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

M Abdeladim, Jean-Philippe Sarrette, J Amorim, André Ricard

► To cite this version:

M Abdeladim, Jean-Philippe Sarrette, J Amorim, André Ricard. 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. Plasma Chemistry and Plasma Processing, 2022, 42, pp.1201-1210. 10.1007/s11090-022-10272-0. hal-03758406

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# 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

M. Abdeladim<fnref n="1" /> · J. P. Sarrette<fnref n="2" /> · J. Amorim<fnref n="3" /> · A. Ricard<fnref n="2" />

Received: 21 February 2022 / Accepted: 10 June 2022 / Published online: 16 July 2022
© The Author(s) 2022

## Abstract

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⁻<fnref n="3" /> s) and inside the 5 L reactor: Long time afterglow (LA), (10⁻<fnref n="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⁻<fnref n="2" /> for N/N₂, but decreased respectively from 10⁻<fnref n="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⁻<fnref n="2" /> s) to a SA(10⁻<fnref n="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⁻<fnref n="3" /> s) appeared to be more efficient for surface treatments than the LA (10⁻<fnref n="2" /> s).

**Keywords** N₂-H₂ microwave discharges · N-atoms · N₂ metastable molecules · N₂⁺ ions · NH radicals and H-atoms

## Introduction

Afterglows of N₂ flowing microwave discharges have been previously studied at medium gas pressures (1–20 Torr) for sterilization of medical instruments by N-atoms [1, 2]. The effect of the introduction of a low percentage of H₂ into N₂ on the paper surface wettability was also previously investigated in the early afterglow [3]. A slight maximum of the N-atom density in the N₂/0.4%H₂ afterglow at 2 Torr, 1 slpm, 100 Watt was correlated to an increase of surface wettability.

<figure><img src="image_2.png" /></figure>

<footnote>M. Abdeladim
mabdeladim@hotmail.com</footnote>

<footnote n="1">¹ LMSE, Faculté de Génie Electrique, U.S.T.O, BP 1505, El Mnaour-Oran, Algeria</footnote>

<footnote n="2">² UPS, INP; LAPLACE (Laboratoire Plasma et Conversion d'Energie), Université de Toulouse, 118 route de Narbonne, 31062 Toulouse, France</footnote>

<footnote n="3">³ Department Fisica, Instituto Tecnológico de Aeronautica, CTA, São José dos Campos 12228-900, Brazil</footnote>
In the present study, the early afterglow flowing from N₂/(0–2.5%) H₂ 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 N₂ active species: N₂(A), N₂ (X,v > 13) metastable molecules, N₂⁺ 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 N₂ 1st positive system at 580 nm (N₂(B,11) → N₂(A,7) vibrational band) and by the N₂ 2nd positive system at 316 nm (N₂(C,1) → N₂(B,0) vibrational band) are measured to obtain the N-atoms, N₂(A), N₂(X,v > 13) metastable molecules and N₂⁺ 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.

## Experimental Setup

The first experimental setup is reported in previous works [1–3, 5]. It is reproduced in Fig. 1.

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 [1–3]. 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.

<figure><img src="image_3.png" /><figcaption>**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</figcaption></figure>
A pink afterglow is observed in the bent part of the 18 mm tube with pure N₂ 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 × 10¹⁵ 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 N₂-H₂ 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 N₂-H₂ 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*<sub>580</sub> 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.

# Kinetic Equations of Active Species in the Mixed Gas of N₂ and H₂(0–2.5%) Early Afterglow

## N-Atom Density

The pure late afterglow emission is produced by a dominant N+N recombination.

<figure><img src="image_4.png" /><figcaption>**Fig. 2** Microwave discharge in a tube of diameter 5 mm, length 20 cm directly connected to a 5 L reactor</figcaption></figure>
The N₂ 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:

$$I_{580} = k_1 \cdot [N]^2 \quad (1)$$

$$\text{with } k_1 = c(580) \cdot V \cdot (h \cdot c/580) \cdot A(580) \cdot k_a \cdot [\text{M}] / (v_{\text{B,11}}^{\text{R}} + [\text{N}_2] \cdot k_{\text{B,11}}^{\text{N2}}) \quad (2)$$

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 $\lambda=580$ nm of tungsten lamp), $h$ and $c$ are respectively the constant of Planck and the light velocity, 580 is for $\lambda=580$ nm, $V$ the afterglow volume observed by the optical fiber of the spectrometer, $A(580)$ the Einstein coefficient of the N₂ (580 nm) transition. The term $c(580) \cdot h \cdot c/580$ is in arbitrary unit.

