Document Type
Article
Publication Date
9-21-2023
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.
Recommended Citation
Esposito, V. J., Allamandola, L. J., Boersma, C., Bregman, J. D., Fortenberry, R. C., Maragkoudakis, A., & Temi, P. (2024). Anharmonic IR absorption spectra of the prototypical interstellar PAHs phenanthrene, pyrene, and pentacene in their neutral and cation states. Molecular Physics, 122(7–8). https://doi.org/10.1080/00268976.2023.2252936
Copyright
Taylor & Francis
Creative Commons License
This work is licensed under a Creative Commons Attribution-Noncommercial 4.0 License
Included in
Biological and Chemical Physics Commons, Other Astrophysics and Astronomy Commons, Physical Chemistry Commons
Comments
This is an Accepted Manuscript version of an article accepted for publication in Molecular Physics, volume 122, in 2024. https://doi.org/10.1080/00268976.2023.2252936 It is deposited under the terms of the Creative Commons Attribution-NonCommercial License (http://creativecommons.org/licenses/by-nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited.