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Title:
Polycyclic aromatic hydrocarbons and the unidentified infrared emission bands - Auto exhaust along the Milky Way
Authors:
Allamandola, L. J.; Tielens, A. G. G. M.; Barker, J. R.
Affiliation:
AA(NASA, Ames Research Center, Space Science Div., Moffett Field, CA), AB(NASA, Ames Research Center, Space Science Div., Moffett Field, CA), AC(SRI International, Menlo Park, CA)
Publication:
Astrophysical Journal, Part 2 - Letters to the Editor (ISSN 0004-637X), vol. 290, March 1, 1985, p. L25-L28. (ApJL Homepage)
Publication Date:
03/1985
Category:
Astrophysics
Origin:
STI
NASA/STI Keywords:
Aromatic Compounds, Hydrocarbons, Infrared Spectra, Interstellar Matter, Milky Way Galaxy, Exhaust Emission, Fluorescence, Molecular Excitation, Particulates, Raman Spectra, Spectral Bands, Spectral Line Width, Vibrational Spectra
DOI:
10.1086/184435
Bibliographic Code:
1985ApJ...290L..25A

Abstract

The unidentified infrared emission features (UIR bands) are attributed to a collection of partially hydrogenated, positively charged polycyclic aromatic hydrocarbons (PAHs). This assignment is based on a spectroscopic analysis of the UIR bands. Comparison of the observed interstellar 6.2 and 7.7-micron bands with the laboratory measured Raman spectrum of a collection of carbon-based particulates (auto exhaust) shows a very good agreement, supporting this identification. The infrared emission is due to relaxation from highly vibrationally and electronically excited states. The excitation is probably caused by UV photon absorption. The infrared fluorescence of one particular, highly vibrationally excited PAH (chrysene) is modeled. In this analysis the species is treated as a molecule rather than bulk material and the non-thermodynamic equilibrium nature of the emission is fully taken into account. From a comparison of the observed ratio of the 3.3 to 11.3-micron UIR bands with the model calculations, the average number of carbon atoms per molecule is estimated to be about 20. The abundance of interstellar PAHs is calculated to be about 2 x 10 to the -7th with respect to hydrogen.

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