of the temperature and, subsequently of the activity of the active species in the same time with that of the energy and density of electrons, which finally implies a sharp decrease of the emission intensity. This leads to the appearance of the non-luminous second zone in which the radiative processes were quasi absent and the active species reached a minimum of their densities. After that, if the temperature continues to diminish, will appear a phenomenon of transfer between ground-state N 2 molecules which results in the new formation of active species such as: N 2 (A), N 2, and high vibration ground state nitrogen molecules (early and pink afterglow discharge) see equations. (30), (31), (34). The third zone, called the late afterglow, is mainly characterized by the dominant CN violet emission which appears together with the weaker nitrogen first positive system radiation. The emission spectrum for (nitrogen +0.1%methanol) plasma post-discharge is presented in Fig. 3. The interaction of the nitrogen active species with the hydrocarbon vapours leads to the appearance of great interest compounds for the post-discharge plasmachemical processes. Among all these reactions products an important role plays the CN red and CN violet spectral systems. A simplified kinetic model describing the CN radical reactions in nitrogen with hydrocarbon traces after-glow plasma is presented in Table 1 [20 21]. Table 1 List of CN chemical basic reactions in a simplified kinetic model of N 2 +hydrocarbon traces after-glow discharge CN chemical basic reactions Reaction constant (k) Eq. number N 2 +CN N 2 +C+N, (2.5±1) exp( 71000±6000)/T cm 3 s -1 (10) N+CN N 2 +C, cm 3 s -1 (11) C+ N 2 CN+N cm 3 s -1 (12) N+C 2 N 2 N+CN undetermined constant reaction (13) N 2 +N+CN N 2 +C 2 N cm 6 s -1 (14) C ( 3 P) + N ( 4 S 0 ) + N 2 CN (B 2 Σ +, v=7) + N 2 (9.4±2.1) cm 6 s -1 (15) N 2 (X 1 Σ + g, v 12)+CN(X 2 Σ) N 2 (X 1 Σ + g)+cn(b 2 Σ + ) (16) N 2 (X 1 Σ + g, v 4)+CN(X 2 Σ) N 2 (X 1 Σ + g)+cn(a 2 Π) undetermined constant reaction (17) N 2 (X 1 Σ + g,v 25)+CN(X 2 Σ) N 2 (X 1 Σ + g)+cn(b 2 Σ +,v 13) (18)

7 1542 L.C. Ciobotaru, I. Gruia 7 The CN radicals emit by radiative transition processes two types of spectral systems [21]: CN(A 2 Π) CN(X 2 Σ) + hν(708.9 nm)-red spectral system, Δv = 3 (19) with a radiative life time τ = 6000 ns (Fig. 4). CN(B 2 Σ + ) CN(X 2 Σ) + hν(388.3 nm)-violet spectral system, Δv = 0 (20) with a radiative life time τ = 60 ns (Fig. 5). In the given experimental conditions, namely the temperature in the lateafterglow zone of about 300 K and the hydrocarbon concentration into nitrogen bigger than %, the CN violet emission becomes dominant, change which can be observed visually by the passing of afterglow colour from red-orange to blue-violet Intensity (a.u.) nm 580.4nm 708.9nm Wavelenghts (nm) Fig. 2 Emission spectrum of pure molecular nitrogen plasma discharge at p = 4 mbar, I = 50 ma nm Intensitatea (u.a.) nm 708.nm Wavelenghts (nm) Fig. 3 Emission spectrum of (nitrogen+ 0.1%methanol) post discharge plasma at p = 4 mbar, I = 50 ma.

