  We present results of wavelength-dependent ultrafast pump-probe experiments
on micelle-suspended single-walled carbon nanotubes. The linear absorption and
photoluminescence spectra of the samples show a number of chirality-dependent
peaks, and consequently, the pump-probe results sensitively depend on the
wavelength. In the wavelength range corresponding to the second van Hove
singularities (VHSs), we observe sub-picosecond decays, as has been seen in
previous pump-probe studies. We ascribe these ultrafast decays to intraband
carrier relaxation. On the other hand, in the wavelength range corresponding to
the first VHSs, we observe two distinct regimes in ultrafast carrier
relaxation: fast (0.3-1.2 ps) and slow (5-20 ps). The slow component, which has
not been observed previously, is resonantly enhanced whenever the pump photon
energy resonates with an interband absorption peak, and we attribute it to
radiative carrier recombination. Finally, the slow component is dependent on
the pH of the solution, which suggests an important role played by H$^+$ ions
surrounding the nanotubes.
