Amorphous carbon (a-C) films have been grown by processes using Ar+ or C+ ions. The films hybridization states were evaluated by Auger electron spectroscopy, X-ray photoelectron spectroscopy and X-ray reflectivity. It was found that the films grown using C+ (Ar+) exhibit a sp(2) (sp(3)) surface, while the deeper monolayers are sp(3) (mixed sp(3)-sp(2)) bonded. These results were compared with simulations of the ion-solid interactions and the results of a phenomenological model which takes into account the balance between the surface and bulk processes. We found two mechanisms of spa formation: on the surface and in the bulk of a-C. We conclude that at medium energy (> 100 eV) the surface effects are negligible (for both C+ and Ar+), while the surface processes dominate when low-energy (similar to40 eV) Ar+ ions are used.