ON THE STATE OF THE EMITTER OF THE 3.3 MICRON UNIDENTIFIED INFRARED BAND - ABSORPTION-SPECTROSCOPY OF POLYCYCLIC AROMATIC HYDROCARBON SPECIES

Experiments directed toward an understanding of the unidentified infrared (UIR) bands have resulted in infrared absorption spectra of the C-H stretch feature of various polycyclic aromatic hydrocarbon (PAH) molecules while a vapor at elevated temperatures. These experiments demonstrate nonpermanent temperature-dependent changes in the character of the feature similar to what has been seen in previous experiments performed with PAHs encapsulated in a KBr matrix. However, there is a distinct difference in peak absorption wavelength of the vapor phase feature from the feature of the PAH in the condensed state. The C-H stretch peak absorption wavelength distinguishes four classes of PAH at elevated temperature: condensed compact, condensed noncompact, vapor phase compact, and vapor phase noncompact PAH species. The compact PAH coronene has a C-H stretch peak wavelength in the condensed state at elevated temperature of 3.290-mu-m, whereas it displays an elevated temperature vapor phase spectrum with peak wavelength at 3.276-mu-m. Vapor phase spectra of other PAH species also exhibit a C-H stretch peak wavelength that is shorter than that of the molecule in the condensed state at elevated temperature. Condensed compact PAH species studied at elevated temperatures have a C-H stretch peak absorption wavelength that is a very good match to the observed 3.289-mu-m UIR band while noncompact species in the condensed state show the feature about 0.01-mu-m shortward of the astronomical data. All vapor phase PAH species studied, consistently show peak absorption shortward (0.01-0.02-mu-m) of the peak exhibited by the species in the condensed state suggesting that much of the interstellar PAHs may not be isolated "free-flying" molecules but exist, rather, in a condensed form.