The 5.25 & 5.7 μm Astronomical Polycyclic Aromatic Hydrocarbon Emission Features

Astronomical mid-IR spectra show two minor polycyclic aromatic hydrocarbon (PAH) features at 5.25 and 5.7 mu m (1905 and 1754 cm(-1)) that hitherto have been little studied, but contain information about the astronomical PAH population that complements that of the major emission bands. Here, we report a study involving both laboratory and theoretical analysis of the fundamentals of PAH spectroscopy that produce features in this region and use these to analyze the astronomical spectra. The Infrared Space Observatory Short Wavelength Spectrograph spectra of 15 objects showing these PAH features were considered for this study, however only four (HD 44179; NGC7027; Orion Bar, two positions) have sufficient signal-to-noise between 5 and 6 mu m to allow for an in-depth analysis. All four astronomical spectra show similar peak positions and profiles. The 5.25 mu m feature is peaked and asymmetric, with an FWHM of about 0.12 +/- 0.01 mu m (similar to 40 +/- 6.5 cm(-1)), while the 5.7 mu m feature is broader and flatter, with an FWHM of about 0.17 +/- 0.02 mu m (50 +/- 5.6 cm(-1)). Detailed analysis of the laboratory spectra and quantum-chemical calculations show that the 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-plane and CH out-of-plane bending fundamental vibrations. The experimental and computational spectra show that, of all the hydrogen adjacency classes that are possible on PAHs, solo and duo hydrogens consistently produce prominent bands at the observed positions, whereas quartet hydrogens do not. In all, this study supports the picture that astronomical PAHs are large with compact, regular structures. From the coupling with primarily strong CH out-of-plane bending modes, one might surmise that the 5.25 and 5.7 mu m bands track the neutral PAH population. However, theory suggests that the role of charge in these astronomical bands might also be important.