Carbon Chain Molecules in Cryogenic Matrices - Polyynes: Synthesis, Properties, and Applications - page 4

FIGURE 1.2 The principle of matrix isolation. The species to be studied (in our case,
carbon vapour molecules shown in black) are co-condensed along with a large excess
of noble gas onto a cold (about 10K for argon) substrate. The highly reactive carbon
molecules are trapped in the ice, cannot move, and stay isolated. Only when the
matrix is warmed up to its sublimation temperature, can the carbon molecules move
and grow to larger chains, and possibly also into rings. We produce the carbon vapor
by resistive heating of two contacting graphite electrodes.

allows no molecular rotations. The cryogenic matrix temperature and the
absence of molecular rotation considerably simplify the absorption spectra
but this comes at a price: compared to the gas-phase spectra, the matrix
spectra are considerably broadened and the absorption maxima are shifted
in wavelength. These distortions make a detailed comparison to gas phase
(e.g. DIB) spectra difficult. The matrix shifts, which depend on the par-
cticular spectral line and can go either into the blue or the red, are usually
relative moderate (<10% in most cases).
Carbon vapor at 3000K consists of atomic and molecular carbon of the
approximate composition C (20%), C 2 (10%), and linear C 3 (70%); the
abundance of larger species is rather low (<1%) [17]. In the range longer
than 200 nm, the optical absorption spectrum of carbon vapor matrix-
isolated in solid argon is usually dominated by the C 3 molecule with its
characteristic relatively sharp and complex structured absorption feature

at 410 nm (see Figure 1.3 ) . This feature, which in the gas phase occurs at

405 nm, originates from the transition from the 1 g ground state to the 1 u

electronically excited state. The complicated substructures of the band come
from bending vibrations of C 3 in the excited state. In the ''as deposited''
spectrum of Figure 1.3, which was taken at a window substrate temperature
of 10K, there are indications of other absorptions — a particular example
being the broad band centered at about 250 nm. Part of this band belongs
to the C 2 molecule (the so-called Mullikan band at 238 nm), but the rest
belongs to some other carrier. Maier and co-workers, who deposited mass