Electronic transitions in single-walled carbon nanotubes: A resonance Raman study

Resonance excitation profiles of the high-frequency peaks in the Raman spec tra of single-walled carbon nanotubes normalized to the scattering intensit y of CaF2 are presented. We find separate resonances of metallic and semico nducting tubes throughout the visible and the near-IR excitation range. The resonance shift of samples with different mean diameters confirm the inver se proportionality of the resonant transition energy to the tube diameter. Smaller diameter tubes are found to have sharper resonances than larger tub es due to a l/d(2) dependence of the Raman cross section.