Formation of Bacteriochlorophyll Anion Band at 1020 nm Produced by Nuclear Wavepacket Motion in Bacterial Reaction Centers

Formation of a nuclear wavepacket on the potential energy surface of the primary electron donor P* induced by 30‐fs excitation of pheophytin‐modified Rhodobacter sphaeroides R‐26 reaction centers leads to a reversible and irreversible appearance of the state P + B A − monitored by the measurements of the 1020‐nm band which is characteristic of the radical anion band of bacteriochlorophyll monomer B A − . The reversible appearance of the state P + B A − is characterized by two (130 and 320 cm −1 ) vibration modes and related to the creation of maximal value of the Franck‐Condon factor when the wavepacket is near an intersection of the P* and P + B A − potential energy surfaces. The irreversible formation of the state P + B A − with a time constant of 3 ps is accompanied by oscillations with frequencies of 9 and 33 cm −1 . These results show that the nuclear vibration modes (9 and 3 3 cm −1 ) on the P* potential energy surface have near their bottom the intersection with the P + B A − surface as well. The electron transfer time between P* and B A was estimated to be in the range of 700 fs. This is larger than the wavepacket intersection time for the 130 cm −1 mode (∼100 fs) and comparable with that for the 33 cm −1 mode (∼600 fs). Therefore the reversible electron transfer with small amplitude can be observed for the 130 cm −1 mode, while the irreversible one is allowed for the 33 cm −1 mode that is probably important for the primary charge separation.