UV irradiated hydrogenated amorphous carbon (HAC) materials as a carrier candidate of the interstellar UV bump at 217.5 nm

Context. Hydrogenated amorphous carbon (HAC) materials have been considered as a laboratory analog of cosmic carbonaceous nanoparticles in the interstellar medium (ISM). In the diffuse ISM, UV radiation can modify the electronic and atomic structure of HAC materials. Aims. Studying structural and optical properties of HAC materials in correlation with UV processing is very important to understand more clearly the effect of the UV radiation on carbonaceous dust grains in the diffuse ISM. This scenario can explain some astronomical spectral features such as the interstellar UV bump at 4.6 mu m(-1). Methods. Laser ablation has been used to produce nano-sized HAC materials which are subsequently irradiated by strong UV doses in a high vacuum. Transmission electron microscope images and spectroscopic analyses show the evolution of the internal structure of the material with the UV irradiation. Results. It is found that hydrogen content and the sp(3)/sp(2) hybridization ratio decrease with the UV irradiation. The graphene layers become longer in processed materials. Also, graphitic fibers are observed in modified materials. The variation in the internal structure leads to dramatic changes in the spectral properties in the FUV-VIS range. The UV irradiation of HAC materials, coresponding to 21-33% of the average dose of the UV radiation in diffuse ISM, has produced a new band centered at 4.6 mu m(-1) (217.5 nm). Conclusions. Consequently, these results confirm for the first time the suggestion by Mennella et al. (1996) that irradiated HAC materials might be considered the carrier of the interstellar UV bump at 4.6 mu m(-1). However, so far the amount of carbon needed to produce the interstellar 4.6 mu m(-1) band is higher than that available for interstellar carbon dust grains. So the ideal structure of irradiated HAC materials that would produce a band of sufficient strength is not yet clear for the interstellar dust.