ISBN 978-80-7378-066-1 © MATFYZPRESS
WDS'08 Proceedings of Contributed Papers, Part II, 20–24, 2008.
Optical Emission Spectroscopy – Passive Method for Plasma
Diagnostics in DC Glow Oxygen Discharge
L. Schmiedt, A. Kaňka and V. Hrachová
Charles University Prague, Faculty of Mathematics and Physics, Prague, Czech Republic.
Abstract. The DC glow discharge in pure oxygen has been studied in pressure
range 150 - 950 Pa and for discharge currents up to 40 mA. Parameters of
discharge - electric field strength and emission spectra - were studied by means of
double-probe method and optical emission spectroscopy. We have focused on
variations of intensities of the oxygen spectral lines and bands in dependence on
pressure. Optical emission spectroscopy has also been employed for determination
of kinetic temperature of neutral particles.
Introduction
The DC glow discharge in oxygen and its mixtures in pressure range of about hundreds of Pascals can be
utilized in miscellaneous applications such as plasma etching [Morimoto, 1993] or plasma oxidation [Wen-an
Loong, 1991]. Therefore the study of properties of oxygen discharge in this pressure range is very important
topic. Two different forms of positive column of DC glow oxygen discharge can be observed at pressures of
about hundreds of Pascals. These forms are defined according to the values of axial electric field strength
[Güntherschulze, 1947]. We distinguish the high-gradient H form with values of axial electric field strength of
about kVm-1, which is typical for positive column in low-pressure discharges. On the other hand, the lowgradient T form with values of electric field strength of about hundreds of Vm-1 starts to appear at pressures of
about hundreds of Pascals. Hence we must take into account possibility of existence of the T form during our
measurements.
Kinetic temperature of neutral particles belongs to important discharge parameters. This temperature
governs rate constants of various processes such as diffusion or dissociation [e.g., Dagdigian et al., 1988;
Gousset et al., 1991]. Direct measurement of this temperature using thermocouple put in active plasma can cause
similar problems as probes. On the other hand, putting thermocouple to the decay or measuring temperature on
the wall of working area can lead to distorted results. Possibility of usage of different methods for kinetic
temperature determination was discussed in past several years. Optical emission spectroscopy has been
employed for our measurements.
Diagnostic Methods
Measurement of Electric Field Strength
Double-probe method has been employed for determination of axial electric field strength. This method is
based on the fact, that voltage on the probes causes zero probe current just if it compensates potential gradient in
the discharge [Raizer, 1991]. The value of axial electric field strength is then the ratio of the probe voltage over
the probe distance.
Temperature Determination
Measurement of the kinetic temperature by means of optical emission spectroscopy is based on
measurement of temperature determined from energy levels of rotationally excited states. Generally supposed
fact is equilibrium between kinetic temperature and the rotational temperature of the ground state. Nevertheless,
since the dipole-allowed transition lifetime of the excited levels used to measure the rotational distribution is
generally much shorter than the collision time in low pressure discharges, it is not possible to assume a priori
that the rotational temperature of an excited state is identical to the kinetic temperature and this equilibrium must
be checked first. Considering oxygen discharge, this condition has already been discussed and verified by
Touzeau et al. [1991]. The possibility of determination of rotational temperature from forbidden atmospheric Aband is analysed in this paper.
Assuming well-resolved rotational spectrum with energy levels governing by Boltzmann law, the intensity
of single spectral line corresponding to the transition between J and J’ rotational states can be written as:
I JJ ' = konst ⋅ν
20
4
JJ '
⋅ SJ ⋅ e
−
EJ
k ⋅Trot
,
SCHMIEDT ET AL.: SPECTRAL DIAGNOSTICS OF OXYGEN DISCHARGE
where νJJ’ is transition frequency, SJ Hönl-London factor [Herzberg, 1957], EJ is energy of J quantum state, k is
Boltzmann constant and Trot denotes rotational temperature. From so-called Boltzmann plot [Touzeau et al.,
1991] (dependence of ln[IJJ’/SJ] on EJ ) one can obtain so-called pyrometric line, whose slope determines
rotational temperature Trot. The rotational temperature Trot was already determined for DC glow oxygen
discharge for pressures up to 300 Pa [Kylián et al., 2003] in our laboratory. The presented contribution spreads
the pressure range up to 530 Pa.
Experimental
The DC glow discharge was sustained in Silica U-shaped discharge tube with inner diameter 22 mm.
