22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Equilibrium and non-equilibrium features in a warm He+H 2 O microwave
plasma at atmospheric pressure
M.A. Ridenti, J. Amorim and A. Dal Pino
Department of Physics, Aeronautical Institute of Technology (ITA), BR-12228-900 São José dos Campos, Brazil
Abstract: A microwave surface-wave discharge at atmospheric pressure was generated
using a He/H 2 O mixture and studied by optical emission spectroscopy (OES). The main
plasma parameters were determined based on the analysis of the spectra when partial
thermodinamical equilibrium could be verified. Experimental evidence and theoretical
arguments suggest that the main source of ionization in this plasma is the electronic impact
ionization of water molecules. A chemical equilibrium model based on the minimization of
the Gibbs free energy was used to gain some insight into the probable chemical species
present in the plasma and to explain the strong emission of H I, O I and OH transitions
observed in the experimental spectra and the absence of He I emissions. Besides that, an
abnormal intensity in the O I [3p3P 0,1,2 ] - [3s3S 1 ] transition was observed, revealing an
overpopulation of the 3p 3P 0,1,2 states. This observation – which may be explored to obtain
stimulated emission – was explained as an effect of the reabsorption of photons generated
in the plasma by the H I Ly β emission by oxygen atoms (O I 3P 2 - 3d 3D 1,2,3 transition), an
effect generally known as photoexcitation by accidental resonance (PAR).
Keywords: atmospheric pressure plasmas, optical emission spectroscopy, photoexcitation
by accidental resonance, non-LTE plasma
1. Introduction
In this paper we report an extensive characterization of
a microwave induced plasma (MIP) by optical emission
spectroscopy (OES). The source used to produce the
discharge was a Surfatron and the carrier gas was a
mixture of helium with water vapour at saturation
concentration. The spectra in the range of 280 nm to 900
nm were fully assigned and some plasma parameters such
as the gas and excitation temperatures and also the
electron density were estimated by means of OES based
techniques. As far as we are concerned, this is the first
time that a MIP in this particular condition is reported and
studied.
Besides that, a model for the plasma chemistry that
assumes chemical equilibrium [1] was applied to describe
the system as a first approximation, giving some hints
about the most probable plasma species expected to be
found in the medium. This approach offers a shortcut to
the complex work of plasma chemistry modelling, at least
as a first approximation, giving a rough but useful picture
of the chemical plasma composition that captures its main
features.
2. Materials and Methods
The discharge was generated by a surfatron surfacewave launcher fed by a Sairem microwave solid state
power supply operating at 2.45 GHz frequency and 120
W power level. A quartz tube (alumina, 3.0 mm i.d., 4.0
mm o.d.) was placed into the launcher coaxial The tube
was connected to the gas feeding system, which delivered
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helium gas (White Martins He 6.0) saturated with water
vapour. A fixed helium flow rate of 0.75 slm, which was
controlled by a flow meter MKS 149A, flowed
continually trough the line
A line-to-spot optical fibre bundle (N.A. = 0.22, spot
size = 1.1 mm) placed radially at a position about 3 mm
far from the lateral tube surface collected the light emitted
by the plasma. Measurements were carried out moving
the fibre axially in steps of 5 mm. The fibre was matched
to a Horiba Jobin-Yvon monochromator iHR550, of
Czerny-Turner type, with 550mm focal length.
Measurements were carried out in the UV range and in
the visible range.
3. Results
In the visible/near-IR range the line with the strongest
intensity corresponded to the 𝐻𝛼 (656.28 nm) emission.
The next three hydrogen lines in the Balmer series
(486.14 nm, 434.05 nm and 410.17 nm) were also
detected. The second most intense emission came from
the triplet atomic oxygen transition at 777.19 nm, 777.42
nm and 777.54 nm. Another intense triplet emission from
atomic oxygen was observed in the spectra at 844.6 nm.
