It is well known that energetic particle bombardment during diamond-like carbon film growth has a significant influence on the resulting film stress, optical transparency, hardness and chemical inertness. It has been commonly recognised that many of the desirable properties of carbon films arise from the sp(3) tetragonal bonding component. Therefore, it is essential to have a deeper knowledge of the role of ion energy on the evolution of the diamond-like phase. In this work, we propose a parameter (P) as a key for the understanding of the nucleation and growth mechanism, which takes into account the number of recoiled atoms and their transfer momentum by argon ion assistance, compared to all atoms present in the volume, described by ion range, plus the atoms coming from the graphite-sputtered target. Thin films of a-carbon were deposited by the dual ion beam sputtering technique. An argon ion beam sputters a graphite target and a second one assists the him during the growth. Sets of samples were produced with different momentum transferred by the assistance beam (P). A spectrophotometric analysis in the visible range is performed to determine the Tauc optical gap E-g, the refractive index and the extinction coefficient. The results show that the films not assisted exhibit a lower density and consequently a low refractive index (n = 1.8 in the visible range), while increasing the momentum transfer parameter (P) the refractive index increases suggesting a densification of the material. The same trend is observed for the Tauc optical gap. In order to get further information on the evaluation of density due to the assistance, X-ray reflectivity measurements have been performed and correlated to the optical results.