On the Origin of the 3.3 Micron Unidentified Infrared Emission Feature
  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.
