SIMULATION OF INFRARED-SPECTRA OF CARBONACEOUS GRAINS

Random covalent network (RCN) theory is applied to describe the infrar ed spectroscopic properties of carbonaceous solids with compositions c ontaining a mixture of aromatic, aliphatic, and diamond-like hydrocarb ons. Application of this technique to carbonaceous dust is equivalent to the synthesis of solids whose structure and bonding satisfy stoiche ometry while minimizing strain energy. The result involves a range of compositions compatible with carbon bonding and the hydrogen concentra tion incorporated in the network. In general, only a limited range of compositions is available rather than the infinite number of possible compositions expected without the inclusion of these constraints. When compositions have been defined in this way, infrared spectra may be s ynthesized for comparison with astronomical spectra of interstellar ca rbonaceous solids. Such spectra contain components corresponding to ab sorption by CHn groups in which n = 1-3. We find, however, that additi onal spectral features, not included in our simple chemical model, mus t be present also in dust. We give plots of such spectra in the 3100-2 800 cm(-1) (3.2-3.6 mu m) region for comparison with infrared spectra of interstellar dust. We have also developed an RCN formalism that inc orporates oxygen into the carbon network as OH groups, and we show tha t this inclusion introduces a strong additional absorption band in the 3300 cm(-1) (3.0 mu m) region.