Integrated Absorbances in the 3.4 μ[CLC]m[/CLC] CH[TINF][CLC][ITAL]n[/ITAL][/CLC][/TINF] Band in Hydrogenated Amorphous Carbon

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Abstract
We have measured the integrated absorbance, A (in units of cm molecule−1), for the spectral components of the 3.4 μm CHn absorption band in hydrogenated amorphous carbon (HAC) for samples deposited at 300 and 77 K. For the asymmetric CH3 stretching band in HAC, A is found to be significantly weaker than that occurring in pure saturated aliphatic hydrocarbons, while the symmetric mode of this group has a comparable intensity in HAC and in pure hydrocarbons. This suggests that the relative strength of CH3 symmetric and asymmetric absorption components may be a useful diagnostic of CH3 bonding in interstellar carbonaceous materials. Our measured absorbances, together with observed optical depths in the 3.4 μm absorption band in the diffuse interstellar medium, suggest that N(C)/N(H) ~ 72-97 parts per million in HAC in diffuse clouds. We discuss the possibility that aromatic carbon particles with CHn edge and defect groups may be the source of both the 2175 Å extinction bump and the 3.4 μm CHn absorption band.