IUCAA Preprints
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Item Computational study of neutral and catoinic catacondensed polycyclic aromatic hydrocarbons(2005-06-01) Pathak, Amit; Rastogi, ShantanuTheoretical calculations have been done for neutral and cationic catacondensed polycyclic aromatic hydrocarbons (PAH) using density functional theory approach. Optimized geometries and charge istributions have been calculated and the change in structure and charge distribution upon ionization of PAHs is studied and discussed. The calculated infrared vibrational modes show systematic variations with size in the linear polyacenes while no regular variation is apparent in non-linear catacondensed PAHs. The prominent features in the spectra of neutral PAHs are due to C–H stretch and C–H wag motions. In the spectra of PAH cations C–C stretch and C–H in plane modes are the most intense. The changes in charge distributions of cations causing these intensity changes have been identified. The C–H stretch intensity depends on the partial charge on peripheral Hydrogen atoms and reduces in cations as Hydrogen atoms become more positive. The prospect of catacondensed PAHs is discussed in the context of Astrophysical Unidentified Infrared bands.Item Computational study for neutral and cationic pericondensed polycyclic aromatic hydrocarbons(2006-02-10) Pathak, Amit; Rastogi, ShantanuQuantum chemical calculations using density functional theory are presented for small to medium sized pericondensed PAHs includ- ing some being reported for the first time. Bond lengths and charge distribution have been computed for these PAHs in both neutral and cationic forms. Upon ionization, significant change in fractional charge on atoms is present particularly for the outer carbon atoms. The charge on the internal carbon atoms tends towards zero in cations. Vibrational frequencies and infrared intensities have been calculated for the optimized structures of PAH neutrals and cations. The drastic intensity alterations occurring upon ionization are discussed and related to specific changes occurring in the charge distribution. The C-H stretch intensity depends on the partial charge on peripheral hydrogen atoms and reduces in cations as hydrogen atoms become more positive. Pericondensed PAHs show better matching with the observed interstellar infrared bands. The co-added model spectra show profiles similar to the observed astrophysical bands.Item Modeling the interstellar aromatic infrared bands with co-added spectra of PAHs(2008-04) Pathak, Amit; Rastogi, ShantanuAims. The observed variations in pro les of the interstellar aromatic infrared bands correlate with the object type and are indicative of PAH populations existing in di erent sources. Spectroscopic studies of PAHs can provide tools for interpreting the variations accompanying the AIBs. As the observed spectra results from a mix of possible species in the region, we try to model this composite spectra by co-adding emissions from PAHs in di erent size groups. Methods. Theoretical IR data of PAHs having 10 to 96 carbon atoms is used to obtain emission spectra. The models are taken in size groups made up of small, medium, and large PAHs. Results. The models show a good pro le match with observations for the 7.7 m complex having sub-features at 7.6 and 7.8 m. The 7.6 m sub-feature dominates in the spectra of medium-sized PAH cations, matching observations from UV-rich interstellar environments. The 7.8 m component is more intense in the spectra of large PAH cations (model III), correlating with observations from benign astrophysical regions. A possible interpretation of the observations of C H out-of-plane bend modes and the weak outliers on the blue side of the intense 11.2 m band is proposed. The models provide pointers to possible PAH populations in di erent regions