Comparative Study of Influence of Experimental Configuration on Densities of Active Species in the Early Afterglows of N2/(0–2.5%)H2 HF Flowing Plasmas

Afterglows of mixed gas of N2 and H2(0–2.5%) flowing microwave discharges in a 5 mm diameter tube connected to a 5 L reactor via a tube of 1.8 cm diameter and 50 cm long, have been studied using optical emission spectroscopy. The obtained results at the entrance of the afterglow tube of 1.8 cm diameter: Short time afterglow (SA), (10–3 s) and inside the 5 L reactor: Long time afterglow (LA), (10–2 s) were then compared. It was found that, in N2 at 2 Torr, 0.5 slpm, the active specie density ratios had a constant value of 10–2 for N/N2, but decreased respectively from 10–3 to 10–4 for N2 (X,v > 13)/N2 and from 10–6 to 10–8 for N+2 /N2. By directly connecting the discharge tube inside the 5 L reactor, the density increases by 10 for N2 (X,v > 13) and by 102 for N2+ by changing the afterglow from LA(10−2 s) to a SA(10–3 s). Moreover and by adding 1% of H2 to N2, the N/N2 and H/H2 ratios had constant values of 1% and 0.2% respectively. The SA(10–3 s) appeared to be more efficient for surface treatments than the LA (10–2 s).

In the present study, the early afterglow flowing from N 2 /(0-2.5%) H 2 microwave plasmas is studied with two different experimental configurations, the first set up is the same as the one already reported in [4]. The second set up is having a new experimental arrangement [5] in which a direct connection with the microwave flowing plasma to the reactor chamber is achieved. With this new set-up, the time necessary for the gas to flow between the discharge and the reactor (post-discharge time) was reduced from 2 × 10 -2 s to (1-3) × 10 -3 s at 2 Torr, 0.5-1.0 slpm.
For this second configuration, it was expected to add to N-atoms other N 2 active species: N 2 (A), N 2 (X,v > 13) metastable molecules, N 2 + ions, NH radicals and H-atoms in the post-discharge chamber. The importance of two configurations i.e. different afterglow residence times which play a crucial role in the generated species were made in evidence.
Intensities emitted by the N 2 1st positive system at 580 nm (N 2 (B,11) → N 2 (A,7) vibrational band) and by the N 2 2nd positive system at 316 nm (N 2 (C,1) → N 2 (B,0) vibrational band) are measured to obtain the N-atoms, N 2 (A), N 2 (X,v > 13) metastable molecules and N 2 + ions absolute densities after NO titration to calibrate the N-atom densities [3]. From the NH(A → X) bands at 336 nm, the possibility to evaluate the NH radical and H-atom densities is analyzed by choosing the appropriated kinetic reactions at the origin of NH 336 nm emission.
The discharge is located inside a 5 mm diameter tube and a length of 20 cm after the surfatron gap. The discharge tube is connected to a bent tube of 18 mm diameter and 50 cm length before a 5 L reactor where the previous surface treatments occurred [1][2][3]. The residence time of the afterglow in the 18 mm tube diameter before the 5 L reactor is in the range of 10 -3 -10 -2 s. Fig. 1 Microwave discharge in a tube of diameter 5 mm, length 20 cm and in a post-discharge bent tube of diameter 18 mm, length 30 cm connected to a 5 L reactor A pink afterglow is observed in the bent part of the 18 mm tube with pure N 2 at 0.5 slpm, p = 5 Torr and 100 Watt. The NO titration of N-atoms in the late afterglows was performed by introducing an Ar-1.5%NO flow after the pink afterglow. At 8 Torr, 0.5 slpm and 100 Watt, z = 3 cm, a N-atom density of 1.0 × 10 15 cm −3 was previously obtained [5].
The experimental setup of Fig. 1 has been modified as shown in Fig. 2 to directly introduce the 5 mm int. diameter discharge tube inside the 5 L reactor. The afterglow residence time at the entrance of the 5 L reactor was then reduced to (1-3) × 10 -3 s [6,7] as for the early afterglow at z = 3 cm in Fig. 1.
The aim of the present study is to compare the density of the N 2 -H 2 active species, firstly in conditions of the same afterglow times (10 -3 s) and secondly with two afterglow times: 10 -3 s and 10 -2 s to check the capacity of the setup shown in Fig. 2 to treat surfaces in the reactor. The importance of the two configurations is to extend the afterglow residence time which plays a crucial role in the generated species, as seen in Table 2.
The N 2 -H 2 microwave plasma is produced by a surfatron cavity at 2450 MHz, 100-200 Watt, 0.5-2 slpm at pressure from 2 to 8 Torr. At 2 Torr, a satisfactory diffusion of the afterglow is produced inside the whole 5 L reactor.
The optical emission spectroscopy across the reactor is performed by means of an optical fiber connected to an Acton Spectra Pro 2500i spectrometer (grating 600 g/mm) equipped with a Pixis 256E CCD detector (front illuminated 1024 × 256 pixels).
The N-atom density is obtained from the I 580 measured intensity after calibration by NO titration with a mixed gas of Ar and NO(1.5%) introduced across the 18 mm diameter tube as shown in Fig. 1.

