Raman and Infrared Emission Spectra of Hydrogenated Amorphous Carbon: Insight into the Origin of the 6-14 Micron Infrared Emission Bands

We have obtained micro-Raman spectra of hydrogenated amorphous carbon (HAC) in particulate and thin-film form. A comparison between Raman and infrared emission spectra of HAC in the 6-14 μm region yields further information concerning the chemical structure of the emitters responsible for the infrared emission bands observed in interstellar and circumstellar sources. In particular, we find that both aromatic and nonaromatic species can contribute to emission at 6.2 μm but that emission in the 7.6-7.8 μm band arises primarily from sixfold aromatic species. Using Raman spectra as a guide, it is suggested that the nanoparticles responsible for the unidentified infrared bands may be three-dimensional, consisting of small stacks of aromatic rings. We find that these particles should emit at wavelengths between 6.17 and 6.25 μm (1620-1600 cm -1 ), coinciding with those observed in types A and B infrared emission sources. We find that a 6.05 μm satellite to the 6.2 μm feature observed in sources such as HD 44179 likely arises as a result of emission from olefinic CC in bridging groups between polycyclic aromatic hydrocarbon molecules. Infrared emission spectra of HAC films also show features at 6.9 and 7.27 μm corresponding to bending vibrations in aliphatic and olefinic groups. Counterparts of these bands in interstellar emission spectra may be useful in indicating the level of hydrogenation of the emitters.