8 8 Study of the carbon atoms production in methanol/ethanol nitrogen flowing post-discharge plasma 1543 This change is also due to the big difference-one hundred times- between the life-time of the CN radical two spectral systems. It can be concluded that the presence of the CN radical represents a good marker for the hydrocarbon presence in a nitrogen plasma discharge. Particularly, the spectral band CN(B, v = 7 X, v = 7) having the band-head with λ = nm, was one of the most intense. Concerning the formation mechanism of this radical it could be considered either the reaction (16) or (17). Reaction (16) is a three-body reaction which has a strong dependence on the pressure value, being in a direct relation of proportionality. As the present experiment was performed at low pressures (< 14 mbar), it can be assumed that the CN(B 2 Σ + ) radical was generated via reaction (17). It is interesting to notice that the C-N bond is rarely present above 323 K. The carbon molecules emit the so-called Swan spectral bands (Δ v = 0 and Δ v =1) situated in nm spectral range (even at temperature of about 370 K) [22 24]. In the after glow-discharge zone, the carbon molecules had been reacted with the nitrogen atoms via the following reaction: N+C 2 CN+C, (21) with the reaction rate k N-C2 = (1.9±0.3) cm 3 s -1. The CH x radicals, produced in the after-glow discharge of ethanol/methanol nitrogen mixtures plasma, reacted with the nitrogen atoms through the agency of two main processes: N+CH3 HCN+H2, (22) with the reaction rate k N-CH3 = cm 3 s -1, N+CH CN+H, (23) with the reaction rate k N -CH = cm 3 s -1. The CH 2 radical does not interact with the nitrogen atoms. The CN radicals produced by the reactions (6) and (8) were quickly destroyed via reaction (12). Low temperature rate constants for the reaction (12), which was calculated using a capture model, are quite large and depend on positive temperatures in agreement to recent experiments. They were also recorded spectra of the nitrogen first positive (1 + ). This spectral system was emitted in the late post-discharge zone (in gaseous homogeneous phase) of the nitrogen plasma because of the re-association process of the nitrogen atoms generated in the active discharge by nitrogen molecules initial dissociation: N( 4 S 0 )+N( 4 S 0 )+N 2 N 2 (B 3 Π g, 11)+N 2 (24) with the reaction rate k N-N-N2 = exp(+500/t g ) cm 6 s -1.

9 1544 L.C. Ciobotaru, I. Gruia 9 The recombination process is produced on the N 2 (B, v < 12) vibration states. The N 2 (B, v 13) state is auto-dissociative, with a rate constant of s -1. The increase of the re-association rate can be explained by the abstraction reactions of adsorbed elements by the nitrogen atoms, processes which produce non-radiative species. Consequently, the atomic oxygen produced by the decomposition of hydrocarbons or by hydroxide groups does not appear in the post-discharge. Another important process present in this zone is the heterogeneous wall recombination: N(4s) + wall ½ N 2 *. (25) Intensity (a.u.) N 2 (B, 2-A,0) λ=775.3nm CN(A,X) λ=708.9nm Wavelenght (nm) Fig. 4 Emission spectrum of CN(A,X) radical in a (nitrogen-0.1% methanol) DC flowing post-discharge nm 358 nm I (a.u.) nm 388 nm CN(B-X), Δ= CH431 C CN λ (nm) Fig. 5 CN(B-X) radical emission spectrum of a (nitrogen-0.1% methanol) DC flowing post-discharge.

10 10 Study of the carbon atoms production in methanol/ethanol nitrogen flowing post-discharge plasma 1545 Within the nitrogen first positive, the most intense observed band was (11 7) corresponding to the λ = nm band head, but this is far less intense than the intensity of the nm CN(7, 7) spectral line [41]. Comparing the two bands intensities CN(7, 7) and N 2 (11, 7) it could be estimated the value of carbon atoms density which was present in the post-discharge of the nitrogen plasma. In this evaluation we have used the value of the nitrogen atoms absolute density determined by NO titration method. This method permits the determination of the atomic nitrogen concentration through the addition of another gas (namely NO) which reacts with the atomic nitrogen, the process leading to a luminescent reaction. When the atomic nitrogen flow becomes equal with the one of the NO, will produce an extinction of the luminescence. The value that has been obtained for the nitrogen atomic concentration was in the range of cm -3, which is in good agreement with literature reported data. The method is available only in the late afterglow where the reaction (24) becomes a dominant one. In the postdischarge zone could also appear the following important reactions between nitrogen molecules and ions which are summarized in the Table 2 [23, 25]. Table 2 Simplified kinetics model of reactions in hydrocarbon / nitrogen afterglow plasma Main chemical reactions Reaction constant (k) Eq. number N 2 (X 1 + g, v 1 )+N 2 (X 1 + g,v 2 ) N 2 (X 1 + g,v 1 ) +N 2 (X 1 + g,v 2 ) (26) N 2 (X 1 + g,v 12)+N 2 (X 1 + g,v 12) N 2 (X 1 + g,v = 0)+N 2 (A 3 + u ) cm 3 s -1 (27) N 2 (X 1 + g,v 16)+N 2 (X 1 + g,v 16) N 2 (X 1 + g )+N 2 (a 3 u ) (28) N 2 (X 1 +,v>24)+n g 2 (X 1 +,v>24) N g 2 (X 1 +,v-δv)+n g 2 (C 3 Π,v) u < cm 3 s -1 (29) N 2 (X 1 + g, v 32) + N 2 (X 1 + g, v 32) e + N 2 (X 1 + g ) + N x exp( 1160 )cm 3 s -1 ) (30) N 2 (X 1 + g,v>4)+n 2 (A 3 + u ) N 2 (B 3 g,v)+n 2 (early after-glow) (3.5 x 1.5) x cm 3 s -1 (31) N 2 (X 1 +, v 19)+N g 2 (A 3 + u ) N 2 (X 1 + g )+N 2 (C 3 Π u ) cm 3 s -1 (32) N 2 (A 3 + u )+N 2 (A 3 + u ) N 2 (X 1 + )+N g 2 (C 3 Π ) u 2 x cm 3 s -1 (33) N 2 (X 1 + g, v 12)+N 2 + (X 2 + u ) N 2 (X 1 + g )+N 2 + (B 2 + u ) cm 3 s -1 (34) T g Reactions (31) and (34) are of Penning type ones and appear in the so-called pink afterglow zone. The most important states involved in production of carbon atoms are of the nitrogen active species N 2 (A 3 + u ) and N 2 (a 3 u ) which appear in equations (27)

