2000ApJ...543L..85G

We have obtained high-resolution (1 cm^–1) spectra of hydrogenated amorphous carbon (HAC) in both emission and absorption. Deconvolution of these spectra into Gaussian-Lorentzian components reveal two new features at 3098 cm^–1 (3.23 µm) and 2890 cm^–1 (3.46 µm) attributable to nonaromatic CCH₂ and tertiary CH groups, respectively. In addition, relatively sharp lines (Δν∼25 cm^–1), which we assign to CH₂ groups in cyclohydrocarbon molecules such as hexahydropyrene, appear on heating HAC to 800 K. We propose that these molecules are the precursors of H_n polycyclic aromatic hydrocarbons seen in low-excitation protoplanetary nebulae. The profile of the 2890 cm^–1 (3.46 µm) tertiary CH feature detected as a component of HAC is very similar to that observed in dark clouds where it has been attributed to interstellar diamond-like material. An absorption/emission feature at 3098 cm^–1 (3.23 µm) should also be present in ``transition'' sources, i.e., regions where the composition of the carbonaceous gas/dust component is evolving from a saturated, CH₂, CH₃-rich material to less saturated aromatic material. The role of H atom reaction and UV irradiation in determining spectra in the 2.8-3.7 µm region is briefly discussed. On the basis of our laboratory data we find that the absence of CH₂, CH₃ absorption in dark clouds can be understood if H atom reactions with HAC are inhibited as a result of an increased rate of gas-phase reactions.