22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Spectroscopy of a miniature spark discharge in the range of 40 - 1100 nm
H. Kersten, S. Klopotowski and T. Benter
University of Wuppertal, Wuppertal, Germany
Abstract: Spectroscopic data of the VUV and UV/VIS/NIR emission of a small spark
discharge, operated with different gas compositions at atmospheric pressure are presented.
The VUV spectra range from 40 to 200 nm and are recorded with a scanning spectrometer,
modified to handle atmospheric pressure conditions. A compact solid state fibre optical
spectrometer is used to additionally cover the UV/VIS/NIR range from 200 to 1100 nm.
Keywords: spark discharge, VUV and UV/VIS emission spectroscopy
1. Introduction
Atmospheric pressure photoionization is a wellestablished method for routine analytical applications in
mass spectrometry [1]. Typically, a RF discharge at
13.56 MHz is maintained inside a small glass tube with
around 1 mbar of a rare gas present. The tube is sealed
with either a MgF 2 or LiF window, which limits the
ionizing radiative output to a minimum of 110 or 105 nm,
respectively. In 2011 our group introduced a miniature
spark discharge lamp, mounted downstream and
windowless inside a capillary atmospheric pressure
sampling stage of a common mass spectrometer [2]. The
discharge gas is flown continuously through two hollow
electrodes with an outer diameter of 0.9 mm and a
spacing between 0.5 and 2 mm. The triggered high
voltage supply with amplitudes of up to 1 kV is provided
by two capacitors on a hand sized circuit board with a
maximum repetition rate of 1.5 kHz. The very small
footprint of the spark assembly assures electromagnetic
compatibility with many peripheral devices.
The
satisfying performance as a photoionization lamp was
demonstrated in terms of temporal and spatial stability of
the discharge, analytical reproducibility and sensitivity.
However, current research focuses on the fundamental
understanding of the discharge process under various
conditions. Two aspects are of major concern: 1. The
temporal evolution of the relevant species and the emitted
radiation during a spark event, and 2. Matching the VUV
emission characteristics with UV/VIS/NIR spectra. The
latter is not only of mechanistic, but also of valuable
practical interest. A spectral database of both regimes
recorded under the same conditions can be used to predict
VUV emissions by simply recording the much easier
accessible UV/VIS/NIR range. In this contribution, we
present systematic measurements in both spectral regimes
under identical experimental conditions.
2. Experimental
A
modified
ActonResearchCooperation-VM502
spectrometer operating at atmospheric pressure is used for
the VUV-emission spectroscopy (Fig. 1). To minimize
interferences with absorbing impurities the spectrometer
is continuously flushed with highly purified helium
P-II-4-16
(<10 ppbV impurities). For windowless operation the
spark discharge-source is placed directly in front of the
spectrometer inlet slit. The discharge gas is provided
through the cathode at around 200 ml/min and a rough
pump is connected to the anode. The required purity of
the supplied gases is achieved with a VICI gas-purifier
HP2-220. A custom scintillator/PMT combination is used
for VUV wavelength conversion to 420 nm and
subsequent detection. The PMT output is monitored with
an active probe connected to an oscilloscope (RT-ZS10E
and RTE 1054, R&S, Cologne, Germany).
A
measureable wavelength range of 40 to 200 nm is
obtained. A fibre optic is mounted on the flange covering
the discharge region and is connected to a compact CCD
spectrometer (AvaSpec-3848, Avantes BV, Eerbeek, The
Netherlands) which covers the spectral range from 200 to
1100 nm.
Fig. 1. Experimental setup.
3. Results and Discussion
Two corresponding emission spectra are exemplary
shown in Figs. 2 and 3. The main discharge gas
component was helium at 1000 mbar with traces of air
and n-hexane. Clearly seen is the broad emission of the
electronically excited dimer He 2 * at 80 ± 10 nm,
generated according to the following mechanism [3]:
He + e- → He* + eHe* + He + M → He 2 * + M
(1)
(2)
1
He 2 * → 2 He + hν (80 ± 10 nm)
(3)
The formation of the helium metastables (He*) can
proceed directly according to equation (1) or indirectly
through radiative relaxation of higher excited states. The
latter process is a precursor emission to the metastable
state and is observed in the UV/VIS/NIR regime. Several
precursor emissions to the formation of He* are indicated
in Fig. 3. At atmospheric pressure, the subsequent
termolecular reaction (2) generating an excited molecular
state is fast [4]. Some excimer states radiatively decay to
the electronic ground state with emissions in the VUV
regime. Since the ground state is repulsive, the observed
emission around 80 nm is very broad. According to the
described cascade, some of the precursor emission lines in
the easily accessible UV/VIS/NIR spectrum can be used
to predict the appearance of the VUV regime, which is
much more elaborate or, in some experimental setups,
even not possible to access.
So far, in all our
measurements at around 1000 mbar only this broad band,
which is assigned to the “second excimer continuum”,
was observed. Pressure variations between 100 and
1000 mbar will clarify the appearance of the smaller “first
continuum”, which is, according to Kurunczi et al. [3],
centred at 60 nm. A well-known effect in the presence of
trace components is near-resonant energy transfer from
the excimers [3]. As shown in Fig. 2 this process
quenches the excimer population very effectively, the
second continuum decreases and characteristic emissions
of the trace components dominate the VUV spectrum. In
the UV/VIS/NIR regime, several characteristic emission
lines and bands of the trace components appear as well
(c.f. Fig. 3). The derivation of the VUV spectrum from
the UV/VIS/NIR regime in the presence of trace
components is also an objective of this contribution.
Fig. 3. Corresponding UV/VIS/NIR spectrum for the
VUV spectrum in Fig. 2. Several precursor emission
lines to the formation of helium metastable states He* are
indicated.
4. Summary
We present systematic emission spectroscopic
measurements of a spark discharge under various
conditions. The measurements cover the spectral range
from 40 to 1100 nm. The observations of different
discharge gas constituents with changing concentration,
as well as the total pressure variation between 100 and
1000 mbar are discussed. A key aspect is the derivation of
the VUV regime from the easier accessible UV/VIS/NIR
spectrum.
5. Acknowledgements
Financial support from the German Research
Foundation (DFG) within project KE 1816/1-1 is
gratefully acknowledged.
Fig. 2. VUV spectrum of a helium-discharge with N 2 , O 2
and n-hexane traces present.µ
2
6. References
[1] J.A. Syage. "Mechanism of [M+H] Formation in
Photoionization Mass Spectrometry". Am. Soc.
Mass Spectrom., 15, 1521-1533 (2004)
[2] H. Kersten, V. Derpmann, I. Barnes, K. Brockmann,
R. O'Brien and T. Benter. "A Novel APPI-MS
Setup for In Situ Degradation Product Studies of
Atmospherically Relevant Compounds: Capillary
Atmospheric Pressure Photo Ionization (cAPPI)".
J. Am. Soc. Mass Spectrom., 22, 2070-2081 (2011)
[3] P. Kurunczi, J. Lopez, H. Shah and K. Becker.
"Excimer formation in high-pressure microhollow
cathode discharge plasmas in helium initiated by
low-energy electron collisions".
Int. J. Mass
Spectrom., 205, 277-283 (2001)
[4] B.G. Yu, V.A. Maiorov, J. Behnke and J.F. Behnke.
"Modelling of the homogeneous barrier discharge in
P-II-4-16
helium at atmospheric pressure". J. Phys. D: Appl.
Phys., 36, 39 (2003)
P-II-4-16
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