$v_{\text{B,11}}^{\text{R}}$ and $k_{\text{B,11}}^{\text{N2}}$ are respectively the radiative frequency and the quenching rate constant of N₂ (B,11) by N₂.

The $k_a$ coefficient is for the following recombination reaction:

$$N + N + N_2 \rightarrow N_2(\text{B}, v' = 11) + N_2 \quad (3)$$

In $k_1$, $c(580)$ is the spectral response of the used spectrometer, $A(580) = (7.8–9.6) \times 10^4 \text{ s}^{-1}$ [8, 9], $k_a = 4.4 \times 10^{-33} \text{ cm}^6\text{s}^{-1}$ [10], $v_{\text{B,11}}^{\text{R}} = (2–2.4) \times 10^5 \text{ s}^{-1}$ [8, 9] and $k_{\text{B,11}}^{\text{N2}} = 3 \times 10^{-11} \text{ cm}^3\text{s}^{-1}$ [11].

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:

$$a_{N+N} \cdot I_{580} = k_1 \cdot [N]^2 \quad (4)$$

With $a_{N+N} \cdot I_{580}$ 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 $a_{N+N} \cdot I_{580}$ 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 N₂/2.5%H₂ 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.

At 2 Torr, 0.5 slpm, N₂/1%H₂, 200 Watt, it is found that $a_{N+N} = 0.85$ and $[N] = 6 \times 10^{14} \text{ cm}^{-3}$ from Eq. (4). With $a_{N+N} = 0.85$, it can be considered that the afterglow was a nearly pure late afterglow (at 85%).
## Densities of N₂(A), N₂(X,v > 13) and N₂⁺

The line intensity ratio method [4, 12] is applied to determine the density of N₂(A) ([A]) from that of N-atoms, by comparing the I₃₁₆ and aₙₙₙ·I₅₈₀ intensities,

where I₃₁₆ is the intensity emitted from the N₂(C,v=1) radiative state. In the afterglow, the following dominant reaction is considered:

$$N_2(A) + N_2(A) \rightarrow N_2(C, v = 1) + N_2(X) \quad (5)$$

As reported in [13], the aₙₙₙ·I₅₈₀ / I₃₁₆ intensity ratio is as follows:

$$a_{N+N} \cdot I_{580} / I_{316} = k_3 \cdot ([N]/[A])^2 \quad (6)$$

The value of k₃ is 2.5 × 10⁻⁷ taken from [13].

The density of all N₂(X,v > 13) molecules ([X, v > 13]) has been obtained by comparing the fractions of late and pink afterglows in the I₅₈₀ intensity: as reported in [13], aₙₙₙ·I₅₈₀ for the late (Eq. (4)) and (1-aₙₙₙ)·I₅₈₀ for the pink where it is considered the following dominant reaction:

$$N_2(A) + N_2(X, v > 13) \rightarrow N_2(B, v = 11) + N_2(X) \quad (7)$$

It comes:

$$a_{N+N}/(1 - a_{N+N}) = k_4 \cdot [N]^2/[A] \cdot [X, v > 13] \quad (8)$$

The value of k₄ is 4–5 × 10⁻⁶ determined from [11].

The N₂⁺ ions density ([N₂⁺]) is determined by comparing the I₃₁₆ intensity from reaction (5) with the I₃₉₁ intensity coming from the N₂⁺(B,v=0) radiative state excited by the following reaction [13]:

$$N_2^+(X) + N_2(X, v > 12) \rightarrow N_2^+(B, v = 0) + N_2(X) \quad (9)$$

The I₃₉₁ / I₃₁₆ intensity ratio is defined as follows:

$$I_{391}/I_{316} = k_5 \cdot ([N_2^+][X, v > 13]/[A])^2 \quad (10)$$

where it is taken the same value for the [X, v > 12] and [X, v > 13] densities.

The value of k₅ is 1.6 × 10⁻² defined in [11].

