22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Processing of plasma deposited analogues of interstellar carbonaceous dust by
UV irradiation, electron bombardment and plasma exposure
M. Jiménez-Redondo, B. Maté, I. Tanarro, M.A. Moreno, R. Escribano and V.J. Herrero
Instituto de Estructura de la Materia, IEM-CSIC, Serrano 123, 28006 Madrid, Spain
Abstract: The stability of hydrogenated amorphous carbon (a-C:H) deposits, generated
through radiofrequency discharges of He+CH 4 mixtures, has been studied. Irradiation with
UV photons (~165 nm), 2 keV electrons, and radiofrequency discharges of pure He and H 2
have been employed to process the dust analogues. The evolution of the samples has been
monitored with infrared spectroscopy.
Keywords: plasma deposition, film processing, interstellar carbon dust analogues
1. Introduction
Carbonaceous compounds, both solids and gas-phase
molecules, are found in very diverse astronomical media
[1]. A significant amount of the elemental carbon is found
in small dust grains. This carbonaceous dust, mostly
formed in the last stages of evolution of C-rich stars, is
the carrier of characteristic IR absorption bands revealing
the presence of aliphatic, aromatic and olefinic functional
groups in variable proportions [2]. Among the various
candidate materials investigated as possible carriers of
these bands, hydrogenated amorphous carbon (a-C:H) has
led to the best agreement with the observations.
Carbonaceous grains are processed by H atoms, UV
radiation, cosmic rays and interstellar shocks in their
passage from asymptotic giant branch stars to planetary
nebulae and to the diffuse interstellar medium. The
mechanisms of a-C:H production and evolution in
astronomical media are presently a subject of intensive
investigation.
In this work we present a study of the stability of
carbonaceous dust analogues generated in He+CH 4
radiofrequency discharges. In order to simulate the
processing of dust in the interstellar environments, the
samples have been subjected to electron bombardment,
UV irradiation, and both He and H 2 plasma processing.
IR spectroscopy is employed to monitor the changes in
the structure and composition of the carbonaceous films.
pressure in the chamber. A quadrupole mass spectrometer
(Balzers, Prisma Plus-QMG220) is installed in a
differentially pumped vacuum chamber in order to sample
through a ~50 µm diaphragm the dissociation degree of
the CH 4 precursor and the stable neutral species produced
in the discharge. Mixtures of 40% CH 4 and 60% He, with
a total pressure of 0.3 mbar, are used as precursor gasses.
Two silicon substrates, 2.5 cm diameter, 1 mm thickness,
are placed in the reactor at different positions (inside the
coil and ~5 cm outside towards the gas exit) to obtain two
types of deposits. Depending on the deposition time (1 –
90 min), the thickness of the carbonaceous films varies
between ~30 nm and 3 µm.
The reactor described above is also used for the plasma
processing, for which either a discharge of pure He or
pure H 2 , at 0.3 mbar and 40 W, is maintained. In this
case, the samples are placed inside the coil.
The processing of the dust analogues with UV photons
and electrons is performed in a different setup. The
samples are placed in a stainless steel cylindrical
chamber, with a residual pressure of 5×10-8 mbar, which
can be cooled down with either a closed-cycle helium
cryostat or liquid nitrogen. The system is coupled to a
Vertex70 FTIR spectrometer in normal incidence
transmission configuration. Depending on the experiment,
a UV lamp (~165 nm photons) or an electron gun (2 keV
electrons) is installed in the chamber.
2. Experimental
The experimental system has been described elsewhere
[3]. It comprises two different setups.
The inductively coupled RF discharge reactor used for
the deposit generation consists of a Pyrex tube (30 cm
length, 4 cm diameter) with a 10 turns Cu coil placed
externally around the central part of the tube. The coil is
fed by a 13.56 MHz RF generator (Hüttinger PFG 300 RF
+ matchbox PFM 1500A) to produce the plasma, which is
maintained at a constant power of 40 W during the
deposition processes. The gas inlet is placed at one of the
ends of the reactor, and at the other end a rotary pump is
connected through a regulating valve to keep a constant
3. Results
3.1 Structure of the carbonaceous deposits
Depending on the position in the plasma reactor in
which the substrate is placed, the infrared spectrum of the
films shows distinct features (Fig. 1).
