Abstract
The mid infrared emissions known as unidentified infrared (UIR) bands are observed ubiquitously in a variety of astrophysical sources and are attributed to emissions from polycyclic aromatic hydrocarbon (PAH) molecules. The observed features result from a composite mix of PAH populations consisting of neutrals, cations, anions and hydrogenated/dehydrogenated species. There are source to source variations in the UIR features indicating the presence of different PAH populations in different astrophysical regions. An understanding of the IR properties of PAHs can, therefore, lead to an understanding of the astrophysical environments. With this view vibrational characteristics using quantum chemical calculations for a large number of PAH neutrals and cations of different shapes and sizes have been studied. The obtained infrared data are used to model emission spectra of diverse PAH groups and compared with the observed bands. The model spectra presented here point out the presence of different size group of PAHs in different interstellar environments.
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