Wall material interaction with the active species in a [627319]

Wall material interaction with the active species in a
nitrogen emethane D.C. flowing discharge
L.C. Ciobotaru*, D.S. Popa
National Institute of Lasers, Plasma and Radiation Physics, Magurele, Ilfov, 077125, Romania
article info
Article history:
Received 6 November 2013Received in revised form6 June 2014
Accepted 8 June 2014
Available online 11 July 2014
Keywords:
Methane addition
Active speciesNitrogen first positive system
Recombination probabilityabstract
The in fluence of a small quantity of methane on the behaviour of radiative species in a (N 2eCH4) D.C.
flowing discharge at values of pressures within the range (0.2 e4) mbar has been studied by optical
emission spectroscopy (OES). The active species identi fied in the discharge zone were the radiative
species N 2(B) and N 2(C). A signi ficant decrease of N 2(B) active species population, due to the quenching
mechanisms was seen while the N 2(C) active species population increased. The atomic nitrogen con-
centration was strongly reduced on the addition of hydrogen. The main product of the plasma echemical
reactions appearing in the (N 2eCH4) mixture D.C. flowing discharge was identi fied as being the cyano
radical, whose concentration dependence on the experimental conditions was also studied. The in fluence
of the wall material on the active species concentration, namely the nitrogen atoms in the post-discharge
zone, was also studied by OES to determine the de-excitation process of the atomic nitrogen for Te flon,
Aluminium, Plastics and Copper. Using a simple method based on the radiation emitted by the nitrogen
first positive system, the de-excitation and the recombination probabilities at the wall were also
calculated.
©2014 Elsevier Ltd. All rights reserved.
1. Introduction
The nitrogen mixtures post-discharge plasma was the subject of
a great industrial interest on different areas, such as nitriting [1],
enhancement of polymer printability, adhesion properties [2e4],
remote plasma enhanced chemical vapour deposition of thin
nitride films[5]etc. Thanks to the large life etime of the dissociated
nitrogen atoms (about 10 s), a homogenous concentration of active
species can be obtained in a larger volume discharge of about one
cubic metre. Another interesting quality of the nitrogen mixtures
post-discharge plasma is represented by the non-equilibrium
thermodynamic character which results in a high ef ficiency of
producing and sustaining plasma echemical reactions involved in
metal surface treatment (e.g. Penning ionization, energy transfer,
molecular dissociation).
For these reasons, in the last 20 years, many studies were
dedicated to the flowing glow/after-glow discharge of a nitrogen-
methane plasma treating different aspects of both fundamentaland applicative fields such as: measurements of the density of
carbon and of active species, mechanisms of methanedecomposition, atmospheric chemistry, kinetics of reactions, ki-
netic temperature determination, surface modi fication and film
deposition and others [6e10].
During industrial nitriding of steel samples, oxidation processes
take place due to the inherent presence of oxygen within the re-
action chamber. In order to prevent surface oxidation of the sub-
strates and to improve the quality of the treated (nitrided or
deposited) surfaces, it was necessary to introduce small quantities
of pure hydrogen or hydrogen-containing compounds, such
as methane, into the nitrogen plasma. As a result, the addition of
hydrogen in nitrogen discharge containing oxygen gives rise to the
appearance of dissociation processes of nitrogen and oxygen mol-
ecules which leads to the formation of chemical species such as NO,
OH, NH and water vapours, avoiding formation of metal oxides. In
our previous papers [11,12] , notable changes in the active species
population were reported to appear in nitrogen D.C. flow dis-
charges on addition of small amounts of hydrogen/methane. For
instance, increases of the N
2(C) population as high as 6 times were
recorded for (N 2exH2) gas mixtures, with “x”ranging between
(0.1e0.5)% [13].
Another important aspect of the nitriding process concerns the
interaction of the far-off after-glow discharge tube wall with the
nitrogen active species which are present in this zone. It is well
known the fact that in the after-glow discharge, because of the*Corresponding author.
E-mail address: catalinaciobotaru@yahoo.com (L.C. Ciobotaru).
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0042-207X/ ©2014 Elsevier Ltd. All rights reserved.Vacuum 110 (2014) 183 e189

absence of an electric source, there are no more generating excited/
ionized species in plasma. After the flow to the sample processing
zone, namely the far-off after-glow discharge zone, only the excited
molecules and atoms nitrogen rest active and could interact with
the material wall.
