22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Measurement of N(4S) and N(2P) absolute densities by optical emission
spectroscopy
A. Salmon1,2, G.D. Stancu1,2, M. Cirisan3 and C.O. Laux1,2
1
CNRS, UPR 288, E.M2.C, Grande Voie des Vignes, 92295 Châtenay-Malabry, France
2
Ecole Centrale Paris, Grande Voie des Vignes, 92295 Châtenay-Malabry, France
3
PlasmaBiotics, 215 rue Jean-Jacques Rousseau, 92136 Issy les Moulineaux, France
Abstract: Bactericidal reactive species N(4S) and N(2P) were measured by optical
emission spectroscopy in the afterglow of a nitrogen pulsed discharge at atmospheric
pressure. Two methods were used and validated: absolute and kinetic methods. The newly
developed kinetic method gives results close to those obtained with the absolute calibration
technique.
Keywords: emission spectroscopy, absolute calibration, nitrogen afterglow, nitrogen atom
density, nitrogen metastable
1. Introduction
Decontamination of heat-sensitive medical devices (e.g.
endoscopes) is still challenging nowadays. Conventional
methods have the disadvantage to damage the material, so
the development of new disinfection techniques is
necessary. Use of the afterglow of nitrogen pulsed plasma
at atmospheric pressure is promising for decontamination
at ambient temperature [1]. This configuration was
studied by several research groups and revealed that the
main bactericidal action was due to the chemical species
produced by the plasma [2]. To investigate further the role
of chemical species, measurement of absolute densities is
necessary.
Optical emission spectroscopy (OES) is a common
technique used to measure the relative densities of excited
states. However, to measure the absolute ground state
density of reactive species by OES, a calibration
technique and a good knowledge of the kinetics are
necessary.
In nitrogen plasma, the mechanisms are sufficiently
well-known to use such an approach. In Ref. [3], kinetic
mechanisms associated with the afterglow emission were
used to measure N, N 2 (A) and O.
Several calibration techniques were used in the
literature. In Ref. [3], calibration was done using an NO
titration technique, but this method is intrusive and
experimentally difficult to use and leads to large
uncertainties at atmospheric pressure [4]. In Ref. [5] a
calibration technique based on the emission signal
resulting from excitation transfer of argon metastable
atoms to nitrogen molecules was used to measure N(2P)
density.
Here, we compare the use of two methods to measure
the axial distribution of the nitrogen ground state N(4S)
and metastable state N(2P) in the afterglow of a pulsed
nitrogen discharge at atmospheric pressure. relative
emission intensity. The absolute method [7] is based on
measurements of the absolute emission intensity of N(2P)
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state and N 2 (B,11) state following nitrogen atoms
recombination (reaction (R2) in Table 1). So, this method
requires absolute calibration of the optical device. The
kinetic method is based on the study of the temporal
decay of N 2 (B,11) emission intensity and therefore only
relied on relative intensities First, we will present both
methods in details, then the experimental set-up. Finally
we will discuss the experimental results and the use of the
two methods in other configurations.
2. Description of the methods
2.1. Kinetic method
The kinetic method is a two-step technique which
requires to work in a pure nitrogen afterglow. From the
N 2 (B,v=11) state emission in arbitrary units and using a
simplified kinetic scheme, one can measure the N density
profile in this configuration, as explained in section 2.1.1.
The main reactions and the associated constants are listed
in Tables 1 and 2. In section 2.1.2., the method is
extended to other wavelengths using relative calibration.
At this stage, the emission can be studied with different
gas mixtures.
2.1.1. N measurement
N 2 (B,11) state is mainly populated by the
recombination reaction (R2) and depleted by quenching
with N 2 (R3). Thus, assuming steady state for the
population of N 2 (B,11), and using (R2-R3) one can easily
find :
[N2 (B, 11)] =
k2
[N(4 S)] 2
k3
(1)
Considering the emission intensity of N 2 (B,11) at 580
nm, equation (1) becomes:
I(580) = K[N(4 S)] 2
(2)
1
with,
k2
hc
K = 𝐴580
C(580)
k 3 580x 10−9
(3)
(4)
where V is the flow velocity, z is the position along the
post-discharge tube and 1/[𝑁(4 𝑆)]0 is a constant.
Using equations (2) and (4), we obtain that I(580)−1/2
is linear along the post-discharge tube:
I(580)
−1/2
=K
−1/2
1
2k1 [N2 ]
� 4
+
z�
[N( S)]0
V
(6)
where G(λ) corresponds to the spectral sensitivity of the
optical device while b includes geometrical parameters
such as the emission column length and the solid angle.
This assumption was previously used in [8].
