On the Origin of the 3.3 Micron Unidentified Infrared Emission Feature

Abstract

The 3.3 $\mu$m unidentified infrared emission feature is commonly attributed to C-H stretching band of aromatic molecules. Astronomical observations have shown that this feature is composed of two separate bands at 3.28 and 3.30 $\mu$m and the origin of these two bands is unclear. In this paper, we perform vibrational analyses based on quantum mechanical calculations of 153 organic molecules, including both pure aromatic molecules and molecules with mixed aromatic/olefinic/aliphatic hydridizations. We find that many of the C-H stretching vibrational modes in polycyclic aromatic hydrocarbon (PAH) molecules are coupled. Even considering the un-coupled modes only, the correlation between the band intensity ratios and the structure of the PAH molecule is not observed and the 3.28 and 3.30 $\mu$m features cannot be directly interpreted in the PAH model. Based on these results, the possible aromatic, olefinic and aliphatic origins of the 3.3 $\mu$m feature are discussed. We suggest that the 3.28 $\mu$m feature is assigned to aromatic C-H stretch whereas the 3.30 $\mu$m feature is olefinic. From the ratio of these two features, the relative olefinic to aromatic content of the carrier can be determined.

33 pages, 14 figures. Accepted for publication in ApJ

References in corpus (4)

• Spatial distribution and interpretation of the 3.3 \mum PAH emission band of the Red Rectangle
• Polycyclic Aromatic Hydrocarbons with armchair edges and the 12.7 μm band
• A Theoretical Study on the Vibrational Spectra of PAH Molecules with Aliphatic Sidegroups
• On the origin of the 11.3 micron unidentified infrared emission feature

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• Morphological study of the nested planetary nebula Hubble 12
• The nature and evolution of a-C(:H) nanoparticle substructures and speculations on the origin of the 3-4$\mu$m emission bands