N-Atom Density
The pure late afterglow emission is produced by a dominant N + N recombination. The N 2 580 nm band head intensity (I 580 ), in arbitrary unit, was measured for constant parameters of the Acton spectrometer (grating 600 gr/mm, entrance slit width of 150 μm and integrating time of 1 s).
I 580 is related to the N-atom density [N] as follows: As mentioned in [6,7], it is expressed in k 1 : The spectral response c(580) of spectrometer (ratio of measured and true intensities at λ = 580 nm of tungsten lamp), h and c are respectively the constant of Planck and the light velocity, 580 is for λ = 580 nm, V the afterglow volume observed by the optical fiber of the spectrometer, A(580) the Einstein coefficient of the N 2 (580 nm) transition. The term c(580) . h·c/580 is in arbitrary unit. υ R B,11 and k N2 B,11 are respectively the radiative frequency and the quenching rate constant of N 2 (B,11) by N 2 .
The fraction a N+N of the N + N recombination in the I 580 intensity has been determined in [6,7] with the conditions of mixed pink and late afterglows (a N+N = 1 in pure late afterglow with a dominant reaction (3) and a N+N = 0 in pure pink afterglow with a minor reaction (3)). Therefore, Eq. (1) becomes: With a N+N ·I 580 is the fraction a N+N of the measured intensity I 580 at emission of the early afterglow.
The N-atom absolute density was calibrated by NO titration and was determined from Eq. (4) by measuring a N+N ·I 580 with constant spectrometer parameters.
The uncertainty on the N-atom density determination is estimated to be 30%. By introducing directly the discharge tube of diameter 5 mm in the post-discharge reactor of 5 L (Fig. 2), an afterglow jet is produced along the whole reactor chamber at pressures of 6-8 Torr, flow rates 0.5-1.0 slpm and injected microwave power of 200 Watt.
The jet disappeared when the gas pressure was reduced to 2 Torr in N 2 /2.5%H 2 for the 0.5 slpm flow rate and power of 200 Watt. These plasma conditions were taken in the following for an afterglow diffusion in the whole 5 L chamber.

Densities of N 2 (A), N 2 (X,v > 13) and N 2 +
The line intensity ratio method [4,12] is applied to determine the density of N 2 (A) ([A]) from that of N-atoms, by comparing the I 316 and a N+N ·I 580 intensities, where I 316 is the intensity emitted from the N 2 (C,v = 1) radiative state. In the afterglow, the following dominant reaction is considered: As reported in [13], the a N+N ·I 580 / I 316 intensity ratio is as follows: The value of k 3 is 2.5 × 10 -7 taken from [13]. The density of all N 2 (X,v > 13) molecules ([X, v > 13]) has been obtained by comparing the fractions of late and pink afterglows in the I 580 intensity: as reported in [13], a N+N ·I 580 for the late (Eq. (4)) and (1-a N+N )·I 580 for the pink where it is considered the following dominant reaction: It comes: The value of k 4 is 4-5 × 10 -6 determined from [11]. The N 2 + ions density ([N 2 + ]) is determined by comparing the I 316 intensity from reaction (5) with the I 391 intensity coming from the N 2 + (B,v = 0) radiative state excited by the following reaction [13]: The I 391 / I 316 intensity ratio is defined as follows: where it is taken the same value for the [X, v > 12] and [X, v > 13] densities.

