Detection of New Infrared Spectral Features in Hydrogenated Amorphous Carbon

Abstract

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 Hn 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.