| Abstract : | Several astronomical objects in the Galaxy and in external galaxies show mid-infrared spectra that are dominated by features at 3.3, 6.2, 7.7, 8.6, and 11.2 μm, known as the unidentified infrared (UIR) bands. These emission features match very well the vibrational modes of polycyclic aromatic hydrocarbon (PAH) molecules (Tielens 2008). In order to completely understand the UIR bands, spectroscopic data on different PAH species are needed including pure PAHs, ionized PAHs, PAHs with side groups, substituted PAHs and PAH clusters. Since there is very limited data on PAH clusters, the present work investigates infrared (IR) spectra and enthalpy of formation of PAH clusters in neutral and ionized forms. The effect of ”packing” on the IR spectra of PAH clusters is discussed. The computed IR spectra are then compared with observations. |
