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The bonding in hard, hydrogenated, amorphous carbon films (a-C:H) prepared by plasma decomposition of benzene was investigated by high-resolution electron-energy-loss spectroscopy. In all asgrown films we find that one-third of the carbon atoms are trigonally bonded and two-thirds of the carbon atoms are tetrahedrally bonded. For the trigonally bonded part, the dielectric properties reveal a transformation from polymeric state for samples prepared with low ion energy to a more graphitic state for films prepared with higher ion energy. Upon annealing, the number of carbon atoms in sp configuration increases. The graphitization of the films is observed in two steps at 200'C and at 400 C. A closing of the optical gap and a delocalization of the m electrons is observed between 400'C and 600'C.

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