WDS'08 Proceedings of Contributed Papers, Part II, 56–61, 2008.
ISBN 978-80-7378-066-1 © MATFYZPRESS
Spectroscopic Characterization of Nitrogen DC Pulsed Discharge
J. Krištof, J. Jašík, V. Martišovitš, and P. Veis
Department of Experimental Physics, Faculty of Mathematics, Physics and Informatics, Comenius University,
Mlynská dolina F2, 84248 Bratislava, Slovak Republic.
C. Foissac and P. Supiot
Laboratoire de Génie des Procédés d’Interactions Fluides Réactifs-Matériaux (U.P.R.E.S.E.A. n°3571),
Bat C5, Université des Sciences et Technologies de Lille, 59 655 Villeneuve d’Ascq, France.
Abstract. The densities of most emitting species and gas temperature (Tg) are
investigated in a nitrogen DC pulsed discharge. The discharge current and gas
pressure are varied from 70 to 150 mA and 133 to 470 Pa, respectively. Tg is
determined through rotational temperatures of first positive (1+) and second positive
(2+) emission systems. These temperatures are deduced by comparison of simulated
and measured spectra. The Tg values range from 450 to 950 K and increase linearly
with increasing the two discharge parameters studying. The densities of N2(B3Πg),
+
N2(C3Πu) and N2+(B3Σ u ) species increase with increasing discharge current. This
behaviour is coherent with a production process by direct impact excitation form the
ground state. The densities of emitting neutral species decrease with increasing
pressure due to fact that the collisional quenching starts to play a significant role at
higher pressure. An opposite trend for ionic species is observed. This fact seems to
prove that other production mechanisms are involved.
Introduction
The study of N2 discharges is currently receiving much attention, both experimental and
theoretical, due to its great impact in applications related to surface treatments such as steel surface
nitriding [Duez et al., 2000] and plasma sources of N atoms [De Souza et al, 1999]. All the
applications require high precision of plasma conditions setting. Most particularly, the knowledge of
the gas temperature is very important for understanding the physical-chemical phenomena in the
discharge. In non-equilibrium plasmas, such as moderated pressure plasmas, the gas temperature
(i.e. the kinetic temperature of heavy particles) can be determined via the rotational temperatures of
emitting species. Generally the rotational analyse of molecular emissions proves that the rotationaltranslational relaxation is sufficiently fast to equilibrate the gas and rotational temperatures.
In this context, we have investigated a nitrogen DC pulsed discharge through the densities of
emitting species and gas temperature. This latter parameter is determinate through rotational
temperatures of first positive (1+) and second positive (2+) systems. The effects of nitrogen pressure
and current discharge were studied.
Experimental set-up
A scheme of experimental set-up is shown in Figure 1. The DC discharge is maintained in a
Simax type tube (27 mm internal diameter, length 54 cm) fitted with one quartz window. A flow of
gas of N2 is from 5.6 sccm to 16.7 sccm for pressures from 133 Pa to 470 Pa in order to allow the gas
to refill completely before the next pulse during all experiments. Purity of nitrogen is 99,990%. The
gas is pumped by an oil rotary pump. The nitrogen pressure, denoted PN2, is measured by a capacitive
gauge (MKS Baratron). The pressure is set by a throttle valve placed at the outlet of the discharge
tube. The nitrogen plasma is created between two internal electrodes. The distance between the
electrodes is 16 cm. The discharge pulse is initiated by a DC voltage source providing up to 3 kV, 300
mA (Statron) using a fast switch. The values of the discharge current (denoted I) are varied from 70 to
150 mA. The discharge pulse width was 100 ms. The repetition frequency of the pulsed discharge is
1 Hz. The plasma emission is observed along the entire 12 cm length of the positive column in the
axial direction by Andor Mechelle-5000 spectrometer coupled with Andor IStar intensified camera
through optic fiber in wavelength range from 200 to 950 nm.
56
KRIŠTOF ET AL.: CHARACTERIZATION OF NITROGEN DC PULSED DISCHARGE
Figure 1. Experimental set-up: MFC – mass flow controllers, HV – high voltage generator,
S – switch, E – electrode, L – lens, W – quartz window, MKS Baratron – capacitive gauge,
spectrometer – Andor Mechelle-5000 spectrometer coupled with Andor IStar intensified camera.
Rotational temperatures of N2(B3Πg) and N2(C3Πu) species
+
The simulation of the N2(B3Πg, v’ = 2 → A3Σ u , v’’ = 0) (1+ system) and
N2(C3Πu, v’ = 0 → B3Πg, v’’ = 0) (2+ system) emission bands allows one to determine the rotational
temperatures of the N2(B3Πg) and N2(C3Πu, v’) species, denoted Tr(1+) and Tr(2+), respectively.
