FIRST EXPERIMENTAL-EVIDENCE OF A C-1S CORE EXCITON IN AMORPHOUS-CARBON FILMS

Amorphous carbon films obtained by pulsed-laser ablation of graphite have been investigated by high-resolution X-ray absorption spectroscopy (XAS). The onset of the 1s --> sigma* transitions lies in the gap between pi* and sigma* states in graphite, indicating a high content of sp(3) hybridization in the films. A sharp peak similar to the core exciton in diamond is observed and assigned to a core exciton in sp(3)-hybridized disordered C atoms. The observed binding-energy shift of similar to 0.5 eV with respect to the diamond core exciton can be explanined in terms of a higher localization of the excited state induced by disorder. This result supports a Frenkel model to describe the core exciton in sp(3)-hybridized carbon.