INFRARED-SPECTROSCOPY OF GAS-PHASE PAH MOLECULES .2. ROLE OF THE TEMPERATURE

Although it is now considered that the Unidentified IR (UIR) emission bands at 3.3, 6.2, 7.7, 8.6 and 11.3 mu m are due to aromatic hydrogenated compounds, there is more controversy over whether the carriers are free molecules, especially polycyclic aromatic hydrocarbons (PAHs), or are associated to larger dust particles. For example, the observed position of the 3.3 mu m band is better reproduced in laboratory by condensed molecules than by gas-phase molecules. On the other hand, free PAHs after the absorption of a single UV photon can easily reach the high temperatures (similar to 1000 K) required to emit at 3.3 mu m In this paper, we report the measurements of IR absorption bands of gas-phase PAHs at various temperatures up to about 1000 K. The band positions clearly depend on the vibrational content of the molecules, which is interpreted in terms of anharmonic couplings. In the particular case of the 3.3 mu m band, it is found that the observed position for this band is consistent with the carriers being free PAHs at high temperatures. It is proposed that this feature is dominated by molecules of the size of coronene (C24H12) which absorb photons of mean energy 6 eV and/or by larger molecules of typically 50 carbon atoms which are exposed to 10 eV photons.