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 Å (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, 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 Å.
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Duley et al. (2004) studied this question.