We present a practical method to compute the vibrational resonant Raman spectra in solids with delocalized excitations. We apply this approach to the study of tetrahedrous amorphous carbon. We determine the vibrational eigenmodes and eigenvalues using density functional theory in the local density approximation and the Raman intensities using a tight binding approximation. The computed spectra are in good agreement with the experimental ones measured with visible and uv lasers. We analyze the Raman spectra in terms of vibrational modes of microscopic units. We show that, at any frequency, the spectra are dominated by the stretching vibrations. We identify a very rapid inversion in the relative Raman intensities of the sp2 and sp3 carbon sites with the frequency of the incident laser beam. In particular, the spectra are dominated by sp2 atoms below 4 eV and by sp3 atoms above 6 eV.
Physical Review B
- Pub Date:
- June 2001
- Density functional theory local density approximation gradient and other corrections;
- Vibrational states in disordered systems;
- Condensed Matter - Materials Science
- 8 figures, 10 pages