Anharmonic IR absorption spectra of the prototypical interstellar PAHs phenanthrene, pyrene, and pentacene in their neutral and cation states

Abstract

Combination band and overtone transitions of polycyclic aromatic hydrocarbon (PAH) cations in the 2000–2900 cm^-1 (5-3.5 µm) region are implicated as carriers of the ‘quasi-continuum’ observed in the near-infrared (IR) emission spectra of many astronomical objects. In neutral PAHs, the strongest absorption features are concentrated in the 700–900 cm^-1 (14–11 µm) range, which are associated with CH out-of-plane bending motions. Upon ionization, this shifts to  the 1000–1600 cm^-1 (10–6 µm) range, where bands are associated with C=C stretches and CH in-plane bends. Anharmonicity is required to accurately characterize the IR absorption spectrum of PAHs, indicated herein by the ability to directly assign the bands in high-resolution experimental absorption spectra of neutral and cationic phenanthrene, pyrene, and pentacene. Neutral PAHs are indicated as the source of the strong 3.3 and 11.2 µm astronomical PAH features, while the broad features in the 6–10 µm region and the ‘quasi-continuum’ from 3.5–5 µm stem from PAH cations. This study reinforces the need for including anharmonicity in the computation of IR absorption and emission spectra of larger and more complex PAHs. This is particularly pertinent to the interpretation of data returned by JWST. 

GRAPHICAL ABSTRACT

KEYWORDS:

• Spectroscopy
• polycyclic aromatic hydrocarbons
• computational methods
• anharmonicity
• astrochemistry

Acknowledgements

This work is dedicated to our dear friend Tim Lee who pioneered high-resolution PAH computational spectroscopy and developed the methods utilised here. He was an integral and central member of the Astrophysics Team at NASA Ames, both scientifically, and more importantly, personally, as a friend, mentor, and inspiration. He is, and will always be, missed by us all.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Notes

1 www.astrochemistry.org/pahdb

Additional information

Funding

V.J.E.’s research has been supported by an appointment to the NASA Postdoctoral Program at NASA Ames Research Center, administered by the Oak Ridge Associated Universities through a contract with NASA. Computer time from the Pleiades cluster of the NASA Advanced Supercomputer (NAS) is gratefully acknowledged. C.B. is grateful for an appointment at NASA Ames Research Center through the San José State University Research Foundation (80NSSC22M0107). C.B., J.D.B., L.J.A. and A.M. acknowledge support from the Internal Scientist Funding Model (ISFM) Laboratory Astrophysics Directed Work Package at NASA Ames (22-A22ISFM-0009). L.J.A., J.D.B. and A.M. are thankful for an appointment at NASA Ames Research Center through the Bay Area Environmental Research Institute (80NSSC19M0193). R.C.F. acknowledges support from NASA grant NNH22ZHA004C and the Mississippi Center for Supercomputing Research supported in part from NSF Grant OIA-1757220. The authors thank Cameron Mackie for his advice and assistance.