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.