2002ApJ...568..448G

The formation of carbonaceous condensates in the atmospheres of asymptotic giant branch (AGB) carbon stars and in the nebulae of post-AGB objects occurs in a chemical environment that also contains oxygen and nitrogen. The extent of incorporation of chemical groups containing these elements in carbonaceous solids will depend on the ratios C/O and C/N. The presence of chemical groups containing O and N can have a significant effect on the infrared spectrum of this material through the introduction of new absorption and emission features and by perturbing spectral lines arising from CH and other hydrocarbon groups. To simulate some aspects of this chemistry, we have prepared samples of hydrogenated amorphous carbon (HAC) containing these elements by laser ablation of graphite in mixed H₂/N₂, H₂/O₂, and H₂/CO gases and in NH₃. Absorption and emission spectra are reported for these materials in the 2.5-20 µm region. We find that the inclusion of oxygen has a profound effect on both 3.4 µm and longer wavelength absorption in HAC even at low concentration. Condensation of HAC in the presence of N₂, CO, and NH₃ produces changes in the profile and relative intensity of components of the 3.4 µm CH_2, 3_ hydrocarbon band, but some of these effects can be reversed by heating to higher temperature. We find that the condensation of a crystalline diamond component is greatly facilitated in the presence of O₂ or NH₃. A number of sharp spectral features attributable to diamond have been observed in these spectra.