SPECTROSCOPY OF PAH SPECIES IN THE GAS-PHASE

AT elevated temperatures, the structured C-H stretch absorption occurr ing at room temperature of polycyclic aromatic hydrocarbon (PAH) molec ules encapsulated in KBr coalesces to a single peak whose wavelength i s a very close match for that of the observed 3.3 mu m unidentified in frared band (UIR) emission feature (Flickinger and Wdowiak, Astrophys. J. 362, L71-L74, 1990). The temperature of approximately 800 K is sig nificant because a PAH molecule in the interstellar radiation field, u pon absorption of an ultraviolet photon, is expected to be excited to an energy equivalent to temperatures of this order (Sellgren, Astrophy s. J 277, 623 1984). Our previous study of PAH molecules in the vapor phase at 600-800 K showed that the C-H stretch PAH feature near 3.3 mu m region C-H stretch feature of napthalene is directly proportional t o temperature over a large temperature range. Laser-excited IR fluores cence studies have also suggested that the wavelength is sensitive to laser photon energy in a direct relationship (williams and Leone, The Diffuse Interstellar Bands Conf., 1994). Comprehensive studies of PAHs thermally excited up to the decomposition temperature are therefore i mportant to carry out. We have utilized a long-path-length optical hea t pipe to confirm that the wavelength of the peak of the C-H stretch b and of naphthalene vapor obeys a direct relationship with temperature as reported by Joblin, adn to demonstrate that this feature undergoes a strengthening with respect to the longer wavelength band due to an o ut-of-plane deformation (782 cm(-1)) as temperature increases. The shi ft from 3067.6 to 3.63.7 cm(-1) over the temperature range of 316-996 K seems to be linear and follows the relationship 1/lambda (cm(-1)) = 3.71.4-7.89 X 10(-3) T(K). From this equivalent to excitation energies in both Type I (3.289 mu m) and Type II (3.296 mu m) UIR objects (Tok unaga et al., Astrophys. J. 380, 452-460, 1991) to be 3925 and 4691 K, respectively. The ratio of the strength of the C-H stretch feature re lative to the C-H out-of-plane deformation feature is observed to incr ease form 0.33 at 316 K to 0.57 at 996 K.