Using equation (3), the constant b can be calculated
while the factor G(λ) is obtained by relative calibration.
We now apply this approach to measure the N(2P) density
from the forbidden line emission at 346.6 nm following
the transition N(2 P ⟶4 S), whose probability A346
equals 0.005 s-1. The emission intensity can be written as
follows:
𝐴580 𝑘2 346 𝐺(580) 𝐼(346.6)
𝐴346 𝑘3 580 𝐺(346)
𝐾
(8)
Table 1. Reaction mechanisms.
N°
Reactions
R2
N(4 S)+N(4 S)+N2 → N2 (X, A, B) + N2
R3
N2 (B, 11) + N2 →N2 + N2
R1
N(4 S)+N(4 S)+N2 → N2 (B, 11) + N2
Table 2. Reaction rate coefficients.
N°
R1
R2
2.1.2. Extension of the method to other wavelengths
We now assume that the calibration factor can be
written as follows:
2
[N(2 P)] =
(5)
From the slope of I(580)−1/2 , it is possible to
determine the constant K. The density of nitrogen atoms
can finally be obtained using equation (2) and keeping
arbitrary units for I(580). The obtained values are only
dependent on the validity of the kinetic mechanism used
and on the uniformity of the flow velocity across the tube
section. However they are independent of the choice of
constants k 2 and k 3 because at this stage, only equations
(2) and (5) are used.
C(λ) = b G(λ)
(7)
Using equations (3) and (6), equation (7) gives:
where A 580 (=77600 s-1) is the Einstein coefficient of
spontaneous emission associated to the transition
N2 (B, 11 ⟶ A, 7), h is the Planck constant, c is the speed
of light, and C(580) is the calibration factor of the optical
device including acquisition parameters and spectral
sensitivity.
In pure nitrogen afterglow, nitrogen atoms are mainly
lost by three-body recombination (reaction R1 in Table
1). Assuming steady state and considering reaction (R1),
we obtain:
1
1
2k1 [N2 ]
=
+
z
4
4
[N( S)] [N( S)]0
V
hc
C(346.6)[N(2 P)]
346x 10−9
I(346.6) = A346
R3
Reaction coefficient
4.4 x 10-33 cm6.s-1
1.7 x 10-32 cm6.s-1
1.5 x 10-32 cm6.s-1
4.4 x 10-34 cm6.s-1
Ref.
[9]
[10]
[11]
[3]
2.8 x 10-11 cm3.s-1
[12]
2.2. Absolute method
The absolute method is based on the measurement of
the intensity of the volumetric plasma source in absolute
units which is defined as follows:
𝐼(𝜆) =
1
𝐿ℎ𝜈𝐴21 𝑛2 𝜙(𝜆)
4𝜋
(9)
where L is the emission column length, A 21 is the
Einstein coefficient of spontaneous emission, φ(λ) is the
normalized line profile, 𝑛2 is the density of the upper state
and ℎ𝜈 is the photon energy. Here, for the comparison
with the kinetic method, we only considered the
emissivity integrated on the column emission length, thus
giving results averaged on the tube section.
The absolute intensity is difficult to obtain because it
depends on the solid angle, the length of collection
volume and the detector response. For this reason, a
calibration lamp of known emissivity is used in place of
the plasma source while the acquisition parameters are
kept constant. From this measurement, conversion from
arbitrary units to absolute units can be done. Then, using
an absolute spectroscopic model (Specair) to fit the
measured intensity, the densities of the excited states can
be calculated.
Here, the N 2 (B,11) state density is calculated from its
measured absolute emission intensity at 580 nm and using
the Specair software. N(4S) density is then calculated
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using equation (1). For N(2P), the density is directly
calculated by Specair from the forbidden line emission at
346.6 nm measured in absolute units.
3. Experimental set-up
The experimental set-up is shown in Figure 1. The
plasma is generated by an industrial reactor working in a
point-to-point geometry (Plasmabiotics, Inpulse One V3).
Both electrodes are encapsulated in a ceramic box. The
gap between the electrodes (few mm) is fed with pure
nitrogen gas (Air Liquide, azote U) at a flow rate of 30
slpm, so that the flow velocity in the post-discharge tube
is about 40 m.s-1. The afterglow emission is collected
using two off-axis parabolic (OAP) mirrors and focused
on the entrance slit of the spectrometer (Acton 2500i).
The spectra are recorded using a ICCD camera (PI-MAX
2).
