| Summary: | The Unidentified Infrared (UIR) bands are a family of emission features seen in dusty objects, which are generally attributed to IR fluorescence of small (~ 50 C-atoms) polycyclic aromatic hydrocarbon (PAH) molecules. However, no specific PAH has so far been identified. The spatial distribution of 3.3 p,m PAH emission in the inner region of the Red Rectangle nebula is studied using an Integral Field spectroscopy technique. The presence of two components at 3.28 and 3.30 p,m with different spatial distributions, originally proposed by Song et al. (2003, 2007) is supported by these data. This implies the presence of two classes of 3.3 p,m band carrier with peak wavelength separation of ",0.02 p,m. From comparison of the 3.3 p,m observations with laboratory and theoretical spectra for a range of PAH molecules it is inferred that the 3.28 p,m and 3.30 p,m components arise from 'bay' and 'non-bay' hydrogen sites, respectively, on the periphery of neutral PAHs. The vibrational spectrum of a large number of PAH molecules are investigated with Density Functional Theory (DFT) calculations to find possible trends which may help in the identification of specific PAHs. It has been found that for acenes (single row), the position of the solo out-of-plane bending mode moves to shorter wavelength with increase number of carbon atoms. A similar behaviour holds also for 2-rows, 3-rows and large compact PAH molecules. Pericondensed and catacondensed PAHs with functional groups are investigated to assess their contribution to the unidentified 21 p,m feature, found in carbon-rich environments. They are unlikely to be responsible for the feature. The results of DFT calculations for large, solo-containing PAHs are then used to model Abstract 4 the asymmetric profile of the 11.2 p,m feature. The IR emission mechanism is taken into account, together with the effects of the DV radiation field in different sources; the temperature dependence of the solo out-of-plane bending mode and position are also considered, using state-of-the-art experimental data. It is found that the model reproduces very well the shape and the val1ations of the 11.2 p,m band in a few environments, supporting the idea that these variations are due to different mass distributions of PAHs, rather than anharmonicity. A similar model is then applied to acenes, which appear to contribute most of the intensity of the 11.0 p,m feature. High-resolution long-slit spectroscopic observations of the 11.2 p,m band in two PNe (NGC 7027 and BD+30° 3639) and in a PDR (The Orion Bar), in a search for fine stmcture have been carried out. While the presence of fine structure cannot ruled out, changes in the 11.2 p,m feature along the Orion Bar, from the molecular cloud to the ionised region, is seen suggesting possible evolution of the carriers of the feature. ..
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