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

Amorphous carbon (a-C) films obtained by pulsed-laser ablation of grap hite have been investigated by X-ray Absorption Spectroscopy (XAS). Th e onset of 1s --> sigma transitions in the films lies in the gap betw een the pi and sigma* bands in graphite and very close to the absorpt ion edge of diamond, indicating a high content of sp(3) hybridization. A sharp feature at this onset is observed and assigned to a core exci ton in sp(3)-hybridized disordered C atoms. Its shift of 0.5 eV with r espect to the core exciton in diamond is probably due to a higher loca lization of the excited electron induced by disorder. A small peak com ing from C-H bonds at the surface is observed and its intensity increa ses with the amount of sp(3)-hybridized atoms in the sample. This can be easily explained by associating a higher amount of dangling bonds a t the surface to a higher sp(3) content. Polarization-dependent XAS me asurements show that the angular distribution of these C-H bonds has a mean value close to the normal to the surface.