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Infrared Absorption and Emission Spectra of Hydrogenated Amorphous Carbon Prepared in the Presence of Oxygen, Nitrogen, Ammonia, and Carbon Monoxide
The formation of carbonaceous condensates in the atmospheres ofasymptotic giant branch (AGB) carbon stars and in the nebulae ofpost-AGB objects occurs in a chemical environment that also containsoxygen and nitrogen. The extent of incorporation of chemical groupscontaining these elements in carbonaceous solids will depend on theratios C/O and C/N. The presence of chemical groups containing O and Ncan have a significant effect on the infrared spectrum of this materialthrough the introduction of new absorption and emission features and byperturbing spectral lines arising from CH and other hydrocarbon groups.To simulate some aspects of this chemistry, we have prepared samples ofhydrogenated amorphous carbon (HAC) containing these elements by laserablation of graphite in mixed H2/N2,H2/O2, and H2/CO gases and inNH3. Absorption and emission spectra are reported for thesematerials in the 2.5-20 μm region. We find that the inclusion ofoxygen has a profound effect on both 3.4 μm and longer wavelengthabsorption in HAC even at low concentration. Condensation of HAC in thepresence of N2, CO, and NH3 produces changes inthe profile and relative intensity of components of the 3.4 μmCH2, 3 hydrocarbon band, but some of these effects can bereversed by heating to higher temperature. We find that the condensationof a crystalline diamond component is greatly facilitated in thepresence of O2 or NH3. A number of sharp spectralfeatures attributable to diamond have been observed in these spectra.
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