PAHS AS THE CARRIERS OF THE 3.3 AND 3.4 MU-M EMISSION BANDS

Using recent laboratory measurements on gas-phase polycyclic aromatic hydrocarbons (PAHs) as well as new astronomical observations, we discu ss the origin of the 3.3 mu m emission band and of its major satellite band at 3.40 mu m. These studies show that the position, width and pr ofile of the 3.3 mu m band are consistent with the carriers being free PAH molecules. The peak position and width of this band are sensitive to the emission temperature (i.e. absorbed UV photon). The observed p eak position and width correspond to an ''average'' emission temperatu re of 1200 K which implies a size of 24 and 48 carbon atoms for a 6 an d 10 eV absorbed photon, respectively. The first overtone of the 3.3 m u m band has been detected at 1.68 mu m in the planetary nebula IRAS 2 1282 + 5050. These observations imply that the hot band is expected at 3.43 mu m rather than at 3.40 mu m. The main 3.40 mu m band is attrib uted to the CH stretching mode from methyl side groups attached to the periphery of PAHs. This assignment is supported by the spectral agree ment with laboratory spectra and by the behavior of the 3.40 mu m band upon UV radiation field in astronomical environments.