The spontaneous fluorescence from the special pair primary electron donor in bacterial photosynthetic reaction centers has been measured at low temperature using fluorescence up-conversion following excitation of the special pair with 80 fs pulses from a mode-locked Ti:sapphire laser. Oscillations are observed during the first few picoseconds of the decay. The frequency of the oscillations and the detection wavelength dependence of their intensity are similar to results obtained by Vos, Martin,

more »

... and co-workers using stimulated emission. This demonstrates that the oscillations are associated with the excited state of the special pair, not with stimulated Raman scattering. Vibrational coherence persists on the time scale of electron transfer. Possible relationships between these results and other observables are discussed.