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

TL;DR

This study presents the first gas-phase infrared spectra of two large, astronomically relevant PAH cations, diindenoperylene and dicoronylene, providing insights into their vibrational modes and comparison with astronomical IR features.

Contribution

It provides the first gas-phase IR spectra of large PAH cations, with experimental data compared to DFT calculations and astronomical observations, advancing understanding of PAHs in space.

Findings

Spectra of DIP+ and DC+ show good agreement with DFT calculations.

Identified specific IR bands linked to PAH edge structures.

Comparison with NGC 7023 IR features reveals both matches and deviations.

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.