Ultraviolet absorption in amorphous carbons: Polycyclic aromatic hydrocarbons and the 2175 extinction feature

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 Angstrom(4.6 mum(-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 pi-pi*plasmon resonance in PAH components having structures similar to coronene. An excellent fit to the laboratory data is obtained with the calculated pi-pi*plasmon in dehydrogenated coronene, C-24. 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 Angstrom.