Ultrafast exciton and excited-exciton dynamics in J-aggregates of three-level porphyrin molecules

We observed the ultrafast response of exciton (S-1-exciton) and excited-exciton (S-2-exciton) in one-dimensional J-aggregates of three-level porphyrin molecules by femtosecond pump-probe spectroscopy. The decay profiles of the nonlinear response can be fitted to a sum of instantaneous response and two exponential decay components with time constants of 1.3+/-0.1 and 40+/-1 ps. The former and latter were found to correspond to the lifetimes of S-2- and S-1-excitons, respectively. The origins of the nonlinearity were attributed to the following three contributions: (1) coherent effects between the pump and probe via one-photon virtual S-1-exciton, (2) induced absorption of real S-2-excitons generated by two photons, and (3) induced absorption of real S-1-exciton. (C) 1999 American Institute of Physics. [S0021-9606(99)50412-1].