Laboratory gas-phase infrared spectra of two astronomically relevant PAH cations: diindenoperylene, C$_{32}$H$_{16}$$^+$ and dicoronylene, C$_{48}$H$_{20}$$^+$

Abstract

The first gas-phase infrared spectra of two isolated astronomically relevant and large PAH cations - diindenoperylene (DIP) and dicoronylene (DC) - in the 530$-$1800 cm$^{-1}$ (18.9$-$5.6 $\mu$m) range - are presented. Vibrational band positions are determined for comparison to the aromatic infrared bands (AIBs). The spectra are obtained via infrared multiphoton dissociation (IRMPD) spectroscopy of ions stored in a quadrupole ion trap (QIT) 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$^{-1}$($\mu$m), in good agreement with 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$^{-1}$($\mu$m), that also agree well with the scaled DFT results presented here. The DIP$^+$ and DC$^+$ spectra are compared with the prominent infrared features observed towards NGC 7023. This results both in matches and clear deviations. Moreover, in the 11.0$-$14.0 $\mu$m 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.