In the UV range, many molecular bands were observed
apart from some atomic lines. The most intense emission
came from the OH band, whith band-head located at
306.4 nm. Two other bands from this system were also
detected at 281.1 nm and 342.8 nm. No He lines were
observed neither on the visible or UV range.
1
Figure 1: (A) Molar fraction of the main plasma species as a function of temperature when chemical equilibrium is assumed in an ideal plasma.
Legend: () He; () H2O; () H2; () OH; () O2; () H; () O; () e-; () H3O+; () H2O+; () H+; () O+; ( ) He+. (B) Energy level
scheme showing on of the possible paths leading to the overpopulation of the state 3p3P by the PAR effect due to the absorption
of the H I Lyβ photons.
The gas temperature was estimated
using
the
Boltzmann plot based on the intensity of the Q 1 branch
lines of the OH (0,0) band. The mean temperature was
found to be 1.5 x 103 K and the maximum variation along
the column did not exceeded 12% of that value. The
excitation temperatures were computed using the
Boltzmann plot of the hydrogen Balmer line series and
the excited argon emission intensities from the 4p states.
The mean excitation temperature obtained in both cases
were compatible and can be represented by a mean value
of 25 x 103 K. The Stark broadening of the 𝐻𝛽 was used
to estimate the electron density. The mean value was
approximately 21013 cm-3 and the total variation was not
higher than 31% of that value. Using the electron density
profile, the LTE temperature was computed in two
different scenarios: ionization occurring exclusively on
water molecules (case A); ionization occurring only on
helium atoms (case B). The average value in the case A
was 3.6 103 K, whereas the average value in the case B
was 6.6 103 K. A global model assuming constant
electron density and temperature based on the power
balance and the electron Boltzmann equation solution was
used to estimate the electron temperature, yielding a value
of 3.5103 K. This is consistent with scenario A. The
relative concentration of the main chemical species were
estimated assuming chemical equilibrium. The results
showed that H atoms are the most abundant atomic
species after He, OH the most abundant after H 2 O (Figure
1 (A)) and it also corroborated the scenario A
As mentioned previously, four intense lines from O I
transitions were observed in the spectra at 777.19 nm,
777.42 nm, 777.54 nm and 844.6 nm. In principle, the
Boltzmann plot based on these lines should also provide a
straight line from which another excitation temperature
2
could be estimated. Surprisingly, as shown in Figure 1
(A), the 844.64 line intensity is much higher than the
value that should be expected if the atomic states were
governed by a Boltzmann distribution with excitation
temperatures of the order of 103 K. Similar behaviour was
observed in the spectra of some astrophysical sources [2].
According to that hypothesis, the accidental resonance
occurs between the H I Ly β emission at 102.57 nm and
the O I transition from the ground state 3P 2 to the
3d 3D 1,2,3 system (102.57 nm). These states are pumped
by the PAR effect and decay by allowed transitions to the
3p3P system (Figure 1 (B)). The radiative lifetime of this
system is ~36 ns, much larger than the radiative lifetime
of its lower state 3s 3S 1 , which is ~ 2.5 ns. This results in
an overpopulation of the 3p 3P triplet and eventually
could lead to a population inversion.
4. Conclusions
A plasma produced under atmospheric pressure
conditions using a surfatron surface-wave launcher and a
He/H2O mixture was studied by means of optical
emission spectroscopy. Using standard techniques of
plasma diagnostics by OES, the electron density and
rotational temperature axial profile were determined. It
was concluded that water ionization by electron impact
was the most probable main charge creation source.
Another interesting result was the observation of an
abnormal strong emission of O I from the triplet system
3p 3P 0,1,2 to 3s 3S 1 . This phenomenon was explained by
the mechanism of photoexcitation by accidental resonance
(PAR) based on the coincidental H I Ly β and O I 3P 2 3d 3D 1,2,3 lines and may be explored to produce stimulated
emission.
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5. References
[1]
S. Gordon and B. J. McBride, NASA reference
publication, Rep. 1311 (1994).
[2]
S. Johansson and V. S. Letokhov, New
Astronomy Reviews, 51, 5 (2007).
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