13 C isotope effects on infrared bands of quenched carbonaceous composite (QCC)

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 12 C/13 C ratios. Peak shifts to longer wavelengths due to the substitution of 12 C by 13 C are clearly observed in several absorption bands. The shifts are almost linearly proportional to the 13 C fraction. New features associated with 13 C 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 ( ...m per 13 C fraction) is detected in the 6.2 (mu) m band, which is attributed to a carbon-carbon vibration. A peak shift ( ... m per 13 C fraction) in an out-of-plane bending mode of aromatic C-H at 11.4 (mu)m is also observed, while only a small shift ( ... (mu) m per 13 C fraction) is detected in the 3.3 (mu) m 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 (mu)m band, should be detectable in celestial objects with low 12 C /13 C 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 (mu)m band complex although the observed shifts in the UIR bands are larger than those inferred from derived isotope ratios for individual objects. The poor quantitative agreement may be attributed partly to large uncertainties in the derived 12 C/13 C, to possible spatial variations of the isotope abundance within the object, and to combinations of other effects, such as hetero-atom substitutions. The present investigation suggests that part of the observed variations in the UIR band peaks may come from the isotopic effects.