Abstract
Laboratory spectra of carbon nanoparticles (CNPs) near 3.29 μm show a band whose profile is an excellent match to that of the aromatic CH feature observed in type A emission sources. This band is associated with the CH stretching vibration in polycyclic aromatic hydrocarbon (PAH) molecules having peripheral aliphatic hydrocarbon substituents. Comparison between experimental and observational spectra suggests that the PAH species responsible for the appearance of a CH absorption feature at 3.275 μm is likely the same as that producing emission at 3.29 μm. We also detect a feature at 3.255 μm, attributed to small PAHs with aliphatic side chains, that corresponds to the aromatic CH band detected in molecular clouds. Our results indicate that the common form for PAHs in the interstellar medium is as rings having one or more aliphatic side chains. The proportion of side chains increases as excitation decreases, so that the overall composition of substituted PAHs in the DISM approaches that of hydrogenated amorphous carbon.
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