The 15–20 <i>μ</i>m PAH emission features: probes of individual PAHs?

Abstract

<i>Context. <i/>Spectral features between about 15-20 <i>μ<i/>m 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.<i>Aims. <i/>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 <i>μ<i/>m and to draw astronomical implications from these spectra.<i>Methods. <i/>We analyse and interpret the spectra of 15 different sources using the NASA Ames PAH IR spectroscopic database.<i>Results. <i/>The bands within the 15-20 <i>μ<i/>m region show large variations. Except for the 16.4 <i>μ<i/>m 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 <i>μ<i/>m 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 <i>μ<i/>m band position.<i>Conclusions. <i/>The 15-20 <i>μ<i/>m 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 <i>μ<i/>m band, coupling with the skeleton core and the inherent strong quartet mode expected around 13.5 <i>μ<i/>m, make it a less viable candidate. The number of large PAHs in the database becomes a limitation in studying the emission between 15-20 <i>μ<i/>m 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.