EXCITED-STATE VIBRATIONAL COHERENCE AND ANISOTROPY DECAY IN THE BACTERIOCHLOROPHYLL A DIMER PROTEIN B820

We have employed dynamic absorption spectroscopy to monitor coherent w ave packet dynamics and anisotropy decays following impulsive excitati on of the B820 subunit of the LH1 light-harvesting complex, which was isolated from Rhodospirillum rubrum G9. When the lower exciton-state t ransition of the bacteriochlorophyll a dimer is pumped, the time-resol ved pump-probe spectrum exhibits contributions from a fully Stokes shi fted stimulated-emission spectrum and a nonstationary vibrational char acter 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 spectr um exhibits strong components only at low frequencies, 20-60 and 180 c m(-1), and a weaker component is observed at 400 cm(-1). The stimulate d-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 d ecay 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 e vent occurs on a very short time scale following optical preparation o f the lower pi --> pi exciton state of the bacteriochlorophyll a dime r at room temperature.