Role of Electronic-Vibrational Mixing in Enhancing Vibrational Coherences in the Ground Electronic States of Photosynthetic Bacterial Reaction Center

We describe polarization controlled two-color coherence photon echo studies of the reaction center complex from a purple bacterium Rhodobacter sphaeroides. Long-lived oscillatory signals that persist up to 2 ps are observed in neutral, oxidized, and mutant (lacking the special pair) reaction centers, for both (0 degrees,0 degrees,0 degrees,0 degrees) and (45 degrees,-45 degrees,90 degrees,0 degrees) polarization sequences. We show that the long-lived signals arise via vibronic coupling of the bacteriopheophytin (H) and accessory bacteriochlorophyll (B) pigments that leads to vibrational wavepackets in the B ground electronic state. Fourier analysis of the data suggests that the 685 cm(-1) mode of B may play a key role in the H to B energy transfer.