Laboratory Simulation of 11-15 μm Spectra Associated with Polycyclic Aromatic Hydrocarbon Molecules

We have obtained 11-15 μm spectra of thin films of hydrogenated amorphous carbon assembled from carbon nanoparticles. Well-defined, narrow features similar to the interstellar emission bands attributed to polycyclic aromatic hydrocarbons (PAHs) are observed at 11.3, 12, 12.8, 13.5, and 14.2 μm. These features are reproduced in many samples prepared under a variety of conditions, indicating that they involve common molecular components. We suggest that these are based on compact ring structures such as coronene, C ₂₄H₁₂, but that these molecules have peripheral substitutions involving additional aromatic rings. They may also be substituted with sp- or sp²-bonded carbon chains. All bands exhibit slight energy shifts in different samples in response to changes in preparation conditions. These shifts replicate variations in wavelength and profile of the 11-15 μm bands seen in different type of emission source. None of the nanoparticle samples exhibit all of the 11.3, 12, 12.8, 13.5, and 14.2 μm features. This implies that the interstellar bands are produced by a limited variety of PAH structures, rather than by a single dominant species.