  We present direct experimental observation of exciton-phonon bound states in
the photoluminescence excitation spectra of isolated single walled carbon
nanotubes in aqueous suspension. The photoluminescence excitation spectra from
several distinct single-walled carbon nanotubes show the presence of at least
one sideband related to the tangential modes, lying {200 meV} above the main
absorption/emission peak. Both the energy position and line shapes of the
sidebands are in excellent agreement with recent calculations [PRL {\bf
94},027402 (2005)] that predict the existence of exciton-phonon bound states, a
sizable spectral weight transfer to these exciton-phonon complexes and that the
amount of this transfer depends on the specific nanotube structure and
diameter. The observation of these novel exciton-phonon complexes is a strong
indication that the optical properties of carbon nanotubes have an excitonic
nature and also of the central role played by phonons in describing the
excitation and recombination mechanisms in carbon nanotubes.
