The relaxation dynamics of the excited electronic states of retinal in bacteriorhodopsin by two-pump-probe femtosecond studies

We present the results of two-pump and probe femtosecond experiments design ed to follow the relaxation dynamics of the lowest excited state (S-1) popu lated by different modes. In the first mode, a direct (S-0 --> S-1) radiati ve excitation of the ground state is used. In the second mode, an indirect excitation is used where the S-1 state is populated by the use of two femto second laser pulses with different colors and delay times between them. The first pulse excites the S-0 --> S-1 transition whereas the second pulse ex cites the S-1 --> S-n transition. The nonradiative relaxation from the S-n state populates the lowest excited state. Our results suggest that the S1 s tate relaxes faster when populated nonradiatively from the S-n state than w hen pumped directly by the S-0 --> S-1 excitation. Additionally, the S-n -- > S-1 nonradiative relaxation time is found to change by varying the delay time between the two pump pulses. The observed dependence of the lowest exc ited state population as well as its dependence on the delay between the tw o pump pulses are found to fit a kinetic model in which the S-n state popul ates a different surface (called Si) than the one being directly excited (S -1). The possible involvement of the A(g) type states, the J intermediate, and the conical intersection leading to the S-0 or to the isomerization pro duct (K intermediate) are discussed in the framework of the proposed model.