Excitation energy dependence of the Raman spectrum of single-walled carbon nanotubes

We investigate the excitation energy dependence of Raman modes in the 600–1200 cm−1 range. The main features are first, two lines around 860 and 1060 cm−1 independent of the energy and second, energy-dependent couples of lines, each of them composed of one first-order mode and one substractive combination band. In each couple, the frequency of the lines is found to follow a linear, strong and opposite energy dependence.