Densities of NH Radicals and H-atoms
The kinetics reactions at the origin of the NH(A) emission shown in Fig. 3 are analyzed.
It is first considered that the NH(A) radiative states in N 2 -H 2 afterglows can be produced by the recombination of N and H atoms. However, the reaction coefficient producing the NH(A) state is unknown. Only the coefficient k e of the following full reaction: where M = N 2 as reported in [14], with k e = 5(± 3)10 -32 cm 6 s −1 .
By considering the NH potential curves [15], the NH(A,v = 0) vibrational level is almost at the same N + H dissociation energy (3.8-4.0 eV). Moreover, a NH( 5 Σ − ) repulsive curve is crossing NH(A 2 П) at 4.2 eV. As a consequence, there is a potential barrier which inhibits the production of NH(A,v = 0) by the N + H recombination.
By taking into account now the N 2 (X,v) + NH → N 2 + NH(A,v = 0) reaction, it is observed that the excitation of the NH(A,0) level from NH(X,0) needing 3.8-4.0 eV will be excited by the N 2 (X,v = 14-15) energy levels.
The line ratio method is applied to determine the NH radical relative density from the following a N+N· I 580 /I 336 intensity ratio, calculated with eqs. (4) and (13) at 2 Torr [13]:  The H-atoms are related to NH by the reaction (11) and the following relationships see [17]:  Fig. 3. The NH(1-1) band at 337 is mixed with the N 2 (0-0) band at 337 nm. Consequently, the I 336 intensity is chosen alone to detect the NH radical. It is measured from the middle of the I 336 and I 337 junction.

Gas Temperature
The gas temperature T g in the discharges and afterglows was estimated by the P 1 /P 2 intensity ratio of the first two rotational bands of the 1st positive emission at 775 nm [18]. If a hot temperature was found in the N 2 discharge (T g = 500-600 K at 8 Torr, 1 slpm [19]), the room gas temperature (300 K) was measured in the present afterglows. Consequently, the rate coefficients above are taken at 300 K as for reaction (3) which is depending on gas temperature [10].

