NEUTRAL AND IONIZED PAHS - CONTRIBUTION TO THE INTERSTELLAR EXTINCTION

A detailed study of the UV-to-near-IR absorption spectra of neutral an d ionized polycyclic aromatic hydrocarbons (PAHs) is reported. The exp erimental results obtained using matrix isolation spectroscopy combine d with the general spectroscopic properties of PAHs derived from theor etical modeling are compared with astronomical observations. From this study, it is concluded that: (i) small neutral PAHs (less than 25 car bon atoms) absorb only in the UV and cannot contribute to the known di ffuse interstellar bands, although they may contribute to the extincti on curve. A search for their spectral signatures should provide a test to the PAH model and should allow us to set significant limits for th e abundances of specific PAHs; (ii) small ionized PAHs (less than 25 c arbon atoms) absorb in the visible and near-IR close to the position o f well-known diffuse interstellar bands. Based on these promising resu lts it is argued that PAH ions constitute good candidates to explain t he diffuse interstellar bands. An assessment of the PAH/diffuse inters tellar band proposal is made and predictions for new diffuse interstel lar bands to be searched for are presented.