Statistics – Computation
Scientific paper
Jan 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009eimw.confe...6b&link_type=abstract
"The Evolving ISM in the Milky Way and Nearby Galaxies, The Fourth Spitzer Science Center Conference, Proceedings of the confere
Statistics
Computation
Astrochemistry, Infrared : Ism, Ism : Molecules, Ism : Molecular Data, Ism : Line And Bands, Line : Identification, Techniques : Spectroscopy
Scientific paper
The mid-infrared spectra of seven very large PAHs (VLPAHs), with C number ranging from 54 to 130, have been determined computationally using Density Functional Theory (DFT). Trends in the dominant band positions and intensities for the major PAH bands in the 5 to 9 micron region as a function of PAH size, charge (neutral, cation, and anion forms) and geometry are discussed. The 6.2 micron CC stretching band for these compact VLPAHs is similar to those of the smaller PAHs previously studied and hence support earlier conclusions: astronomical PAHs must incorporate nitrogen to reproduce the class A 6.2 micron PAH band. In contrast, the 7-9 micron CC stretching modes and the CH in-plane vibrations of the largest of the PAH cations and anions considered here produce strong bands close to 7.8 micron and 8.5 micron. This suggests that the astronomical 7.7 micron band complex is produced by a mixture of small and large PAH cations and anions, with small PAHs contributing more to the 7.6 micron component and large PAHs more to the 7.8 micron component. Moreover, Peeters et al. (2002) found a correlation between the peak position of the astronomical 7.8 micron component and the peak of the 8.6 micron band. The similarity in behavior of these two bands in the compact VLPAH spectra suggests that large, cationic and anionic PAHs contribute strongly to the 7.8 and 8.6 micron astronomical features, with the specific peak position and profile reflecting the local PAH cation-to-anion concentration ratio and the relative intensities reflecting PAH size.
Allamandola Louis J.
Bauschlicher Charles W. Jr.
Peeters Els
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