Carbon Chain Molecules in Cryogenic Matrices - Polyynes: Synthesis, Properties, and Applications - page 3

and thus for an understanding of the interstellar absorption, suitable
laboratory experiments may turn out to be extremely difficult.
Our approach to reproduce the 220 nm interstellar absorption may serve
as an example. We produced graphitic particles by evaporating graphite in
a quenching atmosphere of helium at a pressure of a few torr. Under these
conditions, the carbon vapour re-condenses to form small particles of about
10 nm size. We collected these particles on substrates and recorded the
UV-vis spectra. Naturally, in the process of deposition, the particles heavily
clump together, an effect which distorts the absorption spectra in shape,
width, and position. These distortions render a direct comparison with the
interstellar feature difficult. Strictly speaking, our laboratory approach was
thus too simple. One should have aimed at preparing a non-clumped system
of free or at least matrix-isolated particles, a much more difficult task
(see [ 15 ] for the latter approach). Besides an extremely broad absorption at
around 220 nm, we also found to our surprise some peculiar absorption
features in our samples which were rather unlike to those of graphite parti-
cles. These absorptions turned out to originate from fullerenes. Unfortuna-
tely, C 60 and the other fullerenes did not contribute very much to a better
understanding of the carrier of the interstellar 220 nm absorption. However,
the existence of closed cage fullerenes stimulated research on other related
graphite-like nanostructures. As a result the concept of graphitic onions
emerged. It now appears that such particles are good candidates for the
220 nm carrier [12].
Rather puzzling is the fact that the highly ordered fullerene structures
form with amazing efficiency through the condensation of carbon vapor,
i.e. a chaotic high-temperature process. This demands an explanation. What
is the build-up process and what are the fullerene precursors? Research on
small molecular carbon species may shed some light on this problem.
1.2 MATRIX-ISOLATION EXPERIMENTS OF CARBON VAPOR
Initially, we tried to check the hypothesis that carbon chains may be the
carriers of the DIBs and performed some pilot experiments showing that
such chains did indeed exhibit strong absorptions in the wavelength range
where DIBs are observed. For this purpose we produced carbon vapor by
evaporating graphite rods by resistive heating. To measure the carbon
molecular spectra, we applied the matrix-isolation technique in which the
carbon vapor is co-deposited along with a large excess of a noble gas
(usually argon) on a cryogenic substrate (10K). By this procedure the rather
reactive carbon species are trapped within the frozen matrix of the rare
gas and remain isolated. Weltner and co-workers introduced this carbon
preparation technique in the early 1960s (see [ 16 ] ). For clarity the principle
of matrix-isolation is sketched in Figure 1.2 . Compared to gas phase condi-
tions, the matrix allows molecular vibrations but in the cases of concern here