The 5.25 and 5.7 $μ$m Astronomical Polycyclic Aromatic Hydrocarbon Emission Features - AstrophysicsReport as inadecuate




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Abstract: Astronomical mid-IR spectra show two minor PAH features at 5.25 and 5.7$\mu$m 1905 and 1754 cm$^{ m - 1}$ that hitherto have been little studied,but contain information about the astronomical PAH population that complementsthat of the major emission bands. Here we report a study involving bothlaboratory and theoretical analysis of the fundamentals of PAH spectroscopythat produce features in this region and use these to analyze the astronomicalspectra. The ISO SWS spectra of fifteen objects showing these PAH features wereconsidered for this study, of which four have sufficient S-N between 5 and 6$\mu$m to allow for an in-depth analysis. All four astronomical spectra showsimilar peak positions and profiles. The 5.25 $\mu$m feature is peaked andasymmetric, while the 5.7 $\mu$m feature is broader and flatter. Detailedanalysis of the laboratory spectra and quantum chemical calculations show thatthe astronomical 5.25 and 5.7 $\mu$m bands are a blend of combination,difference and overtone bands primarily involving CH stretching and CH in-planeand CH out-of-plane bending fundamental vibrations. The experimental andcomputational spectra show that, of all the hydrogen adjacency classes possibleon PAHs, solo and duo hydrogens consistently produce prominent bands at theobserved positions whereas quartet hydrogens do not. In all, this a studysupports the picture that astronomical PAHs are large with compact, regularstructures. From the coupling with primarily strong CH out-of-plane bendingmodes one might surmise that the 5.25 and 5.7 $\mu$m bands track the neutralPAH population. However, theory suggests the role of charge in theseastronomical bands might also be important.



Author: C. Boersma, A.L. Mattioda, C.W. Bauschlicher Jr., E. Peeters, A.G.G.M. Tielens, L.J. Allamandola

Source: https://arxiv.org/







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