The C-H stretching features at 3.2-3.5 μm of polycyclic aromatic hydrocarbons with aliphatic sidegroups

Journal Article · Fri Jul 01 04:00:00 EDT 2016 · Astrophysical Journal

DOI:https://doi.org/10.3847/0004-637X/825/1/22· OSTI ID:22868943

Yang, X. J.; Zhong, J. X. ^[1]; Li, Aigen ^[2]

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1. Department of Physics, Xiangtan University, 411105 Xiangtan, Hunan Province (China)
2. Department of Physics and Astronomy, University of Missouri, Columbia, MO 65211 (United States)

The so-called “unidentified” infrared emission (UIE) features at 3.3, 6.2, 7.7, 8.6, and 11.3 μm 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 μm aromatic C–H stretching feature is often accompanied by a weaker feature at 3.4 μm. 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 μm aromatic C–H feature to that of the 3.4 μm 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 μm aromatic C–H stretch (A{sub 3.3}) and the 3.4 μm aliphatic C–H stretch (A{sub 3.4}) are known. While previous studies on the aliphatic fraction of the UIE carriers were mostly based on the A{sub 3.4}/A{sub 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{sub 3.4}/A{sub 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{sub 3.4}/A{sub 3.3} ratio derived from mono-methyl PAHs.

• OSTI ID: 22868943
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 1 Vol. 825; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
ASTROPHYSICS
COSMIC DUST
DENSITY FUNCTIONAL METHOD
EMISSION
ISOMERS
MOLECULES
OSCILLATOR STRENGTHS
POLYCYCLIC AROMATIC HYDROCARBONS
SPECTRA