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Resonant Raman scattering has been used to study amorphous carbon and polycrystalline diamond films. The incident photon energies were varied over the range 2. 2--4. 8 eV. In hydrogenated amorphous carbon films containing both sp^3- and sp^2-bonded carbon, a high-frequency shift is observed for the main Raman peak with increasing photon energies up to 3. 5 eV. This shift is interpreted in terms of scattering from -bonded carbon clusters which is resonantly enhanced for photon energies approaching the -^* resonance of sp^2-bonded carbon. In polycrystalline diamond films excitation with photon energies 3. 0 eV enhances the Raman signal from the sp^3-bonded diamond phase relative to the scattering by sp^2-bonded carbon and with respect to the underlying broadband luminescence. The Raman band arising from scattering by sp^2-bonded carbon shows a high-frequency shift with increasing photon energy for energies 3. 0 eV. Possible models for the structure of this sp^2-bonded carbon phase are discussed on the basis of the present Raman data.
Wagner et al. (Sat,) studied this question.