THE C-H STRETCHING FEATURES AT 3.2-3.5 μm OF POLYCYCLIC AROMATIC HYDROCARBONS WITH ALIPHATIC SIDEGROUPS

The so-called unidentified infrared emission (UIE) features at 3.3, 6.2, 7.7, 8.6, and 11.3 mu m are ubiquitously seen in a wide variety of astrophysical regions. The UIE features are characteristic of the stretching and bending vibrations of aromatic hydrocarbon materials, e.g., polycyclic aromatic hydrocarbon (PAH) molecules. The 3.3 mu m aromatic C-H stretching feature is often accompanied by a weaker feature at 3.4 mu m. The latter is generally thought to result from the C-H stretch of aliphatic groups attached to the aromatic systems. The ratio of the observed intensity of the 3.3 mu m aromatic C-H feature to that of the 3.4 mu m aliphatic C-H feature allows one to estimate the aliphatic fraction of the UIE carriers, provided that the intrinsic oscillator strengths of the 3.3 mu m aromatic C-H stretch (A(3.3)) and the 3.4 mu m aliphatic C-H stretch (A(3.4)) are known. While previous studies on the aliphatic fraction of the UIE carriers were mostly based on the A(3.4) A(3.3) ratios derived from the mono-methyl derivatives of small PAH molecules, in this work we employ density functional theory to compute the infrared vibrational spectra of PAH molecules with a wide range of sidegroups including ethyl, propyl, butyl, and several unsaturated alkyl chains, as well as all the isomers of dimethyl-substituted pyrene. We find that, except for PAHs with unsaturated alkyl chains, the corresponding A(3.4) A(3.3) ratios are close to that of mono-methyl PAHs. This confirms the predominantly aromatic nature of the UIE carriers previously inferred from the A(3.4) A(3.3) ratio derived from mono-methyl PAHs.