22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
On OH radicals production in a glow discharge with water electrode
Q. Xiong1,2 and P. Bruggeman2
1
Chongqing University, School of Electrical Engineering, Shazheng Street 174, Shapingba District, Chongqing,
P.R. China
2
University of Minnesota, Department of Mechanical Engineering, 111 Church St. SE, Minneapolis, MN 55455, U.S.A.
Abstract: Quantitative diagnostics of OH radicals is carried out in an atmospheric pressure
glow discharge with water electrode by broadband UV absorption spectroscopy. When the
water electrode acts as the cathode, it is found that in the positive column the radial profiles
of OH density and gas temperature are broader than the total emission and the emission
intensities from OH (A-X). This indicates that a significant OH radial density is present
outside the plasma column with an even broader temperature profile. The kinetics of OH
species is described in detail.
Keywords: atmospheric pressure discharge, OH radical, water discharge, UV absorption,
absolute density
1. Introduction
The hydroxide (OH) radial is a critical species in
plasma chemistry and playing an important role in many
applications of plasmas [1]. Quantitative determination of
OH attracts widespread interests for obtaining a detailed
insight into the fundamentals of plasma treatments.
Laser-induced fluorescence (LIF) spectroscopy has been
mostly applied to determine absolute OH densities in
various discharge sources in recent years [2-3]. However,
the LIF method usually needs calibration and a model of
the collisional processes affecting the LIF intensity
measurement which strongly depends on the local plasma
conditions. In most measured discharge sources the gas
compositions, such as water vapor concentration in
humidified air, are known. The is however not the case in
plasmas interacting with liquid water as the local water
concentration is unknown and often has a steep gradient
near the water surface due to plasma heating and
evaporation [4]. In this work broadband UV absorption
spectroscopy is applied to absolutely measure the OH
density in a glow discharge with water electrode. This
absorption method has a unique advantage compared to
LIF that it is independent of collision processes. The
accuracy of the method is mostly related to the gas
temperature variation and the accuracy of the
determination of the species density profile by Abel
inversion [5]. The complication induced by complex gas
environments as in the case of discharges in contact with
liquids can be avoided. In this contribution, we illustrate
the method for a measurement of the OH density and gas
temperature profiles in the positive column of an
atmospheric pressure glow discharge in air with water
cathode.
2. Experimental setup and methods
The experiments are carried out in a metal pin-water
electrode configuration, as shown in Fig. 1. A sharp
stainless steel electrode (D = 1 mm) with a cone angle of
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approximately 30° is connected to the DC power supply
through a ballast resistor of 5 kΩ. Tap water contained in
a petri dish (D = 60 mm) is used as the liquid electrode.
The petri dish is connected to a water reservoir to keep
the height of water surface. The distance between the
metal-pin tip to the water electrode is fixed at 2 mm.
A grounded wire through a 25 Ω resistor is inserted in the
water. Voltage and current were monitored in all
measurements and kept at ranges 1.5 ~ 1.6 kV and
13.2 ~ 13.5 mA, respectively.
Fig. 1. Schematic of the experimental arrangements [6].
A laser stabilized broadband lamp (Energetiq EQ99) is
used as the light source. The average intensity variation
less than 1% over 1000 hours working greatly reduces the
experimental uncertainty induced by the light source.
Two convex lenses are arranged to focus the light from
lamp passing through the discharge with a spatial
resolution about 150 µm.
A filter (307 nm,
FHWM = 4 nm) is used passing UV light for the OH
absorption. The light beam is collected by a fiber optics
to a high resolution spectrum system composed of a 1 m
monochromator (ACTON AM510) and a CCD camera
(Andor DU420A). The spectral resolution of the system
1
is about 53 pm FWHM (Gaussian shape) around 310 nm
with a 1200 g/mm grating and 10 µm slit width. OH
absorption spectrum band (X 2 Π → A2 Σ + , 0 − 0)is
measured and averaged over 1000 accumulation by the
CCD with 10 µs integration time. Radial measurements
are performed by moving the discharge setup
perpendicularly to the light beam with an increment of
100 µm. For each position, the measurement was
repeated three times and the averaged absorption signal of
these three measurements was used for data analysis. The
light beam was positioned in the middle of the discharge
gap.
