Mid-infrared spectroscopy of UV irradiated hydrogenated amorphous carbon materials

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 and are sensitive to processing in the harsh interstellar radiation field. The authors 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. With UV irradiation, the mass absorption coefficient of the 3.03 mu m band completely disappears and that of the aliphatic C-H bands 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 6.2 mu m 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. UV irradiation has revealed solo, duo and trio bands that are relatively consistent with those of the A- and B-class PAHs.