Ultraviolet Absorption in Amorphous Carbons: Polycyclic Aromatic Hydrocarbons and the 2175 Å Extinction Feature

Abstract

Amorphous carbons have been prepared by plasma deposition in a vacuum and in the presence of a variety of gases. On subsequent heating in a vacuum, ultraviolet absorption spectra of certain of these materials show the development of a feature centered at 2175 A (4.6 μm-1). We find that the spectrum of amorphous carbon prepared from a hydrogen plasma provides a good match to the interstellar spectrum. In addition, infrared spectra and high-resolution imaging of these deposits indicate the presence of polycyclic aromatic hydrocarbon (PAH) islands in these solids, suggesting that the UV absorption feature can be attributed to the π-π* plasmon resonance in PAH components having structures similar to coronene. An excellent fit to the laboratory data is obtained with the calculated π-π* plasmon in dehydrogenated coronene, C24. Our data suggest that relatively small dehydrogenated PAH molecules similar to molecular groups seen in amorphous carbon may be the origin of the interstellar extinction feature at 2175 A.