Fig. 2. Calibration curve for the kinetic method
4. Results and discussion
The longitudinal profiles of atomic nitrogen density
measured using kinetic and absolute method are shown in
Figure 3. For the kinetic method, three recombination
coefficients were tested. The coefficients measured in
[10] and [11] gave results in good agreement with those
obtained by absolute method. On the contrary, the
constant measured in [9], which was used for kinetic
method in [6], leads to a large difference in the obtained
values. Indeed, according to the kinetic method, the
N(4S) density increases linearly with the constant k 1
whereas the N(2P) density increases quadratically. So, the
measurement is strongly sensitive to the value chosen for
this constant. Comparison with the absolute method
encourages us to choose the value given in Refs. [10] and
[11].
Fig. 1. Schematic of the experimental set-up.
For absolute calibration, the light emitted by a
calibrated tungsten ribbon lamp of known radiance
traceable to NIST standards was recorded keeping the
same conditions as for the acquisitions from the discharge
afterglow. The signal was corrected from a significant
contribution of the stray light. The spectra were then fitted
using the Specair software [13].
For the kinetic method, emission of N 2 (B,v=11) at 580
nm was recorded along the post-discharge tube. In Figure
2 the calibration curve shows a linear profile as expected
in equation (5). The non-linear region is due to a change
in the losses mechanisms of nitrogen atoms. Three-body
recombination is not dominant anymore, so equation (4)
is not valid in this region. The slope of the calibration
curve was measured using a linear fit and was used to
derive N(4S) density according to equation (2). Then,
relative calibration data of a tungsten ribbon lamp were
used to measure N(2P) density according to relation (8).
Fig. 3. N(4S) density distribution along the post-discharge
tube.
In Figure 4, the longitudinal profile of N(2P) is shown.
The observed differences between kinetic and absolute
methods may be due to the imperfect knowledge of the
constant k 1 . Additional research is in progress to explain
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3
this behavior. Globally, the results give the same order of
magnitude as those obtained in [5].
[7] G.D. Stancu, F. Kaddouri, D.A. Lacoste and
C.O. Laux, J. Phys. D: Appl. Phys., 43, 124002 (2010)
[8] C. Yubero, M. C. García, M. D. Calzada, Optica
Applicata, Vol. XXXVIII, No. 2, 2008
[9] I. A. Kossyi, A. Yu. Kostinsky, A. A. Matveyev and
V. P. Silakov, Plasma Sources Sci. Technol. 1, 207-220
(1992)
[10] P. Harteck, R.R. Reeves and G. Manella, J. Chem.
Phys., 29 , 608 (1958)
[11] J.T. Herron, J.L. Franklin, P. Bradt and V.H. Dibeler,
J. Chem. Phys., 30, 4, 879 (1959)
[12] A. Ricard, J. Tétreault and J. Hubert., Phys. B: At.
Mol. Opt. Phys., 24 , 1115 (1991)
[13] C. O. Laux, T. G. Spence, C. H. Kruger and R. N.
Zare, Plasma Sources Sci. Technol., 12, 125 (2003)
Fig. 4. N(2P) density distribution along the post-discharge
tube
5. Conclusions
Absolute densities of N(2P) and N(4S) along the postdischarge tube of a pulsed nitrogen discharge were
measured by OES using absolute and kinetic methods.
Both techniques give close results if the volume
recombination constant of nitrogen atoms k 1 measured in
[10] and [11] is used in the kinetic method. Absolute
calibration requires an absolutely calibrated lamp, and an
absolute intensity spectroscopic model. On the contrary,
the kinetic method is much easier to use because it only
requires relative intensities. However it cannot provide
radial resolution and can only be used in an axisymmetric
tube. This calibration-free technique is still promising to
measure the densities of species inside small-diameter
tubes such as plasma jets.
6. Acknowledgments
This research was supported by DGA grant
DESDEMONA (RAPID contract EJ nr. 2101 241 872)
7. References
[1] J. Ehlbeck, U. Schnabel, M. Polak, J. Winter,Th Von
Woedtke, R. Brandenburg, T. von dem Hagen and K-D
Weltmann, J. Phys. D: Appl. Phys., 44, 013002 (2011)
[2] M. Mols, H. Mastwijk, M. Nierop Groot and T. Abee,
Journal of Applied Microbiology, 115, 689-702 (2013)
[3] A. Ricard, S. Oh and V. Guerra, Plasma Sources Sci.
Technol., 22 035009 (2013)
[4] P. Vasina, V. Kudrle, A. Tálsky, P. Botos, M.
Mrázková and M. Mesko, Plasma Sources Sci. Technol.
13 668–674 (2004)
[5] E. Eslami and N. Sadeghi, Eur. Phys. J. Appl. Phys.,
43, 93–102 (2008)
[6] A-M. Pointu, A. Ricard, E. Odic and M. Ganciu,
Plasma Process. Polym., 5, 559 (2008)
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