Characterizing the infrared spectra of small, neutral, fully dehydrogenated PAHs

Abstract

We present the results of a computational study to investigate the infrared spectroscopic properties of a large number of polycyclic aromatic hydrocarbon (PAH) molecules and their fully dehydrogenated counterparts. We constructed a database of fully optimized geometries for PAHs that is complete for eight or fewer fused benzene rings, thus containing 1550 PAHs and 805 fully dehydrogenated aromatics. A large fraction of the species in our database have clearly non-planar or curved geometries. For each species, we determined the frequencies and intensities of their normal modes using density functional theory calculations. Whereas most PAH spectra are fairly similar, the spectra of fully dehydrogenated aromatics are much more diverse. Nevertheless, these fully dehydrogenated species show characteristic emission features at 5.2$\mu$m, 5.5$\mu$m and 10.6$\mu$m; at longer wavelengths, there is a forest of emission features in the 16--30$\mu$m range that appears as a structured continuum, but with a clear peak centered around 19$\mu$m. We searched for these features in Spitzer-IRS spectra of various positions in the reflection nebula NGC 7023. We find a weak emission feature at 10.68$\mu$m in all positions except that closest to the central star. We also find evidence for a weak 19$\mu$m feature at all positions that is not likely due to C${}_{60}$. We interpret these features as tentative evidence for the presence of a small population of fully dehydrogenated PAHs, and discuss our results in the framework of PAH photolysis and the formation of fullerenes.

References in corpus (24)

• The Spitzer Space Telescope Mission
• The Infrared Spectrograph on the Spitzer Space Telescope
• The rich 6 to 9 um spectrum of interstellar PAHs
• The CH out-of-plane bending modes of PAH molecules in astrophysical environments
• Formation of buckminsterfullerene (C60) in interstellar space
• C${}_{60}$ in Reflection Nebulae
• The profiles of the 3 to 12 $\mu$m PAH features
• Formation of fullerenes in H-containing Planetary Nebulae
• Formation of PAHs and Carbonaceous Solids in Gas-Phase Condensation Experiments
• Evolution of PAHs in photodissociation regions: Hydrogenation and charge states
• The formation of fullerenes: clues from new C60, C70, and (possible) planar C24 detections in Magellanic Cloud Planetary Nebulae
• Detection of C60 in the proto-planetary nebula IRAS 01005+7910
• Interstellar C60+
• Are C60 molecules detectable in circumstellar shells of R Coronae Borealis stars?
• Carbon-rich dust past the asymptotic giant branch: aliphatics, aromatics, and fullerenes in the Magellanic Clouds
• The formation of cosmic fullerenes from arophatic clusters
• The 15 - 20 Micron Spitzer Spectra of Interstellar Emission Features in NGC 7023
• Detection of C60 in embedded young stellar objects, a Herbig Ae/Be star and an unusual post-AGB star
• Carbonaceous molecules in the oxygen-rich circumstellar environment of binary post-AGB stars: C_{60} fullerenes and polycyclic aromatic hydrocarbons
• The 15-20 um emission in the reflection nebula NGC2023
• The 5.25 & 5.7 $\mu$m Astronomical Polycyclic Aromatic Hydrocarbon Emission Features
• Spitzer reveals what's behind Orion's Bar
• Dehydrogenated polycyclic aromatic hydrocarbons and UV bump
• The detection of C60 in the well-characterized planetary nebula M1-11

Cited by in corpus (13)

• Laboratory Photo-chemistry of PAHs: Ionization versus Fragmentation
• Examining the class B-to-A shift of the 7.7 $\mu$m PAH band with the NASA Ames PAH IR Spectroscopic Database
• Polyacenes and diffuse interstellar bands
• On the origin of the 11.3 micron unidentified infrared emission feature
• Formation of interstellar complex polycyclic aromatic hydrocarbons: Insights from molecular dynamics simulations of dehydrogenated benzene
• On Graphene in the Interstellar Medium
• Petroleum, coal and other organics in space
• Interstellar dehydrogenated PAH anions: vibrational spectra
• A systematic study of silicate absorption features in heavily obscured AGNs observed by Spitzer/IRS
• Laser-induced fragmentation of coronene cations
• Spitzer/IRS full spectral modeling to characterize mineralogical properties of silicate dust in heavily obscured AGNs
• Laboratory astrophysics: key to understanding the Universe
• JWST observations of photodissociation regions. IV. Carbonaceous emission band sub-components in NGC 7023 have distinct spatial distributions