  We investigate carbon isotope effects on the infrared bands of a laboratory
analogue of carbonaceous dust, the quenched carbonaceous composite (QCC),
synthesized from a plasma gas of methane with various 12C/13C ratios. Peak
shifts to longer wavelengths due to the substitution of 12C by 13C are clearly
observed in several absorption bands. The shifts are almost linearly
proportional to the 13C fraction. New features associated with 13C are not
seen, indicating that the infrared bands in the QCC are not very localized
vibration modes but come from vibrations associated with rather large carbon
structures. An appreciable peak shift is detected in the 6.2 micron band, which
is attributed to a carbon-carbon vibration. A peak shift in an out-of-plane
bending mode of aromatic C--H at 11.4 micron is also observed, while only a
small shift is detected in the 3.3 micron band, which arises from a C--H
stretching mode. The present experiment suggests that peak shifts in the
unidentified infrared (UIR) bands, particularly in the 6.2 micron band, should
be detectable in celestial objects with low 12C/13C ratios (< 10). The isotopic
shifts seen in the QCC are discussed in relation to the variations in the UIR
band peaks observed in post-asymptotic giant branch stars and planetary
nebulae. The observed peak shift pattern of the UIR bands is qualitatively in
agreement with the isotopic shifts in the QCC except for the 7.7 micron band
complex although the observed shifts in the UIR bands are larger than those
inferred from derived isotope ratios for individual objects. The present
investigation suggests that part of the observed variations in the UIR band
peaks may come from the isotopic effects.
