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WDS'07 Proceedings of Contributed Papers, Part II, 194–197, 2007.
ISBN 978-80-7378-024-1 © MATFYZPRESS
Discharge Generation of Atomic Iodine
I. Picková, V. Jirásek, J. Schmiedberger
Institute of Physics, Academy of Sciences of the Czech republic, Prague, Czech Republic.
Abstract. In this article we describe our new experimental device for generation
of atomic iodine in discharge. This device will be a part of the COIL (chemical
oxygen-iodine laser) and also DOIL (discharge oxygen-iodine laser) system. There
are three main tasks which must be solved for a successful COIL operation:
generation of atomic iodine, generation of singlet oxygen (excited oxygen molecule)
as a source of energy for laser pumping, and efficient mixing of these compounds in
the nozzle. Both atomic iodine and singlet oxygen can be produced chemically or
in electric discharge. In our laboratory we have started the research of a discharge
generation of atomic iodine. For the new discharge method we are going to use the
optical emission spectroscopy diagnostics, by which we will detect species present
in plasma and use this information for improving of iodine generation process.
Introduction
A COIL (chemical oxygen iodine laser) is the CW (continuous wave) high power laser
radiating at the wavelength 1315 nm (a near infrared region). The word chemical in the name
refers to the fact that energy for laser pumping comes from a chemical reaction.
This laser radiates on the transition between excited and ground level of atomic iodine:
I(2 P1/2 ) + hν → I(2 P3/2 ) + 2hν.
(1)
Excited iodine atoms are generated from iodine atoms in ground state by collisions with
singlet oxygen molecule (first excited state of oxygen - O2 (a1 ∆g )). As the energy distance of
the ground and the first excited states of oxygen molecule and iodine atom has nearly the same
value, the near-resonant transfer of energy is possible between them, as is shown in the following
reaction:
I(2 P3/2 ) + O2 (a1 ∆g ) → I(2 P1/2 ) + O2 (a1 Σg ),
(2)
where I(2 P3/2 ) and I(2 P1/2 ) are the ground and the first excited state of iodine atom
respectively, and O2 (a1 ∆g ) and O2 (a1 ∆g ) are the first excited and ground stated of oxygen
molecule respectively. There is also a great advantage that O2 (a1 ∆g ) is the metastable state
with extremely long radiation lifetime of ∼ 70 minutes [Kodymová et. al., 2004].
Singlet oxygen O2 (a1 ∆g ) is produced in convention COIL by chemical reaction of Cl2 gas
and liquid solution of basic hydrogen peroxide (H2 O2 and KOH solution):
Cl2 + 2KOH + H2 O2 → O2 (a1 ∆g ) + 2KCl + 2H2 O.
(3)
In this process it is necessary to achieve the most effective surface reaction area between
liquid and gas due to a very short O2 (a1 ∆g ) lifetime in liquid phase (∼ 2µs only).
Iodine atoms can be generated from different molecules containing iodine. Classical and
still investigated is the iodine molecule I2 , although this molecule has also many disadvantages
for the generation process. For dissociation of one I2 molecule, a collision with more than four
singlet oxygen molecules is needed, and therefore a part of energy, which could possibly be used
for laser pumping, is lost. I2 also quenches excited iodine atoms to the ground state in rather
fast reaction. Another disadvantage is that I2 is solid at room temperature and substantial part
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of device must be heated to avoid its precipitation. For this reasons new source molecules are
examined: HI, CH3 I, CF3 I and similar.
In our laboratory, a method of chemical generation of atomic iodine from HI molecule is
investigated by the substitution mechanism involving atomic chlorine or fluorine [Špalek et. al.,
2002, Jirásek et. al., 2001]. This can be simply described by reaction:
HI + Cl → HCl + I
(4)
HI + F → HF + I
(5)
or
Both singlet oxygen and atomic iodine can be generated also in electric discharge. Main
advantage of the discharge method is that we can avoid many dangerous chemicals like F2
and Cl2 . Different types of discharge and different molecules containing iodine have been
investigated in many laboratories, however improvement of the laser performance by using
these methods is still rather limited.
In our laboratory we intend to study atomic iodine generation in RF discharge from different
iodine donors like I2 , HI, CH3 I, CF3 I and similar ones. A most suitable donor will be selected
by these main criteria: high iodine yield, discharge stability, operation feasibility of donor, and
also cost [Schmiedberger et. al., 2007].
Experimental setup
A main part of the developed experimental device is shown in Fig. 1. A buffer gas (He,
N2 ) is flowing from the channel (left side of the figure) into the cavity where an injector is
located. This injector contains the discharge chamber with RF wire electrode and two channels
for water cooling. The gaseous iodine compounds are introduced into the discharge chamber,
where iodine atoms are generated by electric discharge and then injected through small holes
into the buffer gas flow. The injector body has such a shape that its contour together with the
wall forms a supersonic nozzle, where the gases will be mixed and cooled by expansion.
Two types of RF supply are considered, a CW regime supply with frequency of 13.56 MHz
and a pulsed regime supply with tunable frequency between 20 and 100 MHz (CW regime is
also possible), both with power 25 - 500 W.
Figure 1. Design of new discharge device for atomic iodine generation. From the left: duct of
buffer gas, injector with RF discharge electrode and channel for water cooling. Injector together
with the wall of the cavity forms the supersonic nozzle. Pitot tube is employed for pressure and
flow velocity measurement.