Four spectra from different emission sources were
recorded and the absorption spectrum is obtained as
follows:
A(λ) = 1 −
𝐼𝑝𝑝𝑝+𝑙𝑙𝑙 (𝜆)−𝐼𝑝𝑝𝑝 (𝜆)
𝐼𝑙𝑙𝑙 (𝜆)−𝐼𝑏𝑏𝑏 (𝜆)
,
(1)
2
with A(λ) the fractional absorption of the OH(𝑋 Π →
𝐴2 Σ + , 0 − 0) ro-vibrational transitions, 𝐼𝑙𝑙𝑙 (𝜆) the
emission spectra from the lamp source (plasma off),
𝐼𝑝𝑝𝑝+𝑙𝑙𝑙 (𝜆) the spectra containing the lamp and plasma
emission, 𝐼𝑝𝑝𝑝 (𝜆) the plasma emission spectra (lamp off),
and 𝐼𝑏𝑏𝑏 (𝜆) the recorded background intensity (both lamp
and plasma are off). Fig. 2 shows the absorption spectra
measured from the middle position of the water-cathode
discharge. In the middle of the discharge gap, the
integrated absorption of a specific transition along the
light path can be obtained according to the Beer-Lambert
law:
𝑊(𝑥, 𝑧0 ) = ∫ 𝐴(𝜆)𝑑𝑑 = ∫[1 − exp�−ℎ𝜆0 𝐵 ∙
∫ 𝑛𝑂𝑂 (𝑥, 𝑦) ∙ 𝑓𝐵 �𝑇(𝑥, 𝑦)� . ∅�𝜆, 𝑇(𝑥, 𝑦)� ∙ 𝑑𝑑�] ∙ 𝑑𝑑 (2)
More details about the above formulae can be found in
[5]. 𝑊(𝑥, 𝑧0 ) is calculated under the assumption that the
OH density and gas temperature have a two-dimensional
Gaussian radial distribution in the discharge. This
assumption is made based on the Gaussian profile of the
gas temperature measured by Rayleigh scattering in
previous work [7]. The obtained 𝑊(𝑥, 𝑧0 ) is used to fit
the experimental profiles of 6 OH ro-vibrational lines and
the OH concentration and gas temperature are determined
when the best fitting is achieved. More details about the
fitting procedures can be found in [6].
3. Results and discussion
The radial distributions of the obtained OH
concentration and gas temperature in the positive column
are shown in Fig. 3. The OH density reaches values of
1023 m-3 which is extremely high although consistent with
the expected high water concentration and high gas
temperature. Similar high values have also been obtained
by LIF techniques [8]. The actual values obtained in this
work only depend on absorption cross sections which are
accurately known and are intrinsically more accurate than
densities obtained by LIF.
2
Fig. 2. Absorption spectrum from the middle position of
the water-cathode discharge. Six typical OH(A-X, 0-0)
ro-vibrational lines (marked therein) are selected in the
fitting process to determine the profiles of OH density and
gas temperature [6].
Fig. 3.
Spatial profiles of the OH density, gas
temperature, emission intensities with and without OH
filter in the positive column of a glow discharge with
water cathode. The UV light passes through the middle of
the discharge gap [6].
Electron induced excitation of species mainly occur in
the emitting area. The electron population is low beyond
the radiating column of the discharge. The OH radicals
are primarily produced in the center area of discharge
through electron dissociation of H 2 O or dissociative
recombination of water ions H2 O+ or its clusters H3 O+ by
electrons [9]. It is interesting to observe that the radial
profiles of OH emission (captured with OH filter) and
discharge emission (without OH filter) are narrower
compared to those of OH density and gas temperature.
The diffusion of OH species is more intense at a high gas
temperature as the neutrals diffusion velocity is
proportional to gas temperature. The broad distribution of
the gas temperature further enhances the movement of
OH to a large radial distance. The main loss of OH
radicals is due to recombination process with OH and H
forming H 2 O 2 or H 2 O. The narrow profile of the OH
emission intensity indicates the OH excitation by electron
collisional or other energy transfer processes is
dominantly occurring in the discharge core-area. As in
the positive column the OH density is large and the
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electron temperature is low, the direct electron excitation
of OH(X) will be the dominant production process of
OH(A). The fast quenching effect by water vapor will
prevent the excited OH* moving further outside the
discharge column. As such the OH emission intensity is a
good representation of the actual discharge channel.
4. Conclusion
In this work we studied the spatial distributions of
absolute OH density and gas temperature by broadband
UV absorption in the positive column of an atmospheric
pressure glow discharge with water electrode. Spectralresolved absorption of rotational lines of the OH(A-X)
transition is obtained by a high-resolution spectrometer.
The OH density and gas temperature are determined by
fitting the experimental radial absorption profiles of 6
rotational OH lines. It is assumed that the OH density and
the gas temperature are in good approximation of
Gaussian distributions. It is found that the spatial profiles
of the OH concentration and gas temperature are broader
than that of discharge area. The wide distributions
indicate that the ground-state OH radicals are able to
diffuse significantly beyond the discharge column under a
high gas temperature environment.
5. Acknowledgements
This work was in part founded by the Department of
Energy Plasma Science Center through the US
Department of Energy, office of Fusion Energy Sciences,
Contract: DE-SC0001939 and the University of
Minnesota. One of the authors Q. X. thanks the National
Natural Science Foundation (China Grant No. 11305273)
for support.
6. References
[1] P. Lukes. PhD Thesis. Institute of Chemical
Technology, Prague (2001)
[2] T. Verreycken, et al. J. Phys. D: Appl. Phys., 45,
045205 (2012)
[3] G. Dilecce, et al. Chem. Phys., 398, 142 (2012)
[4] I. Yagi, et al. Plasma Sources Sci. Technol., 24,
015002 (2015)
[5] P. Bruggeman, et al. Plasma Sources Sci. Technol.,
21, 035019 (2012)
[6] Q. Xiong, et al. J. Phys. D: Appl. Phys., submitted
(2015).
[7] T. Verreycken, et al. Plasma Sources Sci. Technol.,
20, 024002 (2011)
[8] A. Nikiforov, et al. Appl. Phys. Express, 4, 026102
(2011)
[9] P. Bruggeman, et al. Plasma Sources Sci. Technol.,
19, 045025 (2010)
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