The present paper deals with the identi fication of these inter-
acting mechanisms and the calculation based on a simple method
of the corresponding reactions rates. The study of this process could
supply us with valuable insight about suitability of certain wall
materials for industrial processing of interest.
2. Experimental method
A schematic view of the experimental system is presented in
Fig. 1 . The discharge was ignited in a Pyrex tube with a 22 mm inner
diameter and a 24 mm outer diameter, between two side-armed
identical hollow Ni eCr cylinder electrodes of 10 mm diameter,
spaced at 400 mm distance. The first electrode, in the direction of
gasflow, was the anode. Spectrally pure gases (99.98%) were used.
Gasflows were regulated and measured by MKS mass flow-metres
with a measurement scale of maximum 1500 sccm for nitrogen and
10 sccm for methane and the pressure was measured at the end of
the discharge tube. 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 zone had two distinct parts: one part with
the same diameter as the discharge tube and a total length of
600 mm, the other part had an inner diameter of 80 mm and an
outer diameter of 82 mm, with the total length of 450 mm (the so-
called far-off after-glow zone).
The second part of the tube was used to study the impact of
transport conditions and wall material on the active species. The
materials chosen in this study were: Te flon, Aluminium, Plastic and
Copper. 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: a Varian-Techtron spectro-
photometer (S) equipped with a grating of 1200 grooves/mm, a
5 mm wide slit and 300 ÷860 nm measurement range, a Hama-
matsu R 585 (PM) Photomultiplier, a quartz optical fibre (OF) and a
recorder (R). The spectral resolution of the system was 0.05 nm.
Alternatively, it was used an Optical Multichannel Analyzer (OMA)
of 220 e900 nm spectral range and a resolution of 1.5 nm. The
integration time could be varied in the 200 e1000 ms range,
depending on the brightness of the emitted radiation.A D.C. power generator able to supply up to 5 kV/0.2 A, adjust-
able power up to 1 kW was used. The experimental parameters for
the (N 2eCH4) discharge were the following:
/C15Total pressure of the gas mixture: 0.2 e4 mbar.
/C15Nitrogen flow rate: 600 e1000 sccm.
/C15D.C. discharge current intensity: 50 e100 mA.
/C15Methane/(nitrogen țmethane) flow rate ratio: 0 e0.2 (%).
/C15Electrical power: 300 e360 W.
An image of the glow discharge zone is presented in Fig. 2 .
3. Results and discussion
3.1. Main active species and reaction products
Research on identi fication of active species, reaction products
and reaction mechanisms in nitrogen emethane plasma discharges
has previously been conducted [14e17]. The measurements were
made in an electric current constant intensity mode, using different
values. The main identi fied nitrogen active species are the
following:
➢N2(B) and N 2(C) radiative species belonging to the N 2neutral
molecule;
Fig. 1. Experimental device (glow and after-glow discharge zones). (D1, D2 eflow-metres: V1, V2, V3 evalves; S espectrophotometer; PM ephotomultiplier; R erecorder; OF e
optical fibre; G ethermal gauge; A eanode; K ecathode; OMA eoptical multichannel analyzer).
Fig. 2. The N 2eCH4flowing glow discharge zone (photo-image).L.C. Ciobotaru, D.S. Popa / Vacuum 110 (2014) 183 e189 184

➢the radiative specie N 2ț(B) which belongs to the N 2țmolecular
ion;
➢the vibrational excited ground-state molecular nitrogen eN2(X,
v);
➢the metastable N 2(A);
➢the atoms of nitrogen.
The active species N 2(X), N 2(A) and N are not distinguished
because their spectral emission does not appear in the
(300e900) nm range. The N 2ț(B) 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 2țmolecular ion is
usually present only near the cathode. The effective radiative spe-
cies identi fied in a pure molecular nitrogen discharge, for our
experimental conditions, were N 2(B) and N 2(C) which emit the
nitrogen spectral systems 1ț, respectively 2ț. Characteristic spectra
for pure nitrogen and for the two systems are presented in Fig. 3 (a
and b).
On addition of 0.1% methane to pure nitrogen, a signi ficant
change in spectral line intensities of the two nitrogen spectral
systems was observed (see Figs. 3 and 4 ).
