Anharmonicity and the IR Emission Spectrum of Neutral Interstellar PAH Molecules

Cameron J Mackie; Alessandra Candian; Timothy J Lee; Alexander G G M Tielens

doi:10.1021/acs.jpca.2c01849

Anharmonicity and the IR Emission Spectrum of Neutral Interstellar PAH Molecules

J Phys Chem A. 2022 May 26;126(20):3198-3209. doi: 10.1021/acs.jpca.2c01849. Epub 2022 May 11.

Authors

Cameron J Mackie^{1

2}, Alessandra Candian³, Timothy J Lee⁴, Alexander G G M Tielens^{5

6}

Affiliations

¹ Kenneth S. Pitzer Center for Theoretical Chemistry, Department of Chemistry, University of California, Berkeley, California 94720, United States.
² Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.
³ van 't Hoff Institute for Molecular Science, University of Amsterdam, P.O. Box 94157, 1090 GD Amsterdam, The Netherlands.
⁴ NASA Ames Research Center, Moffett Field, California 94035-1000, United States.
⁵ Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden, The Netherlands.
⁶ Astronomy Department, University of Maryland, College Park, Maryland 20742, United States.

PMID: 35544706
DOI: 10.1021/acs.jpca.2c01849

Abstract

The characteristics of the CH stretching and out-of-plane bending modes in polycyclic aromatic hydrocarbon molecules are investigated using anharmonic density functional theory (DFT) coupled to a vibrational second-order perturbation treatment taking resonance effects into account. The results are used to calculate the infrared emission spectrum of vibrationally excited species in the collision-less environment of interstellar space. This model follows the energy cascade as the molecules relax after the absorption of a UV photon in order to calculate the detailed profiles of the infrared bands. The results are validated against elegant laboratory spectra of polycyclic aromatic hydrocarbon absorption and emission spectra obtained in molecular beams. The factors which influence the peak position, spectral detail, and relative strength of the CH stretching and out-of-plane bending modes are investigated, and detailed profiles for these modes are derived. These are compared to observations of astronomical objects in space, and the implications for our understanding of the characteristics of the molecular inventory of space are assessed.