The spectra simulation of the N2 spectra is based on the conventional description of the diatomic
molecules spectra analysis e.g. [Herzberg, 1950; Kovacs, 1969]. For the 1+ and 2+ triplet systems,
spectroscopic constants are taken from [Roux et al., 1983] and [Roux et al., 1993], respectively, and
expressions of Höln-London factor for the considered transitions were given in [Kovacs, 1969]. In our
case, the bandpass of the spectrometer is the most important broadening factor. In calculations of
simulated spectra, we used a Gaussian apparatus function with a FWHM of 0.18 and 0.11 nm for 1+
and 2+ systems, respectively. An accurate description of the method has been made in work [Kilianova
et al., 2007].
Tr(1+) and Tr(2+) values are determined from the best fit of experimental and simulated spectra.
(see Figure 2 and 3 respectively). Typical accuracy of the temperature measurements is estimated
about ± 50 K.
e xp e rim e n ta l d ata
sim u la tio n w ith 5 5 0 K
1 .0
Intensity (a. u.)
0 .8
0 .6
0 .4
0 .2
0 .0
766
767
768
769
770
771
772
773
774
775
776
777
778
W a v e le n g th (n m )
Figure 2.
Experimental (PN2= 133 Pa, I = 100 mA and tp=100 ms) and simulated spectra of
+
N2(B Πg, v’ = 2 → A3Σ u , v’’ = 0) transition (1+ system).
3
57
KRIŠTOF ET AL.: CHARACTERIZATION OF NITROGEN DC PULSED DISCHARGE
1.0
experimental data
simulation with 650 K
Intensity (a. u.)
0.8
0.6
(1-1) band
of 2+
0.4
*
0.2
0.0
333
334
335
336
337
338
wavelength (nm)
Figure 3. Experimental (PN2= 133 Pa, I = 100 mA and tp=100 ms) and simulated spectra of
N2(C3Πu, v’ = 0 → B3Πg, v’’ = 0) emission bands (2+ system). The peak denoted by an asterisk peak
located at 336 nm correspond probably to NH(A3Π→X3Σ) emission system [Pearse, 1976], coming
from impurities and the other (at 336.5 nm) could be come from the rotational deviation from the
Boltzman distribution, assumed for the spectral simulation.
Results
Gas temperature
The dependence of rotational temperatures on gas pressure and discharge current is shown in
Figure 4 and 5, respectively. According to uncertainties on measurements, the Tr(1+) and Tr(2+) values
are close from each other. Consequently, the average value between these two temperature values can
be roughly assimilated to the gas temperature, Tg.
In our experimental conditions, the gas temperature values range from 450 to 950 K and increase
linearly with increasing pressure with a slope about 0.4 K/Pa (see Figure 4). The same behaviour is
observed with increasing current. In this case, the slopes about 1.85 and 3.4 K/mA are deduced from
+
Tr(1 )
+
Tr(2 )
700
+
Tr(1 ) and Tr(2 ) (K)
800
+
600
500
100
200
300
400
500
pressure (Pa)
Figure 4. Pressure dependence of the rotational temperatures of N2(B3Πg) and N2(C3Πu) states. The
solid line corresponds to linear fit of the average temperature.
58
KRIŠTOF ET AL.: CHARACTERIZATION OF NITROGEN DC PULSED DISCHARGE
1100
+
Tr(1 ) - 400 Pa
1000
+
Tr(2 ) - 400 Pa
+
Tr(1 ) - 133 Pa
+
Tr(2 ) - 133 Pa
800
+
Tr(1 ) and Tr(2 ) (K)
900
700
+
600
500
400
300
50
75
100
125
150
175
current (mA)
Figure 5. Current dependence of the rotational temperatures of N2(B3Πg) and N2(C3Πu) states. The
solid lines correspond to linear fits of the average temperature. Typical error bars of 50 K have been
assigned to each set of data.
linear fits with the pressure 133 and 400 Pa, respectively (see Figure 5). These results are in good
agreement with the measurements of Tr(2+) obtained by [Kylián et al, 2004] for a DC glow discharge
sustaining in similar conditions.
Densities of emitting species
The dependence of the band heads intensities of the nitrogen emission systems, i.e.
+
N2(B3Πg, v’ = 2 → A3Σ u , v’’ = 0) transition (noted I(1+)), N2(C3Πu, v’ = 0 → B3Πg, v’’ = 0) transition
+
+
(noted I(2+)) and N2+(B3Σ u , v’ = 0 → X2Σ g , v’’ = 0) transition (noted I(1-)), are plotted versus
discharge current and gas pressure in Figure 6 and 7, respectively.
The densities of three considered species increase with increasing discharge current. This result is
coherent with the fact that the main production process of the emitting species is the direct electron
Intensity (a. u.)
1000000
100000
+
I(1 )
+
I(2 )
I(1 )
10000
50
75
100
125
150
175
current (mA)
Figure 6. Current dependence of the three nitrogen emission systems (see text) for PN2 = 400 Pa
and tp = 100 ms.
59
KRIŠTOF ET AL.: CHARACTERIZATION OF NITROGEN DC PULSED DISCHARGE
Intensity (a. u.)
1000000
+
I(2 )
+
I(1 )
I(1 )
100000
10000
100
150
200
250
300
350
400
450
500
pressure (Pa)
Figure 7. Pressure dependence of the three nitrogen emission systems (see text) for I = 100 mA and
tp = 100ms.
impact excitation from the ground state N2(X1Σg+). We can note that the same behaviour is observed
with the gas pressure about 133 Pa. For the discharge current about 70 mA and 150 mA, the ratios
I(1+):I(2+):I(1-) are 1:0.2:0.04 and 1:0.4:0.06, respectively.
As shown in Figure 7, the densities of N2(B3Πg) and N2(C3Πu) species decrease with increasing
gas pressure while the density of nitrogen ion increases weakly. The reduced electric field E/N (with N
denoting the total gas density) is decreased by a pressure increase [Coitout, 1993]. Consequently, at
low pressure (i. e. increase of E/N), the production of N2(B3Πg) and N2(C3Πu) species by electron
impact on N2(X1Σg+) molecules in the discharge zone is more important than at high pressure where
+
collisional quenching also takes a significant role. The opposite but smooth behaviour of N2+(B3Σ u )
+
state seems to prove that an other production mechanism is involved like ionisation via N2(A3Σ u ),
−
N2(a’1Σ u ) or N2(a1Πg) metastable states.
Conclusion
In this work, spectroscopic diagnostics of nitrogen DC pulsed discharge was performed. The
relative densities of the most emitting species and the rotational temperatures of first and second
positive nitrogen systems have been studied. The influence of the discharge current and gas pressureon
these latter quantities has been investigated. For all considered plasma parameters, the evaluated
temperatures from 1+ and 2+ emission systems are relatively closed one to another. So, we can
cnoclude that obtained values give the real gas temperature. This one ranges from 450 to 950 K and
increases with increasing pressure and discharge current. rotational temperatures. The densities of
+
N2(B3Πg), N2(C3Πu) and N2+(B3Σ u ) species increase with increasing discharge current implying that
these species are mainly created by direct electron excitation from the nitrogen ground state. The
densities of N2(B3Πg) and N2(C3Πu) species are decreasing with increasing pressure because
+
collisional quenching starts to play a significant role at higher pressure. Density of N2+(B3Σ u ) is
weekly increasing with increasing pressure because other production mechanism is involved.
Acknowledgments. This research was sponsored by the Science and Technology Assistance Agency under
the contract No. APVV-0267-06 and APVV-0544-07, by the Scientific Grant Agency of Slovak Republic No.
VEGA 1/3044/06, 1/4015/07 and 1/0609/08, Slovak Academic Information Agency (SAIA, N.O.) and Comenius
University UK/363/2008. CF thanks NSP SR for mobility fellowship.
60
KRIŠTOF ET AL.: CHARACTERIZATION OF NITROGEN DC PULSED DISCHARGE
References
Coitout H., PhD Thesis, Université Paris-Sud, Orsay, France, 1993.
De Souza A. R., Digiacomo M., Muzart J. L. R., Nahorny J. and Ricard A., Eur. Phys. J. A 5, 185, 1999.
Duez N., Mutel B., Vivien C., Gengembre L., Goudmand P., Grimblot J., Surf. Coating Technol., 125, 79, 2000.
Herzberg G., “Molecular spectra and molecular structure. I. Spectra of diatomic molecules’’, Van Nostrand
Company, Inc., Princeton, 1950.
Kylián O., Kaňka A. and Hrachová V., Czech. J. Phys. 54, 357, 2004.
Kilianova A., Foissac C., Dupret C., Supiot P. and Veis P., in WDS'07 Proceedings of Contributed Papers: Part
II - Physics of Plasmas and Ionized Media (eds. J. Safrankova and J. Pavlu), Prague, Matfyzpress, pp. 175180, 2007.
Kovacs I., “Rotational Structure in the Spectra of Diatomic Molecules”, Adam Hilger Ltd., London, 1969.
Pearse R. W. B. and Gaydon A. G.:“The Indentification of Molecular Spectra”, Chapman and Hall, London
1976.
Roux F., Michaux F. and Verges J., J. Mol. Spect., 97, 253, 1983.
Roux F., Michaux F. and Vervloet M., J. Mol. Spect., 158, 270, 1993.
61