Transient Absorption Kinetics Associated with Higher Exciton States in Semiconducting Single-Walled Carbon Nanotubes: Relaxation of Excitons and Phonons

We investigate the transient absorption kinetics due to excitons associated with the first and higher transition states in semiconducting single-walled carbon nanotubes at high excitation densities 17 means of femtosecond pump-probe spectroscopy. The time evolutions of absorption changes associated with the first, second, and third excitonic transition bands exhibit similar decay behavior; the decay is much longer than the exciton population decay observed by time-resolved luminescence measurements. From spectral shape analysis of the transient absorption spectra, it is found that the absorption change in the short time regime (<similar to 2 ps) is mainly due to the bimolecular Auger recombination process, reflecting the exciton population decay. The long-lasting decay behavior over similar to 1 ns is attributed to the thermalization process of phonons generated during the inter- and intraband relaxation of excitons promoted to higher states by the Auger recombination process and the subsequent thermal diffusion to the surrounding surfactant and solvent.