11 1546 L.C. Ciobotaru, I. Gruia 11 and (28)-because of the fact that their energies are big enough to break the strong C-H bond (6.2 ev and 8.5 ev, respectively). The intensities dependence of the two spectral bands, CN(7, 7) and N 2 (11, 7) on the percentages of ethanol/methanol added in nitrogen plasma are shown in Fig. 6 and Fig. 7. It can be observed that the N 2 (11, 7) band intensity diminishes faster in the case of the ethanol addition than in the case of the methanol, for an equal concentration of the corresponding hydrocarbon vapours. Contrary, the intensity of the CN(7, 7) band intensity was stronger in the case of the methanol addition. The band intensity I λ is related to the [X] radiative state density by the relation: I λ = (K(λ) Ay/λ) [X], (35) where λ is the wavelength of the emitted radiation, K(λ) is a factor related to the spectral response of the detection apparatus and A y is the transition probability of the radiative state X. In our case X represents the radiative states CN(7, 7) and N 2 (11, 7) respectively CN (7,7) 10 3 CN (7,7) Intensity (a.u.) 10 2 N 2 (11,7) Intensity (a.u.) 10 2 N 2 (11,7) X% ethanol Fig. 6 The intensity dependence of the CN (7, 7) and N 2 (11, 7) spectral bands on the ethanol percentage X% methanol Fig. 7 The intensity dependence of the CN (7, 7) and N 2 (11, 7) spectral bands on the methanol percentage. Thus, the carbon atoms density can be calculated from the ratio between the intensities of the two emission spectral bands CN(7, 7) and N 2 (11, 7), respectively: I CN / I N2 = k[c]/ [N], (36) where [C] and [N] are the denotations for the densities of the carbon and nitrogen atoms respectively. The calculated values of the corresponding reaction coefficients are: k ethanol = (4.8±2) 10 2 for the (N 2 +ethanol) gas mixture, k methanol = (7.9±4) 10 2 for the (N 2 +methanol) gas mixture, (for ethanol/methanol concentrations ratio < 10-2 %).

12 12 Study of the carbon atoms production in methanol/ethanol nitrogen flowing post-discharge plasma 1547 Based on these results were calculated the densities values of the carbon atoms for the two specified gas mixtures. The dependence of the carbon atoms density on the hydrocarbon vapours percentages is presented in Figure Concentration of carbon atoms (cm -3 ) ethanol methanol X% (ethanol/methanol) Fig. 8 The dependence of the atoms carbon density on the ethanol/methanol percentage. The calculated carbon atoms concentration was within the range of (2 6) cm -3 for nitrogen-methanol and ( ) cm -3 for nitrogen-ethanol mixtures, respectively. It can be observed that the carbon atoms density was one order of magnitude bigger in the case of the ethanol-nitrogen mixture than in the one of methanol-nitrogen. We can also notice the fact that the bigger part of the atomic carbon was observed in the afterglow-zone; only a small number of carbon atoms are generated in the active discharge [20]. 4. CONCLUSIONS In this paper we have studied the interaction of the active species from the nitrogen plasma discharge with the ( ) % ethanol/methanol vapours introduced in the later post-discharge zone. We have established the dependence of the spectral emission bands CN(7, 7) and N 2 (11, 7) on the methanol and ethanol concentration into nitrogen, for the two gas mixtures namely nitrogen- methanol and nitrogen-ethanol, respectively. From the ratio of the CN(7, 7) and N 2 (11, 7) two intensities spectral bands of interest, we calculated the carbon atoms concentration in the late post-discharge zone by using a value of the nitrogen atomic concentration, obtained by the NO titration method, in the range of cm -3. This concentration was found to be up to ten times bigger in case of ethanol addition comparative to methanol, namely (2 6) cm -3 for nitrogen-methanol and ( ) cm -3 for nitrogen-ethanol mixtures.

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