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 (20)

• Infrared Emission from Interstellar Dust. I. Stochastic Heating of Small Grains
• Photoelectric Emission from Interstellar Dust: Grain Charging and Gas Heating
• Spitzer's view on aromatic and aliphatic hydrocarbon emission in Herbig Ae stars
• Carbon-rich dust past the asymptotic giant branch: aliphatics, aromatics, and fullerenes in the Magellanic Clouds
• The Carriers of the Interstellar Unidentified Infrared Emission Features: Aromatic or Aliphatic?
• Spitzer Space Telescope spectra of post-AGB stars in the Large Magellanic Cloud ---polycyclic aromatic hydrocarbons at low metallicities
• Spatially Resolved 3 um Spectroscopy of IRAS 22272+5435: Formation and Evolution of Aliphatic Hydrocarbon Dust in Proto-Planetary Nebula
• Polycyclic Aromatic Hydrocarbon in Protoplanetary Disks around Herbig Ae/Be and T Tauri Stars
• High-resolution IR absorption spectroscopy of polycyclic aromatic hydrocarbons in the 3 micron region: role of hydrogenation and alkylation
• The abundances of hydrocarbon functional groups in the interstellar medium inferred from laboratory spectra of hydrogenated and methylated polycyclic aromatic hydrocarbons
• The Carriers of the Interstellar Unidentified Infrared Emission Features: Constraints from the Interstellar C-H Stretching Features at 3.2-3.5 Micrometers
• The Carriers of the "Unidentified" Infrared Emission Features: Clues from Polycyclic Aromatic Hydrocarbons with Aliphatic Sidegroups
• AKARI near-infrared spectroscopy of the aromatic and aliphatic hydrocarbon emission features in the galactic superwind of M 82
• Hydrogenation of PAH cations: a first step towards H2 formation
• 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 Central Region of the Barred Spiral Galaxy NGC1097 Probed by AKARI Near-Infrared Spectroscopy
• The infrared bands of polycyclic aromatic hydrocarbons in the 1.6-1.7 μm wavelength region

Cited by in corpus (8)

• PDRs4All VI: Probing the Photochemical Evolution of PAHs in the Orion Bar Using Machine Learning Techniques
• Deuterated Polycyclic Aromatic Hydrocarbons in the Interstellar Medium: The C--D Band Strengths of Mono-Deuterated Species
• Deuterated Polycyclic Aromatic Hydrocarbons in the Interstellar Medium: The C--D Band Strengths of Multi-Deuterated Species
• PDRs4All VII. The 3.3 $\mu$m aromatic infrared band as a tracer of physical properties of the ISM in galaxies
• Machine-learning identified molecular fragments responsible for infrared emission features of polycyclic aromatic hydrocarbons
• PDRs4All XVI. Tracing aromatic infrared band characteristics in photodissociation region spectra with PAHFIT in the JWST era
• SPHEREx: Aromatics, Aliphatics and PAH Size across the Iris Nebula
• The Influence of Aliphatic Components on the Aromatic Emission Characteristics of Polycyclic Aromatic Hydrocarbons