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 by 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 (<∼2 ps) is mainly due to the bimolecular Auger recombination process, reflecting the exciton population decay. The long-lasting decay behavior over ∼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.