The 15–20 μm PAH emission features: probes of individual PAHs?

Abstract

Context. Spectral features between about 15−20 μm are commonly associated with polycyclic aromatic hydrocarbons (PAHs). With the NASA Spitzer Space Telescope these features are reported routinely, and as such, warrant a deeper molecular explanation. Aims. We aim to determine the characteristics of the group of carriers of the plateau and the distinct sub-features at 15.8, 16.4, 17.4, 17.8 and 18.9 μm and to draw astronomical implications from these spectra. Methods. We analyse and interpret the spectra of 15 different sources using the NASA Ames PAH IR spectroscopic database. Results. The bands within the 15−20 μm region show large variations. Except for the 16.4 μm band, there is also no connection, both in band strength and feature classification, with the mid-IR PAH bands. Of the PAH spectra considered, only those from species containing pendent rings show one "common" characteristic: a band near the astronomical 16.4 μm position. However, coupling with the carbon skeleton's core influences its precise position in the spectrum. Compact PAHs in the size range 50−130 carbon atoms, consistently show a strong band near the astronomical 17.4 μm band position. Conclusions. The 15−20 μm region is the transition zone from PAH nearest neighbour modes to full-skeleton modes. We conclude that a few individual PAHs dominate the astronomical PAH family when clear features are prominent. In the few cases of a broad plateau, the PAH family would be far richer. Although PAHs containing pendent rings showed promise explaining the astronomical 16.4 μm band, coupling with the skeleton core and the inherent strong quartet mode expected around 13.5 μm, make it a less viable candidate. The number of large PAHs in the database becomes a limitation in studying the emission between 15−20 μm and longward. Computation of larger PAH spectra should therefore be stimulated, especially for understanding the forthcoming far-IR data expected from Herschel, SOFIA and ALMA.

Infrared vibrational and electronic transitions in the dibenzopolyacene family

“…This approach has been confirmed [36] in subsequent studies for molecules with sizes up to C 48 H 20 . For the BP86 calculations the scale factor is 0.986 for the 4-31G basis set.…”

The Nasa Ames Polycyclic Aromatic Hydrocarbon Infrared Spectroscopic Database: The Computed Spectra

“…The features that comprise this universal spectrum contain a wealth of information about the physical conditions in the emitting regions and the nature of the PAH carriers (e.g., Joblin et al 1994;Li & Draine 2001;Hony et al 2001;Peeters et al 2002;van Diedenhoven et al 2004;Mulas et al 2006;Bauschlicher et al 2008Bauschlicher et al , 2009Galliano et al 2008;Boersma et al 2009Boersma et al , 2010Mattioda et al 2009;Ricca et al 2010, and references therein). However, exploitation of these features as astrophysical and astrochemical probes has been slow in developing because the IR properties of PAHs under interstellar conditions were largely unknown for at least twenty years after the bands were discovered.…”

The Infrared Emission Bands

“…Figure from Engelbracht et al (2005). these correlations studies are now extended to included the weaker IR emission bands (e.g. Boersma et al 2010;Rosenberg et al 2011;Peeters et al 2012).…”