  Although it is generally accepted that the so-called "unidentified" infrared
emission (UIE) features at 3.3, 6.2, 7.7, 8.6, and 11.3 micrometer are
characteristic of the stretching and bending vibrations of aromatic hydrocarbon
materials, the exact nature of their carriers remains unknown: whether they are
free-flying, predominantly aromatic gas-phase molecules, or amorphous solids
with a mixed aromatic/aliphatic composition are being debated. Recently, the
3.3 and 3.4 micrometer features which are commonly respectively attributed to
aromatic and aliphatic C-H stretches have been used to place an upper limit of
~2\% on the aliphatic fraction of the UIE carriers (i.e. the number of C atoms
in aliphatic chains to that in aromatic rings). Here we further explore the
aliphatic versus aromatic content of the UIE carriers by examining the ratio of
the observed intensity of the 6.2 micrometer aromatic C-C feature (I6.2) to
that of the 6.85 micrometer aliphatic C-H deformation feature (I6.85). To
derive the intrinsic oscillator strengths of the 6.2 micrometer stretch (A6.2)
and the 6.85 micrometer deformation (A6.85), we employ density functional
theory to compute the vibrational spectra of seven methylated polycyclic
aromatic hydrocarbon molecules and their cations. By comparing I6.85/I6.2 with
A6.85/A6.2, we derive the fraction of C atoms in methyl(ene) aliphatic form to
be at most ~10\%, confirming the earlier finding that the UIE emitters are
predominantly aromatic. We have also computed the intrinsic strength of the
7.25 micrometer feature (A7.25), another aliphatic C-H deformation band. We
find that A6.85 appreciably exceeds A7.25. This explains why the 6.85
micrometer feature is more frequently detected in space than the 7.25
micrometer feature.