From the N-atom density of 6×10¹⁴ cm⁻³, at 2 Torr, 0.5 slpm, for an afterglow time of (1–3)×10⁻³ s in the 5 L reactor, with N₂-1%H₂, 200 Watt, the following densities: [A]=4×10¹⁰ cm⁻³, [X, v > 13]=1×10¹³ cm⁻³ and [N₂⁺]=3×10⁹ cm⁻³ are obtained from Eqs. (6, 8 and 10).

## 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 N₂–H₂ 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 kₑ of the following full reaction:

$$N + H + M \rightarrow NH + M \quad (11)$$
<figure><img src="image_5.png" /><figcaption>Fig. 3 Spectrum of the N₂-1%H₂ afterglow in the 5 L reactor (Fig. 2) at 10⁻³ s, 2 Torr, 0.5 slpm and 200 Watt</figcaption></figure>

where M=N₂ 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(⁵Σ⁻) repulsive curve is crossing NH(A²Π) 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 comparing with the N₂/O₂ gas mixtures, the NO(B,0) level being 0.9 eV is lower than the N+O dissociation (6.5 eV), the reaction N+O+M→NO(B,0)+M is exothermic with a rate coefficient of 3×10⁻³⁴ cm⁶ s⁻¹ [6, 7].

By taking into account now the N₂(X,v)+NH→N₂+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 N₂(X,v=14–15) energy levels.

Then the following reaction will be exothermic:

$$N_2(X, v > 13) + NH \rightarrow N_2 + NH (A, v = 0) \quad (12)$$

The rate coefficient of reaction (12) is unknown. $k_f=4×10^{-11} cm^3 s^{-1}$ is chosen as for the two exothermic reactions: N₂(X, v>13)+N₂(A)→N₂+N₂(B,11) and N₂(X, v>13)+N₂⁺→N₂+N₂⁺(B) [6, 7] [12].

The I₃₃₆ intensity is then written as follows:

$$I_{336} = c(336) \cdot (hc/336)A(336) \cdot [X, v > 13] \cdot [NH] \cdot k_f / (v_{NH,A}^R + [M] \cdot k_{NH,A}^M) \quad (13)$$

where c(336) is the spectral response of the used monochromator, A(336)=νRNH,A=2 × 10⁶ s⁻¹ [16], $k_f=4 × 10^{-11} cm^3 s^{-1}$ and kMNH,A is the quenching rate of the NH(A) state by the M molecules. In [16], it is reported that kN₂NH,A<5 × 10⁻¹⁴ cm³s¹ and kH²NH,A=5 × 10⁻¹¹ cm³ s⁻¹.

The line ratio method is applied to determine the NH radical relative density from the following aₙₙₙI₅₈₀/I₃₃₆ intensity ratio, calculated with eqs. (4) and (13) at 2 Torr [13]:

$$a_{N+N} \cdot I_{580}/I_{336} = 7.5 \times 10^{-7} [N^2]/[X, v > 13] \cdot [NH] \quad (14)$$
The H-atoms are related to NH by the reaction (11) and the following relationships see [17]:

$$N + NH \rightarrow H + N_2, \text{ with } k_g=5 \times 10^{-11} \text{cm}^3\text{s}^{-1} \quad (15)$$

$$N(^2D) + H_2 \rightarrow H + NH \quad (16)$$

$$H + NH \rightarrow H_2 + N \quad (17)$$

It comes:

$$[NH] = (k_e \cdot [H] \cdot [N] \cdot [N_2] + k_h \cdot [H_2] \cdot [N(^2D)]) / (k_g \cdot [N] + k_i \cdot [H]) \quad (18)$$

with $[H_2] = [H_2^0] - 1/2[H]$.

The NH(0–0)–(1–1) bands at 336–337 nm are intense as shown in Fig. 3. The NH(1–1) band at 337 is mixed with the N₂(0–0) band at 337 nm. Consequently, the I₃₃₆ intensity is chosen alone to detect the NH radical. It is measured from the middle of the I₃₃₆ and I₃₃₇ junction.

## Gas Temperature

The gas temperature T_g in the discharges and afterglows was estimated by the P₁/P₂ 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 N₂ 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 N₂ and H₂(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(^2D) 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(^2D) densities.

