SPECTRA OF POLYCYCLIC AROMATIC HYDROCARBON MOLECULES AT ELEVATED-TEMPERATURE AND THE INTERSTELLAR UNIDENTIFIED INFRARED BANDS

An experiment in which the infrared absorption spectrum was obtained of the polycyclic aromatic hydrocarbon (PAH) coronene in KBr when the sample is at temperatures as high as 788 K has demonstrated that coronene's spectrum undergoes significant temporary changes with temperature including a change of the complex profile of the aromatic C-H stretch to a simpler profile very similar to that of the 3.3 μm interstellar UIR emission. The spectral change is reversible and cooling the sample to room temperature recovers the initial room temperature (300 K) spectrum. The experiment may duplicate the condition of an interstellar PAH molecule that has been temporarily vibrationally excited by the absorption of an ultraviolet photon. Additional experiments with a variety of PAH species now indicate that the phenomenon first observed for coronene is indeed a general one with possible implication for the assignment of a PAH source for the UIR emission. For all PAH species examined, the profile of the 3.3 μm absorption feature at 300 K can be temporarily changed to a single peak of enhanced strength similar to the 3.3 μm UIR emission profile by heating to some specific elevated temperature. There appears to be a direct relationship between the size of the molecule and the temperature at which conversion to the simpler profile is essentially complete. The results of the temperature experiments on the various PAHs studied are shown and relevance to the interstellar UIR question discussed.