Download Considerations on the use of atmospheric pressure plasma to generate complex molecular environments with relevance for molecular astrophysics

22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Considerations on the use of atmospheric pressure plasma to generate complex
molecular environments with relevance for molecular astrophysics
I. Topala, R. Jijie, A.V. Nastuta, I. Mihaila, I.A. Rusu and V. Pohoata
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Iasi Plasma Advanced Research Center (IPARC), Faculty of Physics, Alexandru Ioan Cuza University of Iasi,
Blvd. Carol I No. 11, 700506 Iasi, Romania
Abstract: Atmospheric pressure plasmas devices are suitable experimental solutions to
generate pulsed molecular environments with various applications. We will discuss here
details related to the chemical mechanism in DBD discharges related to He+H 2 plasma
processing of hydrocarbons in order to simulate the chemical environment in hot cores.
Molecular beam mass spectrometry is employed to characterize the chemical environment
and to compare experimental data with telescope observations.
Keywords: atmospheric pressure plasma, mass spectrometry, molecular astrophysics
1. Introduction
Interstellar medium represents a complex architecture,
with a multiple phase structure. Among these phases,
molecular clouds are of particular interest, even they
represent less than 1% of total medium. The matter
present in the interstellar medium is represented mainly
by atoms and molecules; a small fraction is represents by
carbon and silicon based grains. Biogenic elements (i.e.
O, C and N) abundance is around 0.1%, while H and He
represents more than 99% of atomic matter [1].
Young stellar objects can be detected within giant
molecular clouds and some of these objects are
represented by ambient gas at average temperatures of
300 K, known as hot cores. These are massive regions of
dust and gas (containing a large fraction of molecules),
with a mass range between 10 to 3000 solar masses and a
strong emission in far infrared range. The molecular
hydrogen density in central region of hot cores reaches to
107 cm-3 and the average temperature in the range 100300 K [2]. In contrast, most of the interstellar medium
volume show high values of temperature (T > 103 K) and
in consequence does meet criteria for molecular synthesis.
Around 75 years ago, CH was first molecule discovered
in space. Since then efforts have been made to identify
more and more classes of molecules in interstellar space
or at the surface of various astronomical objects and this
campaign ended with more than 150 interstellar and
circumstellar molecules, with an increase of the molecular
mass and complexity (bond angles, length and atomic
partners) [3]. Scientific data allowing new molecules
discovery are coming mainly from telescope observations
(i.e. rotational transitions data). It is of high interest to
cross check these data with laboratory experiments
results: plasma experiments, crossed molecular beams,
selected ion flow tube mass spectrometry, UV processing
or atomic/ion bombardment of interstellar ice analogues.
From all these techniques, laboratory plasma
experiments are valuable resources to study complex
molecular environments and relatively not expensive
experimental techniques [4]. Many results, in specific
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plasma conditions and experimental arrangements have
shown that plasma can be used for simulations solid state
astrochemical processes, Titan tholin formation,
laboratory mimicking the interstellar dust formation and
charging, synthesis of enol and keto species [5-7].
Our approach to generate complex chemical systems
with relevance for the ongoing processes in hot molecular
cores, is to use a non-equilibrium plasma at atmospheric
pressure. The discharge is feed with a complex mixture of
gases: helium – hydrogen – hydrocarbons (methane,
ethane, propane, butane). Molecular beam mass
spectrometry is used to detect and identify the chemical
population generated in plasma volume.
2. Experimental
A dielectric barrier discharge reactor was designed and
constructed for this study in order to be coupled with the
molecular beam mass spectrometer (HPR-60, Hiden
Analytical). Two rectangular electrodes, supported by 1
mm thick glass discs and 0.74 mm gap, are used to
generate the plasma in controlled gas mixture conditions,
in a volume of about 2.32 cm3. The electrode arrangement
is hosted by a stainless steel reactor. Helium flows
between discharge electrodes with a flow rate of 2.45
L/min, while hydrogen is mixed at a constant flow rate of
1 mL/min. Four types of hydrocarbons were also
independently introduced inside the reactor, with flow
rates of 4.25 mL/min. High voltage sin wave is used to
power one electrode, while the other is grounded (specific
excitation conditions are: 2.45 – 2.6 kV amplitude, 1 kHz
frequency). Multiple electrical breakdown occurs on
every increasing and decreasing edge of the high voltage
wave. This results in individual current pulses (multi peak
discharge), with average amplitude of 68 - 80 mA.
