SIMBAD references

We present the first infrared (IR) gas-phase spectrum of a large and astronomically relevant polycyclic aromatic hydrocarbon (PAH) cation (C₄₂H₁₈⁺, HBC⁺) and its dication (C₄₂H₁₈²⁺, HBC²⁺). The spectra are recorded via infrared multiphoton dissociation (IRMPD) spectroscopy of ions stored in a quadrupole ion trap, using the intense IR radiation of a free electron laser in the 530-1800 cm^–1 (5.6-18.9 µm) 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 (µm), in good agreement with density functional theory calculations after scaling to take the anharmonicities effect into account. HBC²⁺ 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 (µm). Given the similarity in the cationic and dicationic 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 HBC²⁺ at ∼6.5, 7.7, 8.4, and 13.1 µm 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 µm, squarely in the range predicted by theory and as previously found in studies of small (substituted) PAHs. This study therefore supports the use of AIBs observed in the 11-14 µm range as a diagnostic tool for the edge topology of large PAHs in space.