Superhydrogenated Polycyclic Aromatic Hydrocarbon Molecules: Vibrational Spectra in the Infrared

Abstract

Superhydrogenated polycyclic aromatic hydrocarbons (PAHs) may be present in H-rich and ultraviolet-poor benign regions. The addition of excess H atoms to PAHs converts the aromatic bonds into aliphatic bonds, the strongest of which falls near 3.4 $\mu$m. Therefore, superhydrogenated PAHs are often hypothesized as a carrier of the 3.4 $\mu$m emission feature which typically accompanies the stronger 3.3 $\mu$m aromatic C--H stretching feature. To assess this hypothesis, we use density function theory to compute the IR vibrational spectra of superhydrogenated PAHs and their ions of various sizes (ranging from benzene, naphthalene to perylene and coronene) and of various degrees of hydrogenation (ranging from minimal hydrogenation to heavy hydrogenation). For each molecule, we derive the intrinsic oscillator strengths of the 3.3 $\mu$m aromatic C--H stretch ($A_{3.3}$) and the 3.4 $\mu$m aliphatic C--H stretch ($A_{3.4}$). By comparing the computationally-derived mean ratio of $⟨A_{3.4}/A_{3.3}⟩\sim 1.98$ with the mean ratio of the observed intensities $⟨I_{3.4}/I_{3.3}⟩\sim 0.12$, we find that the degree of superhydrogenation --- the fraction of C atoms attached with excess H atoms --- is only $\sim 2.2\%$ for neutral PAHs which predominantly emit the 3.3 and 3.4 $\mu$m features. We also determine for each molecule the intrinsic band strengths of the 6.2 $\mu$m aromatic C--C stretch ($A_{6.2}$) and the 6.85 $\mu$m aliphatic C--H deformation ($A_{6.85}$). We derive the degree of superhydrogenation from the mean ratio of the observed intensities $⟨I_{6.85}/I_{6.2}⟩\sim 0.10$ and $⟨A_{6.85}/A_{6.2}⟩\sim 1.53$ for neutrals and $⟨A_{6.85}/A_{6.2}⟩\sim 1.23$ for cations to be $\lesssim 3.1\%$ for neutrals and $\lesssim 8.6\%$ for cations. We conclude that astrophysical PAHs are primarily aromatic and are only marginally superhydrogenated.

49 pages, 26 figures; accepted for publication in The Astrophysical Journal Supplement Series

References in corpus (7)

• Carbon-rich dust past the asymptotic giant branch: aliphatics, aromatics, and fullerenes in the Magellanic Clouds
• Polycyclic Aromatic Hydrocarbon in Protoplanetary Disks around Herbig Ae/Be and T Tauri Stars
• The C--H Stretching Features at 3.2--3.5 Micrometer of Polycyclic Aromatic Hydrocarbons with Aliphatic Sidegroups
• Photo-stability of super-hydrogenated PAHs determined by action spectroscopy experiments
• Polycyclic Aromatic Hydrocarbons with Aliphatic Sidegroups: Intensity Scaling for the C-H Stretching Modes and Astrophysical Implications
• On the Aliphatic versus Aromatic Content of the Carriers of the "Unidentified" Infrared Emission Features
• The infrared bands of polycyclic aromatic hydrocarbons in the 1.6-1.7 μm wavelength region