Modeling the unidentified infrared emission with combinations of polycyclic aromatic hydrocarbons

The infrared emission band spectrum associated with many different interste llar objects can be modeled successfully by using combined laboratory spect ra of neutral and positively charged polycyclic aromatic hydrocarbons (PAHs ). These model spectra, shown here for the first time, alleviate the princi pal spectroscopic criticisms previously leveled at the PAH hypothesis and d emonstrate that mixtures of free molecular PAHs can indeed account for the overall appearance of the widespread interstellar infrared emission spectru m. Furthermore, these models give us insight into the structures, stabiliti es, abundances, and ionization balance of the interstellar PAH population. These, in turn, reflect conditions in the emission zones and shed light on the microscopic processes involved in the carbon nucleation, growth, and ev olution in circumstellar shells and the interstellar medium.