Charge separation in photosynthetic reaction centers under femtosecond excitation

The process of electron transfer from the primary electron donor P* to the primary electron acceptor B, in the reaction center of Rhodobacter sphaeroides R-26 under 30 fsec pulse excitation was studied in this work with the aim of establishing a relationship between the nuclear subsystem motion and charge transfer. For this purpose the fsec and psec oscillations in the bands of stimulated emission of P* and in the band of reaction product B-A(-) at 1020 nm were investigated. It was established that the reversible formation of the P+BA- state is characterized by two vibration modes (130 and 320 cm(-1)) and connected with an arrival of the wavepacket induced by fsec excitation to the intersection of potential surfaces P*B-A and P+BA-. The irreversible formation of the P+BA- state with the time constant of 3 psec is followed by oscillations with frequencies of 9 and 33 cm(-1). These results show that the irreversibility of electron transfer is determined by two factors: 1) by a difference between the energy width of the wavepacket and the gap between the named surfaces; 2) by a difference between the duration of wavepacket residence near the intersection of the surfaces and the relaxation time of the P+BA- state.