In the two N₂<2.5%H₂ early afterglows of Figs. 1 and 2, identical values of N-atom densities which increased up to 0.4%H₂ from 5 to a maximum value of $7 \times 10^{14}$ cm⁻³ were found. While between 0.4 and 1.0% of H₂, 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(H₂)=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 N₂-1%H₂.

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 aₙₐₙₐₙ factor. Keeping the same densities of N-atoms, a decrease of N₂(X, v>13) density with H₂ and a reduction
<table><caption>**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 N₂-x%H₂ with x=0–2.5%, p=2 Torr, $Q_{tot}$=0.5 slpm and 200 Watt</caption><thead><tr><th>x (%)</th><th></th><th>0</th><th>0.1</th><th>0.4</th><th>1</th><th>2.5</th></tr></thead><tbody><tr><td rowspan="2">a<sub>N+N</sub></td><td>Setup of Fig. 1—10<sup>-3</sup> s</td><td>0.14</td><td>0.3</td><td>0.3</td><td>0.3</td><td>0.3</td></tr><tr><td>Setup of Fig. 2—10<sup>-3</sup> s</td><td>0.5</td><td>0.70</td><td>0.85</td><td>0.9</td><td></td></tr><tr><td rowspan="2">[N] (10<sup>15</sup> cm<sup>-3</sup>)</td><td>Setup of Fig. 1—10<sup>-3</sup> s</td><td>0.5</td><td>0.5</td><td>0.7</td><td>0.6</td><td>0.5</td></tr><tr><td>Setup of Fig. 2—10<sup>-3</sup> s</td><td>0.5</td><td>0.55</td><td>0.7</td><td>0.6</td><td></td></tr><tr><td rowspan="2">[N<sub>2</sub>(X, v>13)](10<sup>13</sup> cm<sup>-3</sup>)</td><td>Setup of Fig. 1—10<sup>-3</sup> s</td><td>5.5</td><td>3</td><td>5</td><td>6</td><td>5</td></tr><tr><td>Setup of Fig. 2—10<sup>-3</sup> s</td><td>6</td><td>2.4</td><td>2</td><td>0.9</td><td></td></tr><tr><td rowspan="2">[N<sub>2</sub><sup>+</sup>] (10<sup>10</sup> cm<sup>-3</sup>)</td><td>Setup of Fig. 1—10<sup>-3</sup> s</td><td>3</td><td>0.3</td><td>0.3</td><td>1</td><td>0.1</td></tr><tr><td>Setup of Fig. 2—10<sup>-3</sup> s</td><td>0.3</td><td>0.05</td><td>0.04</td><td>0.1</td><td></td></tr><tr><td rowspan="2">[NH] (10<sup>9</sup> cm<sup>-3</sup>)</td><td>Setup of Fig. 1—10<sup>-3</sup> s</td><td></td><td>0.15</td><td>0.13</td><td>0.14</td><td>0.15</td></tr><tr><td>Setup of Fig. 2—10<sup>-3</sup> s</td><td></td><td>0.04</td><td>0.07</td><td>0.12</td><td></td></tr><tr><td rowspan="2">[N(<sup>2</sup>D)](10<sup>11</sup> cm<sup>-3</sup>)</td><td>Setup of Fig. 1—10<sup>-3</sup> s</td><td></td><td>&lt;0.25</td><td>&lt;0.08</td><td>&lt;0.03</td><td>&lt;0.01</td></tr><tr><td>Setup of Fig. 2—10<sup>-3</sup> s</td><td></td><td>&lt;0.02</td><td>&lt;0.02</td><td>&le;0.02</td><td></td></tr><tr><td rowspan="2">[H] (10<sup>13</sup> cm<sup>-3</sup>)</td><td>Setup of Fig. 1—10<sup>-3</sup> s</td><td></td><td>&lt;0.2</td><td>&lt;0.2</td><td>&lt;0.2</td><td>&lt;0.1</td></tr><tr><td>Setup of Fig. 2—10<sup>-3</sup> s</td><td></td><td>&le;0.06</td><td>&le;0.1</td><td>&le;0.2</td><td></td></tr><tr><td rowspan="2">[H]/2[H<sub>2</sub>]<sup>0</sup></td><td>Setup of Fig. 1—10<sup>-3</sup> s</td><td></td><td>&lt;20 10<sup>-3</sup></td><td>&lt;4 10<sup>-3</sup></td><td>&lt;2 10<sup>-3</sup></td><td>&lt;0.6 10<sup>-3</sup></td></tr><tr><td>Setup of Fig. 2—10<sup>-3</sup> s</td><td></td><td>&lt;1 10<sup>-3</sup></td><td>&lt;1 10<sup>-3</sup></td><td>&lt;2 10<sup>-3</sup></td><td></td></tr></tbody></table>

