2001MNRAS.324..355P

In this second paper on the 3.2-3.5µm region, we report the results of a theoretical study of the possible contributions of hydrogenated compounds to the observed infrared emission. We consider small aromatic species to which hydrogens have been added directly on the polycyclic frame, in both neutral and ionized forms, to mimic typical structural situations expected for larger polycyclic aromatic hydrocarbons (PAHs). The evolution of the infrared spectra for PAHs having increasingly higher excess H coverage is investigated up to saturation. The present study shows that hydrogenated PAHs produce series of bands that may be at the origin of the broad plateau observed below the 3.4- and 3.5-µm features, together with the features themselves. Ionization produces the usual collapse of the aromatic CH intensities, but it also affects the alicyclic CH intensities, although less drastically, in contrast to the CH intensities of aliphatic substituents.