Infrared Spectroscopy of Pentagon-Containing PAHs: Indenyl and Fluorenyl Anions and Indenyl Cation

Abstract

Polycyclic aromatic hydrocarbon (PAH) ions are crucial intermediates in interstellar chemistry and may play a key role in the infrared emission features observed in space. Here, we investigate the infrared spectra of the indenyl (C${}_9$H${}_7^{-}$) and fluorenyl (C${}_{13}$H${}_9^{-}$) anions and the indenyl cation (C${}_9$H${}_7^{+}$) using infrared pre-dissociation (IRPD) spectroscopy. The experiments were performed in a cryogenic 22 pole ion trap at the FELion beamline of the tunable free-electron laser FELIX. Spectral analysis of the two anionic PAHs, in combination with density functional theory (DFT) computations, revealed key vibrational modes near 1300 cm${}^{-1}$, making these ions potential carriers of the 7.7 μm PAH emission band seen in many astronomical objects. The feature-rich spectrum of cationic indenyl could not be entirely explained by modeling through time-independent anharmonic DFT calculations. Although a better match has been achieved through molecular dynamics simulations, we cannot completely rule out the presence of multiple cationic isomers of the H-loss fragments of indene in the experiments.

Accepted in JPCL

References in corpus (12)

• Detection of Two Interstellar Polycyclic Aromatic Hydrocarbons via Spectral Matched Filtering
• Polycylcic Aromatic Hydrocarbons (PAH's) in dense cloud chemistry
• Discovery of the Pure Polycyclic Aromatic Hydrocarbon Indene ($c$-C${}_9$H${}_8$) with GOTHAM Observations of TMC-1
• Discovery of Interstellar 2-Cyanoindene (2-C${}_9$H${}_7$CN) in GOTHAM Observations of TMC-1
• The Composition and Chemistry of Titan's Atmosphere
• Discovery of two cyano derivatives of acenaphthylene (C${}_{12}$H${}_8$) in TMC-1 with the QUIJOTE line survey
• Discovery of the C7N- anion in TMC-1 and IRC+10216
• Astronomical Detection of the Interstellar Anion C10H- towards TMC-1 from the GOTHAM Large Program on the GBT
• Polycyclic Aromatic Hydrocarbon emission in Spitzer/IRS maps II: A direct link between band profiles and the radiation field strength
• GOALS-JWST: Hidden Star Formation and Extended PAH Emission in the Luminous Infrared Galaxy VV 114
• Cold CAS Ion Trap -- 22 pole trap with ring electrodes for astrochemistry
• Interstellar dehydrogenated PAH anions: vibrational spectra