Resonance effects in Raman scattering from polycrystalline diamond films.

We report on a Raman spectroscopic study of polycrystalline diamond films using a wide range of incident photon energies (1.16-4.82 eV). The scattering intensity of amorphous sp2-bonded carbon is found to vary considerably as a function of the incident photon energy as compared to the strength of the 1332 cm-1 Raman line from sp2-bonded diamond. There is also a frequency downshift of the apparent peak position for scattering from the sp2-bonded carbon with decreasing photon energy. Excitation in the infrared range is shown to give a high sensitivity to scattering from amorphous sp2-bonded carbon whereas ultraviolet excitation enhances the 1332 cm-1 signal from crystalline diamond considerably even in nanocrystalline films.