A quadrupole-based system was used for this purpose
(HPR-60 MBMS, from Hiden Analytical Ltd), with
2500 amu upper mass range. The sampling system
consists of three differentially pumped stages, separated
by aligned skimmer cones. Neutral atomic or molecular
gas population is detected in real time by the molecular
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beam mass spectrometer. Moreover, positive and negative
ion population is sampled from discharge volume and
monitored during discharge operation.
All mass spectrometry data presented in this work are
time averaged, showing the overall effect of plasma on
chemical synthesis and clustering effects.
3. Results and discussions
The capacity of atmospheric pressure plasma generated
based on dielectric barrier principle to generate transitory
molecular populations with relevance for molecular
astrophysics of hot cores was assessed by molecular beam
mass spectrometry analysis.
Fig. 1 shows typical data obtained for helium –
hydrogen – hydrocarbons plasmas at atmospheric
pressure, for both negative and positive ion populations.
Fig. 1. Positive and negative ion mass spectra for various
helium – hydrogen – hydrocarbons plasmas at
atmospheric pressure.
RGA monitoring of the neutral gas was also considered
during our studies. Nevertheless data are not shown here
due to possible interferences of ionization regions: plasma
electrons and spectrometer electron gun.
Ion population have a strong influence on plasma
chemistry, by polymerization, recombination or clustering
processes. Hydrocarbons molecule mass was found to
influence the overall mass spectrum of the plasma, for
both positive and negative ion population. For example,
the positive ion spectra are dominated by grouped mass
signals around C n H 2n-1 , C n H 2n , C n H 2n+1 masses (n up 8).
On the other hand negative ion spectra are more complex
and spread to higher masses. The population appears to be
dominated by water clusters around negative core ions
with general formula A-(H 2 O) n (n up to 26), with A
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represented in various fractions by HO-, O 2 -, HO 2 -, NO 2 -,
NO 3 -, CO 3 -.
4. Conclusions
The atmospheric pressure plasma generated in helium –
hydrogen – hydrocarbons represents a good solution to
non-equilibrium
chemical
population,
with
a
representative fraction of molecular clusters. Relatively
high mass molecules are dissociated and recombination
phenomena lead to many types of molecular families,
some of them being also identified in molecular hot cores
(i.e. hydrocarbon radicals or complex hydrocarbons).
5. References
[1] D. Wooden, Composition and evolution of interstellar
clouds. In Comets II, M. C. Festou, H. U. Keller, and H.
A. Weaver (eds.), University of Arizona Press, 33‐66
(2004)
[2] R. Rolffs, Structure of Hot Molecular Cores. PhD
thesis, Köln University (2011)
[3] J. Nuth et al., Chemical Processes in the Interstellar
Medium: Source of the Gas and Dust in the Primitive
Solar Nebula. In Meteorites and the Early Solar System
II, D. S. Lauretta and H. Y. McSween Jr. (eds.),
University of Arizona Press, 147‐167 (2006)
[4] D. Schram, Astro chemistry – Plasma chemistry.
Some plasma aspects: processes and questions, Annual
meeting of the Astrophysical Chemistry Group of the
RSC and RAS on progress in astrochemistry, intro 2,
(2011)
[5] H. Thejaswini et al., Plasma chemical reactions in
C 2 H 2 /N 2 , C 2 H 4 /N 2 , and C 2 H 6 /N 2 gas mixtures of a
laboratory dielectric barrier discharge. Advances in Space
Research 48, 857–861, (2011)
[6] W. Goedheer, Nucleation in plasmas, charge and
dynamics of dust, Annual meeting of the Astrophysical
Chemistry Group of the RSC and RAS on progress in
astrochemistry, inv 4, (2011)
[7] J. Wang, et al. Interstellar enols are formed in plasma
discharges of alcohols. The Astrophysical Journal, 676,
416‐419, (2008)
Acknowledgments: This work was supported by
Romanian Space Agency (ROSA) under the project
STAR CDI ID 349/2014-2016.
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