302 research outputs found
On the Aliphatic versus Aromatic Content of the Carriers of the "Unidentified" Infrared Emission Features
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.Comment: 18 pages, 10 figures, 3 tables; accepted for publication in MNRA
Polycyclic Aromatic Hydrocarbons with Aliphatic Sidegroups: Intensity Scaling for the C-H Stretching Modes and Astrophysical Implications
The so-called unidentified infrared emission (UIE) features at 3.3, 6.2, 7.7, 8.6, and 11.3 μm ubiquitously seen in a wide variety of astrophysical regions are generally attributed to polycyclic aromatic hydrocarbon (PAH) molecules. Astronomical PAHs may have an aliphatic component, as revealed by the detection in many UIE sources of the aliphatic C-H stretching feature at 3.4 mm. The ratio of the observed intensity of the 3.4 mm feature to that of the 3.3 μm aromatic C-H feature allows one to estimate the aliphatic fraction of the UIE carriers. This requires knowledge of the intrinsic oscillator strengths of the 3.3 mm aromatic C-H stretch (A3.3) and the 3.4 μm aliphatic C-H stretch (A3.4). Lacking experimental data on A3.3 and A3.4 for the UIE candidate materials, one often has to rely on quantum-chemical computations. Although the second-order Møller-Plesset (MP2) perturbation theory with a large basis set is more accurate than the B3LYP density functional theory, MP2 is computationally very demanding and impractical for large molecules. Based on methylated PAHs, we show here that, by scaling the band strengths computed at an inexpensive level (e.g., B3LYP/6-31G), we are able to obtain band strengths as accurate as those computed at far more expensive levels (e.g., MP2/6-311+G(3df,3pd)). We calculate the model spectra of methylated PAHs and their cations excited by starlight of different spectral shapes and intensities. We find that (I3.4/I3.3)mod, the ratio of the model intensity of the 3.4 μm feature to that of the 3.3 μm feature, is insensitive to the spectral shape and intensity of the exciting starlight. We derive a straightforward relation for determining the aliphatic fraction of the UIE carriers (i.e., the ratio of the number of C atoms in aliphatic units NC,ali to that in aromatic rings NC,aro) from the observed band ratios (I3.4/I3.3)obs: NC,ali/NC,aro ≈ 0.57 x (I3.4/I3.3)obs for neutrals and NC,ali NC,aro ≈ 0.26 x (I3.4/I3.3)obs for cations
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