A plasmon resonance in dehydrogenated coronene (C24Hx) and its cations and the origin of the interstellar extinction band at 217.5 nanometers

Analysis of the wavelength dependence of extinction in the ultraviolet is shown to provide additional insight into the physical properties of the carrier of the interstellar absorption feature at 217.5 nm (5.7 eV). In particular, it is found that this band has a cutoff at both long and short wavelengths, supporting the assignment of this feature to a plasmon band. The photon energies corresponding to these cutoffs are highly diagnostic of the carrier of this feature and suggest that the absorber is a seven-ring aromatic molecule. Comparison with laboratory data, together with theoretical calculations of the plasmon resonance in small dehydrogenated aromatic-ring structures, supports the assignment of this feature to the pi-pi* plasmon resonance in dehydrogenated coronene molecules, C-24 H-x and its cations, where x <= 3. It is shown that this assignment is consistent with available observational data on the 217.5 nm feature, including such properties as a constant central wavelength, variable width, its relation to E( 1250-V)/E(B-V), and the carbon abundance in the interstellar medium. The pi and pi* energy levels in the 217.5 nm carrier, derived from an analysis of interstellar extinction, have been used to construct its energy-level diagram, enabling comparison with other observational data.