Following the method used by Chiar et al. [2], it is
possible to analyse the hydrocarbon structure by
estimating the relative abundances of hydrogen and the
two bonding types for carbon (sp2 and sp3). To perform
this, the synthetic spectrum has to be obtained by
identifying the different bands (Fig. 2).
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1
0.06
Inside the coil
Outside the coil
0.05
absorbance
0.04
0.03
0.02
0.01
0.00
-0.01
3000
2500
2000
1500
wavenumber cm-1
Fig. 1. Spectra of two carbonaceous deposits grown in
different positions of the plasma reactor.
The results obtained show that the hydrocarbon deposits
grown inside the coil have a larger abundance of sp2 C
compared to the ones grown outside. The spectrum of the
latter is similar to the ones that can be observed in the
interstellar medium [2,4]. These results can be
represented in a ternary diagram showing the relative
abundances of hydrogen atoms, sp2 bonds and sp3 bonds
in the hydrocarbon structure (Fig. 3).
Fig. 3. Ternary diagram showing the sp2, sp3 and H
abundances in the samples. a-C:H 1 corresponds to the
film grown outside the coil and a-C:H 2 to the one grown
inside. The results for interstellar hydrocarbon grains
from [2] are also shown (black symbols).
0.04
0.4
t=0 min
t=10 min
0.03
absorbance
Absorbance
0.3
0.2
0.02
0.01
0.1
0.00
0.0
3200
3000
3000
2800
1800
1600
1400
wavenumber (cm-1)
Fig. 2. Experimental (black) and synthetic (magenta) IR
spectra of a carbonaceous deposit.
3.2 Sample processing
The processing of the samples by UV photons,
electrons and plasma has been monitored through their IR
spectra. An example of the a-C:H deposit spectrum before
and after processing by a H 2 plasma can be seen in Fig. 4.
The variations observed in the spectrum evince changes
in the structure and composition that are different
depending on the processing the sample is subjected to.
2
2500
2000
1500
wavenumber cm-1
Fig. 4. Spectra of the carbon deposit before (black) and
after (red) a 10 minute processing by a H 2 plasma.
Electron bombardment is shown to have little effect in
the structure of the deposit. Electrons erode the films
layer by layer, thus leaving the composition of the
hydrocarbons unchanged. UV photons, on the other hand,
change the structure of the carbonaceous deposit,
producing a dehydrogenation of the carbon structure.
Processing with He and H 2 plasmas has a pronounced
effect on the aliphatic sp3 part of the structure, which is
strongly depleted, whereas the aromatic component is
hardly affected. The gas used for the processing does not
seem to have a big influence in the result of the
processing, which is the reduction of the hydrogen
abundance and the increase in sp2 type bonds.
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4. Summary and conclusions
Deposits of carbonaceous dust analogues have been
grown in inductively coupled RF discharges of He+CH 4 .
The samples have been processed with electron
bombardment, UV irradiation, and exposure to plasmas of
He and H 2 .
The position of the substrate in the plasma reactor is
shown to have a major impact in the structure of the
deposited hydrocarbon. Films grown outside the coil bear
a greater similarity in composition to those found in the
interstellar medium.
Bombardment with 2 keV electrons causes the film to
be eroded layer by layer, without modifying the structure
and composition. UV irradiation and plasma processing
produce a dehydrogenation of the carbonaceous deposit,
and a decrease in the amount of sp3 bonds. The gas used
for the plasma processing, He or H 2 , does not seem to
impact the result.
5. Acknowledgements
This work has been funded by the MCINN of Spain
under grants FIS2013-48087-C2-1-P and the Consolider
Astromol, CDS2009-00038. We thank also the European
Research Council for additional support under ERC-2013Syg 610256-NANOCOSMOS. MJR has received funding
from the FPI program of the MCINN.
6. References
[1] A.G.G.M. Tielens. Rev. Mod. Phys., 85, 1021 (2013).
[2] J.E. Chiar, A.G.G.M. Tielens, A.J. Adamson and A.
Ricca. Astrophys. J., 770, 78 (2013).
[3] B. Maté, I. Tanarro, M.A. Moreno, M. JiménezRedondo, R. Escribano and V.J. Herrero. Faraday
Discuss., 168, 267 (2014).
[4] E. Dartois, T.R. Geballe, T. Pino, A.T. Cao, A. Jones,
D. Deboffle, V. Guerrini, P. Brechignac and L.
d'Hendecourt. A&A, 463, 635 (2007).
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