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Astron. Astrophys. 357, 1013-1019 (2000)

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4. C-C-C bending modes of PAHs

Within the framework of the PAH model, emission is expected longward of about 15 µm arising from the C-C-C bending modes which cause in- and out-of-plane distortion of the carbon skeleton. Given that these PAH modes are spread out over this region, but tend to congest at the shorter wavelength end, a low-lying, long wavelength continuum emission was expected from PAH sources since all PAHs-large and small-will emit here (ATB). This initial expectation was based on a very limited set of laboratory PAH spectra, comprised principally of small species (C10 to C24). In recent years, many more PAHs have been measured in the laboratory and they consistently show IR modes in this wavelength region. Hudgins and coworkers (e.g. Hudgins & Sandford 1998, and references therein) have reported the spectra of 23 neutral and 20 cationic PAHs between 2.5 and 20 µm isolated in rare gas matrices at 10 K. Moutou et al.(1996) present a Far IR spectral collection of about 40 neutral PAHs suspended in CsI pellets between 14 and 40 µm. Further, Zhang et al. (1996) have directly studied the far-IR emission of a few PAHs in the gas phase. Together, these data allow us to refine the earlier expectations. Overall, the individual bands in this region typically have intensities no more than 5-10% of the most intense bands in the spectrum (ATB; Langhoff 1996; Hudgins & Sandford 1998 and references therein). Consider, for example, the fluoranthenes, a PAH family defined by the incorporation of a pentagonal cyclopentadienyl ring in their carbon skeletons. For the neutral fluoranthenes individual band intensities in the 15 to 30 µm region typically range from a few to 10% those of the dominant C-H out-of-plane bends in the 11 to 15 µm range and are comparable to the C-C stretches in the 6 µm region. The laboratory data also shows that, despite the dramatic effect of ionization on the global band intensity pattern across the mid-IR, the absolute intensities of the bands in the 15-30 µm range are only moderately affected, if at all, by ionization. Specifically, absorption intensities lie in the range 2-30 km/mol for neutral PAHs and 6-30 km/mol for PAH cations (Langhoff 1996). Bearing in mind that a PAH has typically between 3 and 10 features in this wavelength region, the total integrated strength from all of these long wavelength features taken together exceeds that of the 6.2 µm band.

Fig. 5 shows the average of the observed plateaus reported here, compared to the broad feature produced by simply coadding the laboratory absorption spectra of a few molecules from the Hudgins dataset, assigning each band a roughly 30 wavenumber width as expected for emission from highly vibrationally excited molecules (ATB). The specific molecules chosen to produce the broad laboratory feature is not unique. Given that larger interstellar molecules up to sizes of [FORMULA] C-atoms can contribute to this region as well (Schutte et al. 1993), a pseudo continuum (emission plateau) is indeed expected as part of the PAH model.

[FIGURE] Fig. 5. Comparison between the average of all plateaus and laboratory spectra. The average was obtained by coadding all interstellar emission plateaus normalized to the integrated intensity. The first laboratory spectrum (mix 1) is produced by the coadded spectra of anthracene (33%), 1,2-benzo(a)anthracene (33%), and pentacene (33%). The second laboratory spectrum (mix 2) is a spectrum produced by the mixture: benzo(k)fluoranthene (20%), pentacene (40%), anthracene (20%), and 1,2 benzanthracene (20%). The clear feature around 16.4 µm in the second laboratory spectrum is caused by the two fluoranthenes.

While there is some variation in the substructure of the emission plateaus observed, there is a distinct, slightly stronger component centered near 16.4 µm in some sources (Fig. 4). Infrared activity in this spectral region corresponds to in-plane C-C-C bends whereas bands longward of about 17 µm correspond to out-of-plane warping (Bauschlicher, private communication). Of the molecules represented in the Moutou et al. and Hudgins et al. datasets, the species which consistently show a band between 16 and 17 µm which could build up and generate a striking feature at this location are the fluoranthenes (cf. mix 2, Fig. 5). Analysis of the specific vibrations corresponding to each feature in this region for these molecules (Bauschlicher, private communication) indicates that the prominent 16.4 µm band may arise from the vibration of a pendant hexagonal ring (not pentagonal ring, although these may have a band here in some cases) and in the cases studied here, most of the intensity at this position involves an in-phase, planar vibration of the two opposite carbons of the pendant ring (i.e. at the para positions) along the line which is parallel and adjacent to the fused bond with the rest of the molecule's carbon network (see Fig. 6). Analysis of the vibrations of molecules, without a pentagonal ring, but showing a band near 16.4 µm confirms the vibration of a pendant ring as origin of this band. We note that all of the fluoranthenes in the NASA Ames dataset possess a very strong band near 14 µm because they contain hexagonal rings, included the pendant ring responsible for the band near 16.4 µm, with 4-adjacent hydrogen atoms. Since these are small molecules, this dominates the spectrum in the CH out-of-plane region between 10.5 and 14 µm. There is a weak feature at this position in the interstellar spectra (Hony et al. in preparation). Nevertheless, the smallest members of the fluoranthene family cannot be major carriers of the interstellar 16.4 µm emission band. Alternatively, the distinctive symmetry of the pentagonal ring might just induce IR activity in otherwise weak or forbidden transitions and, as such, pentagons may represent only one possible way in which broken symmetry can lead to enhanced IR activity at this position. If the latter is the case, this effect may also become particularly important for asymmetric, non-condensed PAHs. This distinctive substructure is likely a characteristic of smaller PAHs because, as PAH species increase in size, the influence of the center of asymmetry in a molecule on its spectrum becomes diluted by the constant increase in the number of regular PAH modes.

[FIGURE] Fig. 6. The in-plane C-C-C bending of the pendant ring that characterizes vibrational modes near 16 µm. The intensity at this position involves an in-phase, planar vibration of the two opposite carbons of the pendant ring (i.e. at the para positions) along the line which is parallel and adjacent to the fused bond with the rest of the molecule's carbon network.

In summary, together with the laboratory data discussed in this section, the astronomical detection of the 15-20 µm feature reported in this paper provides further credence to the PAH model and can be used to deduce specific characteristics of the interstellar PAH population in various regions. Substructure in the plateau, such as the 16.4 µm feature, may be particularly useful in this respect since they are likely carried by the small end of the PAH size distribution.

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© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000
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