The rapid electronic state dynamics that occur prior to charge separation in the photosynthetic reaction center of Rhodobacter sphaeroides R-26 have been investigated by """"""""two-color"""""""" wavelength-resolved pump-probe and anisotropy measurements. A narrow band (40 fs duration transform limited) pump pulse is used to selectively excite reaction center pigments the accessory bacteriochlorophyll (B), the upper excitonic state of the special pair (Py+), or the lower excitonic state of the special pair (Py-). Population dynamics are then measured with a 12 fs duration probe pulse across the entire Qy absorption spectral region as a function of time. wavelength, and polarization. Excitation of either Py- or B results in the formation of a distinct optical band at 825 nm exhibiting polarization characteristics consistent with those expected for Py+, the band appears instantaneously upon excitation of Py- with a negative anisotropy and appears somewhat delayed after excitation o f B. The dynamics observed following direct excitation of the Py+ absorption band suggests that internal conversion between the excitonic states of P is rapid, occurring with a 65 fs time constant. Excitation of the accessory BChl (i.e., populating the excited state, B*) provides a detailed answer for the mechanism of energy transfer within the bacterial reaction center. The process proceeds via a two-step mechanism, flowing sequentially from B* to Py+ to Py- with time constants of 120 and 65 fs, respectively. These results, in conjunction with a range of the transient transmission spectra, suggest that the initially excited state of the zeroth order chromophores, i.e., B* and Py-, is delocalized at the earliest times, consistent with a supermolecular picture of the reaction center.
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