The electronic spectra of fullerene structures in high frequency discharge are studied in the plasma chemistry laboratory of the Faculty of Science of Masaryk University in Brno. The ultraviolet and visual spectra are investigated in order to be compared with the diffuse interstellar bands and interpreted within the theory of quantum mechanics. The preliminary results of the study are presented here in the form of a poster

Janca, J.

Solc, M.

Vetesnik, M.

English

NASA Technical Reports Server

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LABORATORY SPECTRA OF C6o AND RELATED MOLECULAR STRUCTURES
J. Jan_a,* M._olc,** and M.Vete_nfk***
*Laboratory of Physics of Plasma, Masaryk University, 61137 Brno, Czech republic
**Astronomical Institute, Charles University, _v_dsk_ 8, Prague, Czech republic
***Dept. of Theoretical Physics and Astrophysics, Masaryk University, 61137 Brno, Czech republic
The electronic spectra of fullerene structures in high frequency discharge
are studied in the plasma chemistry laboratory of the Faculty of Science
of Masaryk University in Brno. The ultraviolet and visual spectra are
investigated in order to be compared with the diffuse interstellar bands
and interpreted within the theory of quantum mechanics. The preliminary
results of the study are presented here in the form of a poster.
It is more than two centuries since the Swiss geometrician Leonhard Euler proved
the existence of closed spheroidal surfaces composed of twelve pentagonal and a vari-
able number of hexagonal planar faces. In the past decade, the structures occupied
by carbon atoms and ions engrossed theorists in the molecular dynamics, and the
mathematical theory of Euler began to be used for theoretical modelling of special,
highly stable molecular clusters termed fullerenes. It was found that there exist at
least one or two configurations of carbon atoms predisposed to form soccer-ball-like
molecules characterized by a high degree of symmetries, namely C60 and C70 .
The fullerenes and other related structures soon became a centre of interest
of astrophysicists who explained some specific properties of absorbing matter in the
interstellar space with the presence of these strange, large molecules. Even though
contradictory facts still exist, it seems compelling enough to accept that the absorp-
tion peak at 217 nm, which has long been attributed to small particules of graphite
(Huffman, 1977), might be produced by the giant spheroidal molecules composed of
sixty atoms of carbon and structured in a pentagonal and hexagonal polyhedral net,
characteristic of the very fullerenes (KrXtschmer et al., 1990).
Because polyhedral carbon clusters appear to be excellent candidates for car-
riers of diffuse interstellar band opacities observed in the circumstellar environment
(Ldger et al., 1988), a laboratory determination of their ultraviolet and visual absorp-
tions would be most useful. In spite of grave experimental difficulties, very sophis-
ticated techniques have been developed that now enable the synthesis of fullerenes
53
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in macroscopic quantities by vaporizing graphit under specific thermodynamic con-
ditions. Thus, polyhedral carbon molecules can now be produced by laser irradiation
or with resistive heating of graphit and they can be formed in a sooting flame or in
an electric arc. It was discovered that various stable fullerenes could be synthetized
in this way (with 32 and even more carbon atoms arranged at the same number of
vertices of truncated icosahedrons) but only C60 and C70 can be prepared in quanti-
ties enabling an investigation of their physical properties. This paper describes two
experiments designed to produce fullerene molecules in the laboratory as well as to
study the processes of their formation.
The first experiment was fulfilled in the arrangement used many times for pro-
ducing superconductive films by the deposition technique. In contrast to the common
low pressure chemical vapour deposition, our radio frequency deposition of C60 works
at atmospheric pressure, and, without requiring a high vacuum, permits a wide va-
riety of processing environments. The carbon films containing fullerene molecules
were produced by thermal vapour depositions in a radio frequency unipolar plasma.
Using the radiofrequency of 27 MHz, carbon films, rich on fullerenes, were pro-
duced on a mono-crystalline silicon substratum by the technique of depositions on
the plasmatically vaporized chemical aerosols. The ions contained in the aerosol mist
were generated in an ultrasonic nebuliser and were injected through an electrode into
the plasma region. The high frequency generator of the power output of 1.5 kW work-
