Integrated Absorbances in the 3.4 μm CH_n Band in Hydrogenated Amorphous Carbon

We have measured the integrated absorbance, A (in units of cm molecule⁻¹), for the spectral components of the 3.4 μm CH_n 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 CH_n edge and defect groups may be the source of both the 2175 Å extinction bump and the 3.4 μm CH_n absorption band.