Infrared Spectra of Hexa-peri-hexabenzocoronene Cations:HBC+ and HBC2+

Abstract

We present the first infrared (IR) gas phase spectrum of a large and astronomically relevant PAH cation (C${}_{42}$H${}_{18}$${}^{+}$, HBC${}^{+}$) and its di-cation (C${}_{42}$H${}_{18}$${}^{2+}$, HBC${}^{2+}$). The spectra are recorded via infrared multi-photon dissociation (IRMPD) spectroscopy of ions stored in a quadrupole ion trap, using the intense infrared radiation of a free electron laser in the 530$-$1800 cm${}^{-1}$ (5.6$-$18.9 $\mu$m) range. HBC${}^{+}$ shows main intense absorption peaks at 762 (13.12), 1060 (9.43), 1192 (8.39), 1280 (7.81), 1379 (7.25) and 1530 (6.54) cm${}^{-1}$($\mu$m), in good agreement with DFT calculations that after scaling to take the anharmonicities effect into account. HBC${}^{2+}$ has its main absorption peaks at 660 (15.15), 766 (13.05), 1054 (9.49), 1176 (8.50), 1290 (7.75), 1370 (7.30) and 1530 (6.54) cm${}^{-1}$($\mu$m). Given the similarity in the cationic and di-cationic spectra, we have not identified an obvious diagnostic signature to the presence of multiply charged PAHs in space. While experimental issues associated with the IRMPD technique preclude a detailed comparison with interstellar spectra, we do note that the strong bands of HBC${}^{+}$ and HBC${}^{2+}$ at $\sim$ 6.5, 7.7, 8.4 and 13.1 $\mu$m coincide with prominent aromatic infrared bands (AIBs). HBC has only trio CH groups and the out-of-plane CH bending mode of both HBC cations is measured at 13.1 $\mu$m, squarely in the range predicted by theory and previously found in studies of small (substituted) PAHs. This study therefore supports the use of AIBs observed in the 11$-$14 $\mu$m range as a diagnostic tool for the edge topology of large PAHs in space.

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• Multiply charged naphthalene and its C10H8 isomers: bonding, spectroscopy and implications in AGN environments
• Gas-phase spectroscopy of photostable PAH ions from the mid- to far-infrared
• Infrared action spectroscopy of doubly charged PAHs and their contribution to the aromatic infrared bands
• Laboratory formation and photo-chemistry of fullerene/anthracene cluster cations
• IRMPD spectroscopy of a PAH cation using FELICE: The infrared spectrum and photodissociation of dibenzo[a,l]pyrene