ing at 27 MHz was connected through a matching network with a graphite electrode.
The toluene diluted by argon was used as the carrier gas. Once the liquid precursors
were evaporated into the plasma, they decomposed and reacted in the boundary layer
over the substrate to form a fullerene abundant carbon film.
As the extensive heat flux from the plasma plume to the substratum rose during
the deposition processes, the substratum immersed in the upper part of the unipolar
high-frequency plasma had to be thermally connected with a special air cooled holder.
Such effective cooling of the substratum is crucial for controlling the microstructure
of the film and to minimize the film-substratum interactions. It is worth noting that
the substrate temperature could also be controlled by a special arrangement in the
flow of the carrier gas.
Different substrate temperatures in the range of 200 up to 10000°C were used
to deposite the fullerene rich film of carbon onto the substratum. Depending on
the concentration of the toluene in the carrier gas flow and on the position of the
substratum in the high-frequency plasma, tile deposition rate was between 300-120
nm/min, and the film thickness varied between 0.4 - 2 #m .
The deposited samples of fullerenes were treated for laboratory measurements in
the way normally used in chemical practice. They were extracted from the substrac-
tion target in redestilled, hot CHCI3, then the solution was evaporated in a vacuum
and the residue solid extract was dried and finally dissolved in n-hexane. The UV
absorption spectra of the resulting solution were measured in a quartz cell. Conta-
mination by "UV impurities" made it very difficult to evaluate these spectra, because
54
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of the peak of Cs0 at 213 nm that might be blended by the band corresponding to
trichlormethan that was not completely removed. For this reason, the reference solu-
tion was prepared in the same manner so that the corresponding absorption curves
could be compared to each other. From the smooth course of the reference curve, it
could be deduced that the peak at 252 nm had been produced with a new constituent
in the solution. Later, after evaporating the hexane, the solution could be seen in a
crystalic form through a microscope as expressing all the features of a genuine ful-
lerite, The original record of the absorption curve of the fullerite is given in Figure
2. It can be compared with the absorption curve of the pure C60 in Figure 3 .
This experiment has shown that besides many sophisticated ways developed to
synthetize fullerenes for practical use, there may still exist other techniques suitable
for producing them in macroscopic quantities. From this point of view, the results
may be of interest to those who are engaged in the technological aspects of forming
these large molecules in an artificial way. But there are other aspects that may give
a new importance to this technique, especially regarding the possibility of under-
standing better and explaining the processes that lead to the formation of fullerenes.
This very aspect is important for astrophysicists because, to a certain extent, it can
contribute to interpretations of the carriers of the diffuse interstellar bands. From
the astrophysical point of view, the experiment surely might attract the interest of
those who are engaged in solving the problems of the formation of fullerene molecular
structures in a close stellar environment or even in the interstellar space. Except
for the high frequency electromagnetic field, one can suppose that the conditions for
the formation of fullerene molecules in our experiment should be similar to those
prevalent in atmospheres of some types of highly evolved stars producing a carbon
constituent of the interstellar matter (e.g. of carbon stars). That is why we decided
to rearrange and to repeat the experiment once again and direct it this time at the
spectroscopical investigation of problems connected with the formation of fullerene
structures in plasmatic environments.
The arrangement of the new experiment is similar to that already realized, only
the cylinder demarcating the space of the plasmatic discharge has been replaced with
one of quartz. For the spectrometric measurements, the apparatus was completed
with a Seya-Namioka monochromator with concave difraction grating enabling the
study of spectral features of the discharge in the range of 200 to 600 nm. Using an
accessory source 0f high temperature contirmum light, one also has the possibility
of getting the absorption spectra of carbon molecular clusters. The experiment is
schematically described in Fig.1 and is now at a stage of testing. The first results
