Spectroscopy of PAH species in the gas phase

At elevated temperatures, the structured CH stretch absorption occurring at room temperature of polycyclic aromatic hydrocarbon (PAH) molecules encapsulated in KBr coalesces to a single peak whose wavelength is a very close match for that of the observed 3.3 μm unidentified infrared band (UIR) emission feature (Flickinger and Wdowiak. Astrophys. J.362, L71-L74. 1990). The temperature of approximately 800 K is significant because a PAH molecule in the interstellar radiation field, upon absorption of an ultraviolet photon, is expected to be excited to an energy equivalent to temperatures of this order (Sellgren, Astrophys. 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 μm is at a shorter wavelength than in the condensed state (Flickinger et al., Astrophys. J.380, L43-L46, 1991). Recent work by Joblin et al. (1st Symp. on the Infrared Cirrus and Diffuse Interstellar Clouds. 1994) in a short path length diamond cell indicates the gas phase wavelength of the 3.3 μm region C-H stretch feature of naphthalene is directly proportional to temperature over a large temperature range. Laser-excited IR fluorescence 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 important to carry out. We have utilized a long-path-length optical heat pipe to confirm that the wavelength of the peak of the C-H stretch band of naphthalene vapor obeys a direct relationship with temperature as reported by Joblin, and to demonstrate that this feature undergoes a strengthening with respect to the longer wavelength band due to an out-of-plane deformation (782 cm−1) as temperature increases. The shift from 3067.6 to 3063.7 cm−1 over the temperature range of 316–996 K seems to be linear and follows the relationship 1/λ (cm−1) = 3071.4–7.89 × 10−3T (K). From this relationship we calculated the temperatures equivalent to excitation energies in both Type 1 (3.289 μm and Type 11 (3.296 μm) UIR objects (Tokunaga 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 relative to the CH out-of-plane deformation feature is observed to increase from 0.33 at 316 K to 0.57 at 996 K.