of 10 times of $N_2^+$ ions density was noticed. The H-atom density was the same with 1%H₂ 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 ($\gamma=10^{-4}-10^{-5}$) in comparison to those of $N_2(X, v>13): \gamma=10^{-2}-10^{-3}$ 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 H₂ dissociation rate, the afterglows of Ar-N₂-H₂ gas mixtures of Ar-N₂-H₂ gas mixtures specifically the Ar-2%(N₂-5%H₂) has been studied [13, 20, 21]. The

<table><caption>**Table 2** A $a_{N+N}$ factors and active species densities in the $N_2$ afterglows of Fig. 1 at times $10^{-3}$ s and $10^{-2}$ s at $p=2$ Torr, $Q_{tot}$=0.5 slpm and 100 Watt</caption><thead><tr><th></th><th>a<sub>N+N</sub></th><th>[N] (10<sup>15</sup> cm<sup>-3</sup>)</th><th>[N<sub>2</sub>(X,v>13)] (10<sup>13</sup> cm<sup>-3</sup>)</th><th>[N<sub>2</sub><sup>+</sup>] (10<sup>10</sup> cm<sup>-3</sup>)</th></tr></thead><tbody><tr><td>Setup of Fig. 1—10<sup>-3</sup> s</td><td>0.14</td><td>0.5</td><td>5.5</td><td>3</td></tr><tr><td>Setup of Fig. 1—10<sup>-2</sup> s</td><td>0.85</td><td>0.5</td><td>0.5</td><td>0.04</td></tr></tbody></table>
plasma length increased from 4 cm with N₂-xH₂ 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 N₂ at 4 Torr, 1 slpm, 100 Watt, an afterglow time of 3 10⁻³ s, a N/N₂ ratio higher than 10% has been previously observed [5].

In the Ar-2%N₂-10⁻³ H₂ gas mixture at 2 Torr, 1 Slpm, 150 Watt, afterglow time of 1 10⁻³ s, it has been measured with the Fig. 2 setup in the 5 L reactor: [N]=2 × 10¹⁴ cm⁻³, [N₂, v>13]=0.4 × 10¹³ cm⁻³, [N₂(A)] = 1.7 10¹⁰ cm⁻³, [N₂⁺]=2 × 10⁹ cm⁻³, [NH]=0.2 × 10⁹ cm⁻³, [H]≤0.4 × 10¹³ cm⁻³. The corresponding [N]/[N₂] and [H]/[H⁰₂] ratios were 15% and ≤6% respectively. By comparison in N₂-(0.4–1)%H₂, 2 Torr, 0.5 slpm, 200 Watt (Table 1), there is thus a sensitive increase of the [N]/[N₂] and [H]/[H⁰₂] ratios with the 98%Ar dilution.

## Conclusion

By connecting the diameter 5 mm discharge tube directly to the 5 L reactor, an homogeneous short afterglow (SA of 10⁻³ s) in the whole 5 L reactor with the mixed gas of N₂ and H₂(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/2N₂=(5–7) × 10⁻³. By considering a rate coefficient k_c=4 × 10⁻¹¹ cm³ s⁻¹ for the N₂(X,v>13)+NH reaction, the H/2H⁰₂ 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 N₂(X, v>13) and N₂⁺ active species than in a long afterglow (LA of 10⁻² s). For the experimental conditions of N₂-1%H₂ at 2 Torr, 0.5 slpm, the N/N₂ and H/N ratios had constant values of 1% and 0.3% respectively.

To increase the N and H atom densities, an Ar-2% N₂-10⁻³H₂ gas mixture was experimented, giving in the 5 L reactor N/N₂ and H/N ratios of 7.5% and 2%, nearly one order of magnitude higher than in N₂-(0.4–1)%H₂.

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