enabling a theoretical interpretation of the formation of large molecular clusters of
carbon from their less complex fragments are expected to be obtained in the second
half of the year.
55
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First note added in proof:
When discussing the fullerene structures as possible carriers of the DIBs, one
should take into account that the spherical fullerenes are not the only possible iso-
mers of the polyatomic carbon type molecules. It seems a puzzle to regroupe the
carbon pentagons and hexagons in fullerene cages into another configurations but
the fact is that, in this manner, we can get tubules at both sides closed by two co-
vers each composed of six pentagons and unlimited number of hexagons. In Fig.4,
which graphically describes the rod-like cage carbon structures C(_.12) (n = 2,3,4,...)
, the graphitic tubule lattices arise by wrapping the honeycomb lattice on a cylinder
whereas each cover is formed from a hexagon surrounded by six pentagons. One can
object that the stability of the covers must not be too high (Kroto 1987) but the free
bounds at the ends of the tubules can be occupied with hydrogen atoms and a special
type of polyatomic hydrocarbon tubules formed. It is beyond question that this kind
of hypothetical molecular structures should express specific polarizing features in the
interstellar space. Does anybody succeed in trying to compute their stability and
their electronic rotation-vibration spectra?
Second note added in proof:
Only little changes are needed to rearrange the experiment in order to form
fragments of fullerene-like cage structures based on silicon. It is worth noting at this
place that silicon clusters are currently of growing interest as new materials with
interesting properties (Kobayashi, Nagase, 1993) and that the cosmical abundance
of silicon is comparable with those of carbon. Thus, even though there is a strong
chemical bond between silicon and oxygen in astrophysical environments, some atoms
of silicon may form fullerene-like structures. Why not to take them into consideration
also in investigating the carriers of DIBs?
Third note added in proof:
Even if there is no evidence that fullerenes are present in cometary atmospheres
which are otherwise rich on the carbon compounds, one can think about the way how
to detect them by IR spectroscopic measurements. It is known that the light from
stars is affected only a little when passing through a dilute cometary coma. However,
in case of "occultation" of an early type star by the innermost region of coma, some
absorption features of cometary origin could be seen in the spectrum.
An extensive list of comets is being prepared for selection of target objects to be
observed by the Infrared Satellite Observatory (launch scheduled on Autumn 1995).
During next 18 months (the assumed active lifetime of the satellite) comets P/Kopff,
P/Churyumov-Gerasimenko, P/Wild 2, and also P/Schwassman-Wachmann 1 and
3, P/Encke, P/d'Arrest, P/Honda- Mrkos-Pajdusakova are expected to be bright
56
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and observable. Spectrophotometry in the region of 2-12 microns by ISO/ISOPHOT
instrument and simultaneous Earth-based visible spectroscopy and photometry of
comets will include also the observations of selected stars "occulted" by comets. Un-
fortunately, due to various perturbations in the motion of comets and due to the
hardly predictable cometary activity in developing atmospheres no definitive selec-
tion of stars with suitable spectra can be made. The candidate stars, located in the
about 10' wide strip on sky along the predicted path for every of the target comets, are
surprisingly few, only 2 to 10 for each target comet. So it will deserve a cooperation
between observatories to fulfill this program.
Thus, even if there is no evidence that fullerenes are present in cometary atmos-
pheres, one may hope that these proposed observations could bring at least upper
limits for the concentration of DIB's carries of any kind in comets.
References:
Ajie, H., Alvarez, M.M., Anz, S.J., Beck, R.D., Dietrich, F., Fostiropoulos, K., Huff-
man, D.I-{., Krgtschmer, W., Rubin, Z., Schiver, K.E., Sensharma, D. and Whet-
ten, R.L.: 1990, Jour. Phys. Chem. 94, 8630
Huffman, D.R.: 1977, Adv. Phys. 26, 129
Krgtschmer, W., Lamb, L.D., Fostiropoulos, K. and Huffman, D.R.: 1990, Nature
347, 354
Kroto, H.W.: 1987, Nature 329., 529
Kobayashi, K. and Nagase, S.: 1993, Bull. Chem. Soc. Jpn. 66, 3334
Ldger, A., d'Hendecourt, L., Verstraete, L. and Schmidt, W.: 1988, Astron.& Astro-
phys. 203,145.
57


Laboratory spectra of C60 and related molecular structures

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