2012A&A...544A.107G

Mid-infrared (MIR) bands are characteristic for the short-range and medium-range structure of hydrogenated amorphous carbon (HAC) materials that exist in the interstellar medium (ISM) and are sensitive to processing in the harsh interstellar radiation field. We study the variability of the MIR features from the spectrum of non-processed to that of UV-processed HAC materials and compare them to spectra of interstellar carbonaceous materials. Nano-sized HAC materials produced by laser ablation were irradiated by vacuum-UV photons with doses comparable to those relevant for interstellar processing. They were subsequently analyzed by IR spectroscopy. In the MIR range, the spectra of HAC materials show many absorption bands such as the sp³ aliphatic =C-H stretching vibration at 3.03µm, the sp³ aliphatic -C-H stretching vibration at 3.4µm and also the sp³ aliphatic C-H bending vibration at both 6.85µm and 7.25µm. All these are recognizable bands of HAC materials. Other absorption bands such as the sp² aromatic =C-H stretching vibration at about 3.3µm and the sp² -C=C stretching vibration close to 6.25µm are observed. The HAC materials also possess bands which represent the aromatic out-of-plane bending at 11.65, 12.46 and 12.9µm in addition to the aromatic -C-C-C in-plane bending at 15.87µm. With UV irradiation, the mass absorption coefficient of the 3.03µm band completely disappears and that of the aliphatic C-H bands (3.4, 6.85 and 7.25µm) decreases. This reduction shows that the UV radiation destroys most of the aliphatic C-H bonds inside the HAC structure. On the other hand, the strength of the aromatic 6.2µm band increases, which is evidence of the partial graphitization within UV-irradiated HAC materials. Because UV irradiation is not uniform, this band agrees well with the C-class PAH toward HD 100764. The C-H out-of-plane vibration bands are strongly affected by UV irradiation. Bands at 11.35, 12.14 and 12.64µm (solo, duo and trio, respectively), which are found for PAHs of many interstellar spectra were observed partially and were compared to those of non-processed materials, in particular, those at 11.35µm, which represent the aromatic structures. UV irradiation has a variable effect on both aliphatic and aromatic bands in the MIR region. The aliphatic C-H structure decreases while the aromatic C=C structure, which might lead to the graphitic colonies for PAHs, increases. UV irradiation has revealed solo, duo and trio bands that are relatively consistent with those of the A- and B-class PAHs.