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Title:Light on the 3 μm emission band from space with molecular beam spectroscopy
Author(s):Maltseva, Elena
Contributor(s):Buma, Wybren Jan; Lee, Timothy; Huang, Xinchuan; Oomens, Jos; Tielens, Xander; Petrignani, Annemieke; Candian, Alessandra; Mackie, Cameron J.
Abstract:The majority of interstellar objects shows IR emission features also known as unidentified infrared (UIR) emission bands. These UIR bands are attributed to IR emission of highly-excited gaseous polycyclic aromatic hydrocarbons (PAHs). To understand the physical conditions and chemical evolution of the interstellar environment a precise identification of the emission carriers is desired. The 3 $\mu$m UIR feature is represented by a strong band at 3040 cm$^{-1}$, a plateau from 3150 to 2700 cm$^{-1}$ and a number of weak features within this plateau. The 3040 cm$^{-1}$ component is assigned to fundamental CH-stretch vibrations of PAHs, but there still remain many questions on the origin of the other features. In this work we have studied experimentally the 3 $\mu$m region of regular, hydrogenated and methylated PAHs (up to 5 rings), combining molecular beam techniques with IR-UV ion dip spectroscopy, and theoretically by density functional theory (DFT) calculations within the harmonic and anharmonic approximation. We find that (a) the 3 $\mu$m region of PAHs is dominated by Fermi resonances and thereby cannot be treated within the harmonic approximation
(b) the periphery structure of the molecules strongly affects the shape of the 3 $\mu$m band. In particular, the two-component emission interpretation can be explained by the presence of molecules with and without bay-hydrogens
(c) due to strong Fermi resonances of fundamental modes with combination bands regular PAHs can significantly contribute to the 3 $\mu$m plateau in the 3150-2950 cm$^{-1}$, while hydrogenated and methylated species are primarily responsible for features in the 2950-2750 cm$^{-1}$ region.
Issue Date:2016-06-23
Publisher:International Symposium on Molecular Spectroscopy
Genre:Conference Paper/Presentation
Rights Information:Copyright 2016 by the authors
Date Available in IDEALS:2016-08-22

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