The presence of hydrogen molecules in the discharge is the
result of the following equation:
CH4țN2ðAȚ/H2țN2țCH2 (1)
The constant reaction k 1¼1.35/C210/C013cm3s/C01(estimated) [18]
Fig. 4 shows the emission spectrum of (N 2ț0.1% CH 4) gas
mixture discharge. It was observed that the 1țsystem intensity hadasignificant decrease which is the result of the quenching of the N 2
(B) active species population. Consequently, this behaviour is due to
the quenching reaction of the N 2(A) metastable with the methane
molecule. Extinction of the N 2(A) metastable has taken place via
the following excitation transfer reaction:
300 400 500 600 700 800 9001000020000300004000050000Intensity (a.u.)
Wavelenghts (nm)pure N2 ; Q= 1000 sccm
600 650 700 750 800 850 9001000020000
1+
Δv = 11+
Δv = 41+
Δv = 3
1+
Δv = 2I (a.u.)
(nm)pure N 2
300 320 340 360 380 400 420 440 46040006000
2+
Δv = -32+
Δv = 12+
Δv = 0
2+
Δv = -1
2+
Δv = -2I (a.u.)
(nm)pure N 2ab
Fig. 3. The emission spectrum of pure molecular nitrogen. (a) The spectral system 1țin a pure molecular nitrogen discharge at Q¼1000 sccm. (b) The spectral system 2țin a pure
molecular nitrogen discharge at Q¼1000 sccm.200 300 400 500 600 700 800 90050001000015000200002500030000I (a.u. )
(nm)N2 + 0.1% CH4
315.8 nm336.4 nm
356.88 nm
387.73
Fig. 4. Spectrum of a (nitrogen ț0.1% methane) gas mixture discharge at
Q¼1000 sccm.L.C. Ciobotaru, D.S. Popa / Vacuum 110 (2014) 183 e189 185

N2ðAȚțCH4/N2ðXȚțCH4 (2)
having a reaction rate k2of 1 /C210/C014cm3s/C01[18].
As a result, the main mechanism of creating the N 2(B) species
became inef ficient:
N2A3Sțu;v0¼0;1/C16/C17
țN2A3Xț
uv¼0;1/C16/C17
/N2B3Y
gv¼1-11/C16/C17
țN2XðȚ “the pooling reaction ” ðȚ
(3)
with the reaction rate k3¼1.1/C210/C09cm3s/C01[19,37] .
Thus, reduction of the N 2(A) metastable concentration on
addition of a small quantity of methane to molecular nitrogen had a
significant effect on the reduction of the N 2(B) species
concentration.
Fig. 5 presents the dependence of the N 2(B) species concen-
tration on the concentration of methane. As it can be observed, the
N2(B) concentration is directly proportional to the1țnitrogen
system intensity . This is due to the radiative transition process:
N2/C16
B3Pg;v0/C17
/N2/C16
A3Sț
u;v/C17
țhn/C16
1ț/C17
(4)
with an emission probability of A4¼2.4/C2105[20].
In the (N 2ț0.1% CH 4) gas mixture discharge, a signi ficant in-
tensity increase of the 2țnitrogen system (300e420 nm, the head
band at l¼337.1 nm) was observed ( Fig. 6 ). This fact is bound to the
increase of the N 2(C) active species population through the agency
of two consecutive reactions:
– the electronic excitation of the nitrogen molecular energy
ground-state
ețN2/C16
X1Sț
g/C17
/N2/C16
C3Pu/C17
țe (5)
with the reaction rate k5¼7.5/C210/C012cm3s/C01[21]
– the radiative transitionN2/C16
C3Pu/C17
/N2/C16
B3Pg/C17
țhn/C16
2ț/C17
(6)
with the radiative life-time t¼(45.4 ±4.0/C01.5±0.5) /C210/C09s for
pressures varying in the (10/C03e1.0132 /C2103) mbar range [22].
At equilibrium, the N 2(C) population, which is obviously pro-
portional to the intensity of the nitrogen second positive system, is
given by the relation:
N2ðCȚ¼kC
ene½N2ðXȚ/C138=A (7)
where kC
eis the excitation coef ficient by electronic collisions, ne, the
electronic concentration and A, the Einstein probability of a spon-
taneous transition between the excited states C and B of the ni-
trogen molecule.
The behaviour of N 2(C) active species on addition of small
amounts of methane can be explained by the fact that the kC
eco-
efficient is very sensitive to the modi fication of the reduced electric
field of the positive column which is in the (6 e10/C210/C016Vc m2)
range, in the speci fied experimental conditions [23]. In the same
time, the reduced electric field (E/N) increases up to 30 e47% on
addition of hydrogen due to the increase of both electric field and
gas temperature, as shown in our previous paper [13]. These two
processes, namely the increase of the electric reduced field and the
strong dependence of the electronic collisions excitation coef ficient
on the reduced electric field, lead to a signi ficant increase of the
N2(C) concentration, and consequently to the nitrogen second
positive system intensity. More details regarding the reaction
mechanisms involved in this behaviour (ionization by associative
processes and the direct ionization) are covered by Refs. [24e26].
This phenomenon was also observed in nitrogen ehydrogen gas
mixture discharges and can be explained by reaction mechanisms
described in the following.
Dissociation of a nitrogen molecule in atoms is produced, in the
given experimental condition, mainly by V eV (vibration evibra-
tion) transfer, as a transition from the last bond level v¼45 on a
pseudo-level situated in the continuum zone, via collision with a
vibrational excited molecule, as follows:
N2ðX;vȚțN2ðX;v¼45Ț/N2ðX;v/C01ȚțNțN (8)
The presence of pure hydrogen or compounds containing
hydrogen, even in a very small quantity, produces a strong de-
activation of the vibrational higher energy levels via V-T transfer
(vibration-translation) between molecular nitrogen and molecular0.00 0.05 0.10 0.15 0.20789101112131415B
Linear Fit of Data1_B
Linear Fit of Data1_BN2 (B) concentration (%)
CH4 concentration(%)
Fig. 5. Concentration dependence of N 2(B) species vs. CH 4concentration in a (CH 4ț
N2) mixture plasma discharge.0.00 0.05 0.10 0.15 0.200102030405060N2(C) concentration (%)
CH4 concentration ( % ) experimental data
fitting curve
Fig. 6. The concentration dependence of N 2(C) species on CH 4concentration.L.C. Ciobotaru, D.S. Popa / Vacuum 110 (2014) 183 e189 186

hydrogen, a process which becomes dominant for the high vibra-
tion numbers:
H2țN2ðX;vȚ/H2țN2ðX;v/C01Ț (9)
The result is a strong diminution of the dissociation rate of the
molecular nitrogen on addition of increasing amounts of hydrogen,
this resulting in a decrease of the concentration of nitrogen atoms.
In nitrogen-methane gas mixture discharges, decomposition of
the methane molecule takes place by breaking the C eH strong
chemical bond [20,27,28] . The most important plasma echemical
reactions occurring in this process are the following:
– Dissociation by electronic collisions.
– Dissociation by excited molecular nitrogen collisions.
– Reactions of radicals with nitrogen atoms.
The main reaction products obtained in these reactions are the
CHxradicals and the NH/CN species. The emission band of the NH
species, situated around l¼336 nm, was not observed due to the
great intensity of the nitrogen spectral line l¼337.1 nm which
belongs to the 2țnitrogen spectral system. Nevertheless, the violet
emission of the CN radical occurring in the radiative transition
process was observed:
CNðBȚ/CNðXȚțhn (10)
with a probability of emission of A4¼1.4/C2107[20]
As could be observed from Fig. 7 , the CN radiation emitted due to
the transition Dv¼0 (B, v0¼7/C0X,v00¼7), having the band-head
with l¼388 nm, was very intense and appeared as a result of
the following recombination process:
CțNțN2/CNðB;7ȚțN2 (11)
with k11¼(9.4±2.1) /C210/C033cm6s/C01[20].
The spectrum presented in Fig. 7 is a consequence of the CN
radical generation reactions, as follows:
CH2țN/CNțH2 (12)
with the reaction rate k12¼1.6/C210/C011cm3s/C01[29].CHțN/CNțH (13)
with the reaction rate k13¼2.1/C210/C011cm3s/C01[29].
Obviously, the CH 2and CH hydrocarbon radical concentrations
were directly proportional to the methane concentration.
An increase of the CN radical emission band intensity Dv¼0
(which is proportional to the CN radical population) with
increasing methane concentration was observed and presented in
Fig. 8 .
