Hydrogenated amorphous carbons (HACs) are considered as laboratory analogues to cosmic
carbonaceous nanoparticles in the interstellar medium (ISM). The optical properties of
nano-sized HACs may be influenced by the UV processing. The variation of the internal
structure leads to dramatic changes in the spectral properties in the FUV-VIS range. This
scenario can explain some astronomical features such as the interstellar UV bump at 4.6
μm-1. The spectrum of HACs, irradiated by a dose of UV
irradiation that corresponds to 21–33% of the average dose of the UV radiation in diffuse
ISM, exhibits a new band centered at 4.6 μm-1. This result
confirms, for the first time, the suggestion by Mennella et al. (1996)
that irradiated HACs might be considered as the carriers of the interstellar UV bump at
4.6 μm-1. However, the amount of carbon needed to reproduce
this band is higher than that available for interstellar carbon dust grains. So the ideal
structure of irradiated HACs that would produce a band of sufficient strength has still to
be searched for.