On The Origin Of The 3.3 Micron Unidentified Infrared Emission Feature

Abstract

The 3.3 micron 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 micron 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 hybridizations. We found 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 micron features cannot be directly interpreted in the PAH model. Based on these results, the possible aromatic, olefinic and aliphatic origins of the 3.3 micron feature are discussed. We suggest that the 3.28 micron feature is assigned to aromatic C-H stretch whereas the 3.30 micron feature is olefinic. From the ratio of these two features, the relative olefinic to aromatic content of the carrier can be determined.