X-RAY-ABSORPTION SPECTROSCOPY STUDY OF PULSED-LASER-EVAPORATED AMORPHOUS-CARBON FILMS

Amorphous carbon (a-C) films obtained by pulsed-laser ablation of graphite have been investigated by X-ray Absorption Spectroscopy (XAS). The onset of 1s --> sigma* transitions in the films lies in the gap between the pi* and sigma* bands in graphite and very close to the absorption edge of diamond, indicating a high content of sp(3) hybridization. A sharp feature at this onset is observed and assigned to a core exciton in sp(3)-hybridized disordered C atoms. Its shift of 0.5 eV with respect to the core exciton in diamond is probably due to a higher localization of the excited electron induced by disorder. A small peak coming from C-H bonds at the surface is observed and its intensity increases with the amount of sp(3)-hybridized atoms in the sample. This can be easily explained by associating a higher amount of dangling bonds at the surface to a higher sp(3) content. Polarization-dependent XAS measurements show that the angular distribution of these C-H bonds has a mean value close to the normal to the surface.