Activation of the 3.47 mm band by H atom irradiation of carbon grains covered with a water ice layer at 12 K

We present the first results of laboratory research on the processing of nano-sized carbon particles under simulated dense-medium conditions. The experiments aim to study the variations of the degree of hydrogenation of carbon particles covered with a water ice layer in response to H atom exposure. The effects of H atom processing have been studied by means of IR spectroscopy. The results show that the formation of C-H bonds in the aliphatic CH2 and CH3 functional groups does not take place. The activation of a band at 3.47 mu m is observed. It is assigned to the C-H stretching vibration of tertiary sp(3) carbon atoms. The band profile is compatible with that present in the spectrum of sources embedded in dense clouds. We propose that the carbon grain population, which is responsible for the interstellar aliphatic 3.4 mu m band in diffuse regions, absorbs at 3.47 mu m in dense clouds.