3.2. The in fluence of wall material on the behaviour of active
species
The wall material of the after-glow discharge tube has a decisive
influence on the nitrogen active species evolution due to their
interaction. This fact could bring important consequences for the
kinetics of the after-glow flowing discharge [30e36], where the
active species are transported from the source (electric discharge)
to the place where the treatment of the sample is accomplished.
The result of this transport could be strongly affected by the
interaction between the active species and the wall of the reaction
chamber.
In the post-discharge zone there is no electric field and therefore
the electronic collisions are no longer the main creating mechanism
of the active species. For this reason, the excited species disappear
either by radiative processes or by excitation transfer to the atoms
and molecules of nitrogen in the ground-state. The life-time of
these species is of the order of milliseconds and, as the velocity is
that of the thermal gas, all excited species in the post-discharge
disappear within a few mm. As the treatments in post-discharge
usually are made hundreds mm away from the source of activespecies, it is obvious that in this zone remain, as active species, only
nitrogen atoms obtained by dissociation, and vibrational excited
nitrogen molecules in energy ground-state.
The present paper presents the de-excitation process of atomic
nitrogen. The spectral emission of a far-off after-glow discharge was
investigated by placing the samples used for plasma treatment at a
certain distance from the end of the discharge tube (more than
600 mm). The inner wall of the discharge tube was entirely covered
with the material of interest for our studies, rolled into a cylinder of
200 mm length. The emission spectra were recorded by means of
an optical fibre which was alternatively placed at the entrance and0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0. 16 0.18 0.20051015202530CN concentration (%)
CH4 concentration ( % ) experimental data
fitting curve
Fig. 8. Concentration dependence of CN radical on CH 4concentration.
Fig. 7. The CN(B-X) radical emission spectrum of a (nitrogen- 0.1% methane) DC
flowing discharge.L.C. Ciobotaru, D.S. Popa / Vacuum 110 (2014) 183 e189 187

respectively at the exit of the covered zone. Within the 1țsystem
spectrum, the most intense observed band was (11-7) corre-
sponding to the l¼580.46 nm band-head. The intensity of this
band decreased exponentially when the discharge tube was
covered inside, due to the de-excitation process at the wall. In order
to compare the de-excitation function on different wall materials,
the ratios RM¼IM
11-7=IPx
11-7(where IM
11-7and IPx
11-7are the intensities
of the (11-7) bands) with, and also without the material placed
inside the discharge tube, were calculated. In the first case, the
active species interacted only with the Pyrex glass. The results
obtained are presented in Table 1 . When using Te flon as interacting
material, the data reveal the fact that the interaction between ni-
trogen atoms and the Te flon was of the same order as the inter-
action with the Pyrex glass. On the contrary, on interaction with
plastics, aluminium and especially copper, the nitrogen atoms
disappeared. These results could be explained by considering the
nitrogen atoms recombination kinetics in the after-glow discharge.
Based on the values listed in Table 1 , de-excitation probabilities and
nitrogen ewall material interaction process rates were calculated.
The nitrogen atoms generated in the discharge by nitrogen mole-
cule dissociation processes disappeared in the far-off after-glow
discharge zone because of the two main quenching mechanisms, as
follows:
– The recombination process in gaseous homogeneous phase:
NțNțN2/N2/C16
B;v0¼11;v00¼7/C17
țN2ðXȚ (14)
[29] [37] [38] , with the reaction rate k14¼8.3$10/C034exp ( ț500/
Tg)c m6s/C01.
– The heterogeneous wall recombination:
Nð4sȚțwall/1=2N*
2 (15)
with the reaction rate kp¼5$10/C02s/C01.
