  The analysis of the Raman scattering cross section of the radial breathing
modes of double-wall carbon nanotubes allowed to determine the optical
transitions of the inner tubes. The Raman lines are found to cluster into
species with similar resonance behavior. The lowest components of the clusters
correspond well to SDS wrapped HiPco tubes. Each cluster represents one
particular inner tube inside different outer tubes and each member of the
clusters represents one well defined pair of inner and outer tubes. The number
of components in one cluster increases with decreasing of the inner tube
diameter and can be as high as 14.
