Raman scattering of light and IR absorption in carbon nanotubes

Raman light scattering and IR absorption spectra of samples containing multilayer carbon nanotubes in different stages of purification by the selective oxidation technique have been investigated. It was found that the Raman spectra of carbon nanotubes exhibit softening of the mode at 1582 cm(-1) corresponding to E-2g vibrations of graphite hexagons and a line at 120 cm(-1) due to the radial vibrations of nanotubes. In IR absorption spectra measured in the region of 0.07-0.3 eV, several sets of lines with a spacing of 15 meV (120 cm(-1)) between lines of each group have been detected. We suggest that each group corresponds to electron transitions generating electron-hole pairs in semiconducting nanotubes and contains a phononless 00-line and its phonon replicas with spacing between them equal to the "breathing" mode energy of 120 cm(-1). Measurements of electric conductivity at a frequency of 9300 MHz indicate that, in addition to semiconducting nanotubes, the samples contain nanotubes with properties of a highly disordered semimetal. (C) 1998 American Institute of Physics. [S1063-7761(98)02305-1].