1995A&A...299..835J

Although it is now considered that the Unidentified IR (UIR) emission bands at 3.3, 6.2, 7.7, 8.6 and 11.3µm 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 3.3µm 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 (∼1000K) required to emit at 3.3µm. In this paper, we report the measurements of IR absorption bands of gas-phase PAHs at various temperatures up to about 1000K. 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 3.3µm 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 6eV and/or by larger molecules of typically 50 carbon atoms which are exposed to 10eV photons.