The central part of the discharge tube (length 340 mm) was equipped with head-on plane windows and two pairs
of cylindrical platinum probes (length 5 mm, diameter 0.1 mm) used for measurements of electric field strength.
The electric circuit utilized for our measurements is schematically shown in Fig. 1.
Spectra of emitted radiation were analysed by means of monochromator Jobin Yvon-Spex Triax (focal
length 550 mm) using plane grating (1200 grooves/mm) with spectral resolution 0.024 nm. The monochromator
is equipped with thermo-electricly cooled MTE CCD 1024x256-16 detector linked to the CCD 3000 controller
connected to the PC. This arrangement allowed us to detect emission spectra in the range 200 - 1050 nm.
Spectra were detected in axial direction as can be seen in Fig. 2.
Figure 1. Electric circuit for measurements of axial electric field strength.
Figure 2. Experimental set-up for spectral measurements in axial direction.
Great attention has been devoted to the purity of the experimental system to avoid affecting plasma
properties by presence of impurities [Hrachová et al., 2004]. Discharge tube was heated up to 420 °C before
each measurement and it was pumped for several hours, using turbomolecular pump pre-pumped by diaphragm
pump. The pressure in vacuum system checked after this procedure by full range gauge was better than 5.105
Pa.
Our measurements were realized in spectrally pure oxygen of Linde production (declared purity better than
10 ppm) in pressure range 150 - 950 Pa and for discharge currents up to 40 mA.
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SCHMIEDT ET AL.: SPECTRAL DIAGNOSTICS OF OXYGEN DISCHARGE
Results
Study of H and T form
First we have focused on study of range of existence of both forms. The H form was detectable in the
whole pressure range while the T form was found for pressures above 600 Pa only. Existence of particular forms
was verified by measurements of axial electric field strength. Example of course of electric field strength in both
forms is to be seen in Fig. 3. Similar behaviour was observed in the whole range where both forms were
detected.
Figure 3. Dependence of electric field strength on discharge current (p = 800 Pa).
Identification of the spectra
Due to the simultaneous presence of both forms in the discharge tube for higher pressures, spectral
measurements were realized for pressures up to 530 Pa, when only the H form of the discharge exists.
Our attention was paid to emitted radiation. Emission spectra were detected in the range 200 - 900 nm.
Spectral lines of atomic oxygen triplet 777.2 nm and atomic oxygen 844.7 nm and atmospheric A-band of
molecular oxygen were the most intensive parts of oxygen emission spectra in this spectral range.
Pressure dependences of above mentioned lines and band for several values of discharge current are shown
in Fig. 4 and 5. The increase of intensities of all studied lines and band was observed. Moreover, intensity of the
head of atmospheric A-band was found to increase with increasing pressure, while decrease of intensities of the
both atomic lines was observed. This effect can be simply explained with decreasing of oxygen molecule
dissociation with increasing pressure as a result of shortening of the mean free path of the electrons and hence
diminution of their energy gained from acceleration by the electric field.
Figure 4. Pressure dependence of intensities of atomic 777.2 nm line and head of A-band at 759.4 nm.
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SCHMIEDT ET AL.: SPECTRAL DIAGNOSTICS OF OXYGEN DISCHARGE
Figure 5. Pressure dependence of 844.7 nm line.
Figure 6. Pressure dependence of the rotational temperature Trot.
Temperature Determination
Rotational temperature Trot of O2 molecules has been determined from well-resolved atmospheric A-band.
Pressure dependence of rotational temperature determined from PP branch of Boltzmann plot [Touzeau et al.,
1991] is shown in Fig. 6. As can be seen, the increase of the rotational temperature Trot with increasing pressure
and discharge current has been observed. That can be explained by increasing power fed to the discharge.
Conclusion
Optical emission spectroscopy has been employed for study of DC glow discharge sustained in pure
oxygen. Spectral measurements were analysed for pressures up to 530 Pa, since two different forms of positive
column of oxygen discharge were detected for higher pressures. It has been found, that intensities of all studied
lines increase with increasing discharge current, while different behaviour of pressure dependences for particular
lines was observed.
The rotational temperature Trot of oxygen molecules has also been determined from the atmospheric Aband. Its increase with increasing pressure and discharge current has been observed. This behaviour is in a good
agreement with previous results.
Acknowledgments. This research has been supported by the research plan MSM 0021620834 that is
financed by the Ministry of Education of the Czech Republic.
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SCHMIEDT ET AL.: SPECTRAL DIAGNOSTICS OF OXYGEN DISCHARGE
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