Final design of the device configuration, mainly the shape of the injector body was based
on computational modeling, evaluating the flow properties (Mach number). For this task the
CFD code Fluent was used.
As a diagnostic tool we will use the optical emission spectroscopy, an optical spectrum
analyzer Advantest Q 8384 will be used for this task. Its spectral range is from 600 to 1700 nm
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(red and near infrared region) and maximum resolution 10 pm. Emitted light will be collected
by optical fibre both from the discharge chamber and from the supersonic cavity outside the
discharge region.
Emission spectroscopy diagnostics
Emission spectroscopy is the diagnostic technique widely used for measuring plasma parameters (temperature) and plasma composition i.e. species present in plasma like atoms, molecules
and ions. A great advantage of emissive spectroscopy is that it is passive and therefore nondisturbing method i.e. does not change the state of plasma. According to measured wavelength
we can employ the UV, optical, infrared and also other types of emission spectroscopy. It can
be also divided to atomic and molecular spectroscopy.
In our experiments we expect to have different atoms like e.g. I, H, F (depending on iodine
donor used), atoms of buffer gas, and also different molecules: I2 , HI, H2 , CH3 , F2 , and some
more. Atomic spectra are much simpler than molecular spectra due to a fact that molecule can
rotate around its axes and distances between atoms in molecule can change (vibrations). On
the other hand, there are many interactions also in atoms, so even theory of atomic spectra can
be complicated [Martin et. al., 1996]. The theory of spectroscopy (mostly based on quantum
mechanics) can be found in many books [Griem, 1964, Svanberg, 1992, Lochte-Holtgreven, 1968].
Tables with atomic and molecular spectra or spectral images can be also found in various
Internet databases, such as HITRAN, NIST, VPL and JPL database (see References). Fig. 2.
shows for example a part of the hydrogen iodide (HI) spectrum, which is of our interest. In
databases only some of molecular spectra can be found, because many molecules have not been
investigated yet. New data can be usually found in articles, for example in [Fantz, 2004].
Figure 2. Selection of hydrogen iodide (HI) spectral lines. Different sets of spectral lines
forming molecular bands can be seen. Source: HITRAN database.
To analyze the spectral correctly, we need to adopt some theory about the plasma behavior.
Very important is usually the fact whether plasma is in thermodynamic equilibrium or at least
local thermodynamic equilibrium (LTE). In thermodynamic equilibrium all the parts of the
temperature of the whole system can be defined. LTE criteria can be adopted, when parts of
the system are near the thermodynamic equilibrium and changes are very small, so temperature
of each part can thus be defined and measured. Conditions for LTE are not always fulfilled,
which is usually a problem of the low-pressure plasma characterized by a small amount of
collisions.
Conclusion
We intend to continue in chemical oxygen-iodine laser and start the discharge oxygen-iodine
laser research by employing RF discharge for atomic iodine generation. The effects of different
modes of the discharge operation, different gas flows and also different iodine donors and buffer
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gases on atomic iodine yield will be investigated. As a diagnostic tool we shall use optical
emission spectroscopy to detect different emission lines of iodine, buffer gas, donor molecule
and also different fragments and ions. Interpretation of these spectral measurements will be
based on performing a detailed literature inspection of published tables or images of atomic and
molecular spectra of species probably present in the discharge and also on spectroscopic theory.
Chemical generation of both singlet oxygen and atomic iodine by chemical way will also
continue in our laboratory.
References
Kodymová, J, O. Špalek, V. Jirásek, M. Čenský, Advances in the development of the oxygen-iodine laser,
Czechoslovak Journal of Physics, 54, 561-574, 2004
Špalek, O., V. Jirásek, M. Čenský, J. Kodymová, I. Jakubec, G. D. Hager, Chemical generation of atomic
iodine for chemical oxygen-iodine laser. II. Experimental results, Chemical physics, 282, 147-157, 2002.
Jirásek, V., O. Špalek, J. Kodymová, M. Čenský, Chemical generation of atomic iodine for chemical
oxygen-iodine laser. I. Modeling of reaction systems, Chemical physics, 269 , 167-178, 2001.
Schmiedberger, J., V. Jirásek, J. Kodymová, K. Rohlena, Advanced Concept of Discharge Oxygen-Iodine
Laser, AIAA Plasmadynamics and Lasers conference, 2007.
Martin, W. C., W. L. Wiese, Atomic spectroscopy, A Compendium of Basic Ideas, Notation, Data and
Formulas, National Institute of Standards and Technology (http://physics.nist.gov/Pubs/AtSpec/index.html),
originally chapter 10 of the book of G. W. F. Drake (editor): Atomic, Molecular and Optical Physics
Handbook, AIP Press, Woodbury, NY, 1996.
Griem, H. R., Plasma Spectroscopy, McGraw-Hill, 1964
Svanberg, S., Atomic and molecular spectroscopy, Springer-Verlag, Berlin Heidelberg, 1992.
Lochte-Holtgreven W., Plasma Diagnostics, North-Holland Publishing Company, Amsterdam, 1968.
Fantz, U., Emission Spectroscopy of Molecular Low Pressure Plasmas, Contribution Plasma Physics, 44,
508-515, 2004.
NIST database, http://www.nist.gov/
HITRAN database, http://www.cfa.harvard.edu/hitran/
VPL database, http://vpl.ipac.caltech edu/spectra/
JPL database, http://spec.jpl.nasa.gov
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