The Experimental Results
In Table 1, the first and second lines show the results of active species density in mixed gas of N 2 and H 2 (0-2.5%) with the Figs. 1 and 2 setups for the same plasma and afterglow conditions (10 -3 s, 0.5 Slpm, 200 Watt).
The N( 2 D) and H atom densities reported in Table 1 are deduced from Eq. 18 calculations as being the upper density limits to obtain a positive value of H and N( 2 D) densities.
In the two N 2 -< 2.5%H 2 early afterglows of Figs. 1 and 2, identical values of N-atom densities which increased up to 0.4%H 2 from 5 to a maximum value of 7 × 10 14 cm −3 were found. While between 0.4 and 1.0% of H 2 , the density of the N-atoms slowly decreased. These results are similar to that of Ref. [3]. The weak maximum of the N-atom density at x(H 2 ) = 0.4% has been previously correlated to a best surface wettability.
For the other active species, lower densities were found with the Fig. 2 setup except for NH and H in N 2 -1%H 2 .
By comparing the results obtained with the setup of Fig. 1 at z = 3 cm and of Fig. 2 in the 5 L reactor at the same afterglow times of 10 -3 s (lines 1 and 2), it was observed with the setup of Fig. 2 an increase of the late afterglow condition by the a N+N factor. Keeping the same densities of N-atoms, a decrease of N 2 (X, v > 13) density with H 2 and a reduction (15) N + NH → H + N 2 , with k g =5 × 10 −11 cm 3 s −1 of 10 times of N 2 + ions density was noticed. The H-atom density was the same with 1%H 2 into N 2. Thus the Fig. 1 setup at z = 3 cm was in favour of N 2 + ions. Table 2 shows the comparison of the results in pure N 2 at 2 Torr, obtained with the setups of Fig. 1 at 10 -3 s (z = 3 cm-line 1) and at 10 -2 s (across the 5 L reactor-line 2).
In the Fig. 1 setup a constant N-atom density is kept from an afterglow time of 10 -3 s and 10 -2 s in that a huge reduction, at time 10 -2 s by a factor 10 for N 2 (X,v > 13) and 10 2 for N 2 + active species density, were observed. A long afterglow time in the Fig. 1 setup (10 -2 s) is thus detrimental to high N 2 (X, v > 13) and N 2 + active species density in the 5 L reactor. Such a result comes from the very low destruction probability of N-atoms on the tube wall (γ = 10 -4 -10 -5 ) in comparison to those of N 2 (X, v > 13): γ = 10 -2 -10 -3 for N 2 (X,v > 13) [4] and a full wall recombination for N 2 + . For a surface treatment with high densities of N 2 (X, v > 13) and N 2 + added to constant N and H atoms, the Fig. 2 setup, with an afterglow time of 10 -3 s, appears to be the most appropriate setup. Still higher N 2 + ions density should be obtained inside the 18 mm diameter tube, before the 5 L reactor, where a sample could be introduced.
To increase the H 2 dissociation rate, the afterglows of Ar-N 2 -H 2 gas mixtures of Ar-N 2 -H 2 gas mixtures specifically the Ar-2%(N 2 -5%H 2 ) has been studied [13,20,21]. The Table 1 A a N+N factors (from the I 11/9 band ratio) and active species densities along the 5 L diameter in the afterglows of N 2 -x%H 2 with x = 0-2.5%, p = 2 Torr, Q tot = 0.5 slpm and 200 Watt plasma length increased from 4 cm with N 2 -xH 2 to 20 cm with a 98%Ar dilution. Then a jet was observed at 2 Torr inside the 5 L reactor. The present optical measurement was performed 2 cm above the jet, expecting a full afterglow diffusion inside the 5 L reactor. In N 2 at 4 Torr, 1 slpm, 100 Watt, an afterglow time of 3 10 -3 s, a N/N 2 ratio higher than 10% has been previously observed [5].

Conclusion
By connecting the diameter 5 mm discharge tube directly to the 5 L reactor, an homogenous short afterglow (SA of 10 -3 s) in the whole 5 L reactor with the mixed gas of N 2 and H 2 (0-2.5%) at 2 Torr, 0.5 slpm, 200 Watt was obtained. By comparison with a 1.8 cm diameter early afterglow in the same plasma and afterglow time condition of 10 -3 s, the part of the N + N recombination in the 5 L afterglow increased, with a N-atom density unchanged and a dissociation rate N/2N 2 = (5-7) × 10 -3 . By considering a rate coefficient k c = 4 × 10 -11 cm 3 s −1 for the N 2 (X,v > 13) + NH reaction, the H/2H 0 2 dissociation rates were found to be ≤ (1-4) × 10 -3 in the same range for the two considered setup.
The purpose of using a direct connection of 5 mm diameter discharge tube to the 5 L reactor was to obtain a more efficient surface treatment in a short afterglow (SA of 10 -3 s) with more N 2 (X, v > 13) and N 2 + active species than in a long afterglow (LA of 10 -2 s). For the experimental conditions of N 2 -1%H 2 at 2 Torr, 0.5 slpm, the N/N 2 and H/N ratios had constant values of 1% and 0.3% respectively.
To increase the N and H atom densities, an Ar-2% N 2 -10 −3 H 2 gas mixture was experimented, giving in the 5 L reactor N/N 2 and H/N ratios of 7.5% and 2%, nearly one order of magnitude higher than in N 2 -(0.4-1)%H 2 .
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