  We examine the excitonic nature of high-lying optical transitions in
single-walled carbon nanotubes by means of Rayleigh scattering spectroscopy. A
careful analysis of the principal transitions of individual semiconducting and
metallic nanotubes reveals that in both cases the lineshape is consistent with
an excitonic model, but not one of free-carriers. For semiconducting species,
side-bands are observed at ~200 meV above the third and fourth optical
transitions. These features are ascribed to exciton-phonon bound states. Such
side-bands are not apparent for metallic nanotubes,as expected from the reduced
strength of excitonic interactions in these systems.
