Abstract
Abstract The first gas-phase infrared spectra of two isolated astronomically relevant and large polycyclic aromatic hydrocarbon (PAH) cations—diindenoperylene (DIP) and dicoronylene (DC)—in the 530–1800 cm−1 (18.9−5.6 μm) range—are presented. Vibrational band positions are determined for comparison to the aromatic infrared bands. The spectra are obtained via infrared multiphoton dissociation spectroscopy of ions stored in a quadrupole ion trap using the intense and tunable radiation of the free electron laser for infrared experiments (FELIX). DIP+ shows its main absorption peaks at 737 (13.57), 800 (12.50), 1001 (9.99), 1070 (9.35), 1115 (8.97), 1152 (8.68), 1278 (7.83), 1420 (7.04), and 1550 (6.45) cm−1(μm), in good agreement with density functional theory (DFT) calculations that are uniformly scaled to take anharmonicities into account. DC+ has its main absorption peaks at 853 (11.72), 876 (11.42), 1032 (9.69), 1168 (8.56), 1300 (7.69), 1427 (7.01), and 1566 (6.39) cm−1(μm), which also agree well with the scaled DFT results presented here. The DIP+ and DC+ spectra are compared with the prominent infrared features observed toward NGC 7023. This results both in matches and clear deviations. Moreover, in the 11.0–14.0 μm region, specific bands can be linked to CH out-of-plane (oop) bending modes of different CH edge structures in large PAHs. The molecular origin of these findings and their astronomical relevance are discussed.
Highlights
Strong emission features at 3.3, 6.2, 7.7, 8.6, and 11.2 μm dominate the infrared (IR) spectrum of many astronomical sources
They play an important role in the energy and ionization balance of the interstellar medium (ISM) and may serve as a catalyst for the formation of molecular H2 in photo-dissociation regions (PDRs; Tielens 2013, and references therein)
As a set of the chemically most stable polycyclic aromatic hydrocarbon (PAH) species that are able to survive in the harsh conditions of the ISM. This idea was motivated by the observation of highly similar aromatic infrared bands (AIBs) spectra observed toward very different interstellar sources as well as the limited number of bands in the 15–20 μm range, which is a region in which structural features of PAHs are expected to show up (Boersma et al 2010)
Summary
Strong emission features at 3.3, 6.2, 7.7, 8.6, and 11.2 μm dominate the infrared (IR) spectrum of many astronomical sources These bands are commonly known as the aromatic infrared bands (AIBs) and are generally attributed to IR fluorescence of large (roughly more than 40 C atoms containing) polycyclic aromatic hydrocarbon (PAH) molecules and their related families. This idea was motivated by the observation of highly similar AIB spectra observed toward very different interstellar sources as well as the limited number of bands in the 15–20 μm range, which is a region in which structural features of PAHs are expected to show up (Boersma et al 2010). These spectra are compared to both density functional theory (DFT) predictions and astronomical spectra
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