On the electronic structure of isolated mono-dehydrogenated polyaromatic hydrocarbon ions and their astrophysical relevance
Number of pages
Faculty of Science (FNWI)
Van 't Hoff Institute for Molecular Sciences (HIMS)
The attribution of the unidentified infrared bands to polycyclic aromatic hydrocarbon (PAH) molecules is a key argument for
their abundant occurrence in interstellar environments, which has important implications for interstellar chemistry. In contrast
to terrestrial conditions, their transient forms are of importance in the low-density astrophysical environments. Here, the
gas-phase IR spectra of three PAH molecules in a carbocation state (naphthyl+, C10H7 +; phenanthryl+, C14H9 +; and pyrenyl+,
C16H9 +) are investigated by action spectroscopy methods using an infrared free electron laser and an ion-trap mass spectrometer.
The IR spectra of the mono-dehydrogenated PAH+ (aryl) ions in the 6-18 μm spectral range are compared to computed IR spectra
for various structural isomers of the aryl ions; the comparison indicates that the most stable structures under isolated conditions
have a triplet electronic configuration. Electronic structure calculations on systems as large as the mono-dehydrogenated
circumcoronene cation (C54H17 +) provide further evidence for the higher stability of a triplet state as compared with the
singlet state. Moreover, the gas-phase IR spectra reveal that the IR signatures of a PAH cation before and after H-atom loss
are very similar, in particular in the 6-9 μm region involving the skeletal CC stretching modes, so that triplet mono-dehydrogenated
PAH ions are similarly compliant with the general match between PAH mid-IR features and the interstellar unidentified infrared
emissions. The establishment of a triplet electronic ground state suggests that interstellar scenarios should consider the
possible influence of triplet aromatic chemistry as well as the possible influence of the altered optical properties of triplet
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