Nuclear Wavepacket Motion between P and P+BA - Potential Surfaces with Subsequent Electron Transfer to HA in Bacterial Reaction Centers. 1. Room Temperature

Formation and coherent propagation of nuclear wavepackets on potential energy surfaces of the excited state of the primary electron donor P* and of the charge transfer states P+BA - and P+HA - were studied in native and pheophytin-modified Rhodobacter sphaeroides R-26 reaction centers (RCs) induced by 25 fs excitation (where BA and HA are the primary and secondary electron acceptors, respectively). The processes were monitored by measuring coherent oscillations in kinetics of the time evolution of the stimulated emission band of P* at 935 nm, of the absorption band of BA - at 1020 nm, and of the bleaching band of HA at 760 nm. It was found that the nuclear wavepacket motion on the 130−140 cm-1 surface of P* is directly induced by light absorption in P. When the wavepacket approaches the intersection between P* and P+BA - surfaces at 120 and 380 fs delays, the formation of intermediate mixed-state emitting light at 935 nm (P*) and absorbing light at 1020 nm (P+BA -) takes place. At the latter time, the wavepacket is transferred to the 32 cm-1 mode which can belong to the P* hypersurface effectively transferring the wavepacket to the P+BA - surface or can represent a diabatic surface which is formed by the states P* and P+BA -. The wavepacket motion on the P+BA - surface or on the P+BA - part of the mixing surface is accompanied by irreversible electron transfer to HA. This process is monitored by the kinetics of 1020 nm band development and 760 nm band bleaching (delayed with respect to 1020 nm band development) which both have the enhanced 32 cm-1 mode in Fourier transform (FT) spectra. The mechanism of wavepacket transfer from the 130−140 cm-1 to the 32 cm-1 mode is discussed.