The reaction coef ficient k pof this process is de fined by the
relation:
kp¼gv
2R(16)
where gis the recombination probability at the wall, vis the nitrogen
atoms thermal velocity and Ris the discharge tube radius (2 R¼the
hydraulic diameter). The thermal velocity was calculated with the
classical formula:
v¼/C08kTg/C14pm/C11=2(17)
where mis the nitrogen atomic mass, kis the Boltzmann constant
andTgthe gas temperature. The recombination probability gde-
pends on the nature, the state and the temperature of the wall
surface. At the room temperature Tg¼300 K, the calculated thermal
velocity is v¼500 m/s. For Pyrex glass, the values of gvaried in the
(0.65 e30)10/C05range, the most used value in literature being2$10/C05[38]. Using these values, a calculated recombination prob-
ability of kp¼5$10/C02s/C01was obtained for the interaction with the
Pyrex tube wall. On the other side, the temporal variation of the
nitrogen atom density resulted from the relations (14) and (15) is
described by equation:
d½N/C138=dt¼/C0kN½N/C1382½N2/C138ekp½N/C138 (18)
from which, after integration, the following expression was
obtained:
1/C14½N/C138z¼1/C14½N/C138zoexp/C0kpz=v/C1ț/C0kN½N2/C138/C14kp/C1/C2exp/C0kpz=v/C1/C01/C3
(19)
where zand zowere the distances to the entrance and, respectively,
to the exit of the covered zone, measured from the end of the
discharge tube. In the mentioned experimental conditions, for
pressures in the range (0.2 e4) mbar, the molecular nitrogen den-
sity was [N 2]~ 1 017cm/C03and for the residence time similar of those
from Table 1 , the equation (19) can be approximated as:
1/C14½N/C138z¼1/C14½N/C138zoțkN½N2/C138/C14kp (20)
This approximation permitted calculation of the ratio RMas:
RM¼IM
11-7.
IPx
11-7¼exp/C0/C02kpz=v/C1(21)
Calculation of recombination probabilities for aluminium and
plastics is based on the experimental values of RM(11-7) ratio pre-
sented in Table 1 . By comparing our calculated recombination
probability with published data for aluminium and aluminium-
oxide [39] it can be deduced that the layer deposited on the covered
wall was made of aluminium and not of aluminium-oxide. The
corresponding reaction rates can also be calculated using these
values for recombination probabilities in the case of aluminium and
also plastics.
By comparing published and calculated values of recombination
probabilities [39] and considering the fact that the intensity I 11-7
values obtained were exceedingly small ( R11-7<0.1)e(equation
(21)), it can be deduced that the layer formed on the wall was made
of copper and not of copper eoxide ( Table 2 ).
In fact, these calculations represent a simple method to deter-
mine the nature of the layer, which could be formed on the device
wall during the discharge, by using some physical parameters easy
to access, such as the distance measured from the end of the
discharge tube and the thermal velocity of the nitrogen atoms. This
method provides the advantage to be non-expensive and rather
accurate and the results obtained are in good agreement withpublished data.
4. Conclusion
The concentrations of radiative species N
2(B) and N 2(C) (which
emit the 1țand 2țnitrogen spectral systems) and also those of CN
species and CH xradicals are very sensitive to the addition of even
small amounts of methane. On addition of a quantity of up to 0.2%Table 2
Comparison of calculated and published data ( gand kp).
Wall mater g(calc.) g(publ.) kp(calc.)
Plastics (3 ±1)/C210/C040.75±0.37 s/C01
Al (6 ±3)/C210/C04(6.5±2.5) /C210/C041.50±0.70 s/C01
Al2O3 (1.6±0.4) /C210/C04
Cu (4.0 ±2)/C210/C026.8/C210/C02
Cu2O2 /C210/C02e10/C03Table 1
The RMratio for different wall materials ( Q¼theflow rate, Dt¼the residence time).
Q(cm3s/C01) Dt(s) Te flon Plastics Al Cu
2.5 1 /C210/C011.02 0.41 0.2 0
5.0 5 /C210/C021.00 0.43 0.2 0
7.5 3.3 /C210/C021.04 0.55 0.23 0
10.0 2.5 /C210/C021.15 1.02 0.45 0L.C. Ciobotaru, D.S. Popa / Vacuum 110 (2014) 183 e189 188

methane to nitrogen, a decrease of N 2(B) concentration to a half of
the initial value and a simultaneous six-times increase of the con-
centration of N 2(C) active species were observed.
The de-excitation process of the atomic nitrogen at the wall in
the far-off after-glow discharge zone proved to have a signi ficant
influence. For instance, the loss of nitrogen atoms by re-association
at the wall is a process strongly dependent on the nature of the
wall. For Pyrex and Te flon materials, this process had negligible
effects due to the very small dissociation probability ( g<10/C05). On
the contrary, for wall materials consisting of Aluminium or Copper,
destruction of nitrogen atoms took place in a signi ficant proportion
(g>10/C03).
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