Excited-state vibrational coherence and anisotropy decay in the bacteriochlorophyll a dimer protein B820

We have employed dynamic absorption spectroscopy to monitor coherent wave packet dynamics and anisotropy decays following impulsive excitation of the B820 subunit of the LH1 light-harvesting complex, which was isolated from Rhodospirillum rubrum G9. When the lower exciton-state transition of the bacteriochlorophyll a dimer is pumped, the time-resolved pump-probe spectrum exhibits contributions from a fully Stokes shifted stimulated-emission spectrum and a nonstationary vibrational character within 40 fs of excitation. Coherent wave packet motion in bath the ground state and the excited state is observed via modulations of single-wavelength transients. The photobleaching portion of the spectrum exhibits strong components only at low frequencies, 20-60 and 180 cm(-1), and a weaker component is observed at 400 cm(-1). The stimulated-emission portion of the spectrum exhibits weak modulation components at 20-60 and 180 cm(-1). but strong components are observed at fairly high frequencies: 360, 400, 470, 600, and 730 cm(-1). An anisotropy decay observed in the stimulated-emission region reports a prompt > 20 degrees tilt of the photoselected transition-dipole moment. A possible explanation for these results is that an intradimer charge-transfer event occurs on a very short time scale following optical preparation of the lower pi --> pi* exciton state of the bacteriochlorophyll a dimer at room temperature.