Abstract
Abstract— Primary processes of photosynthesis in the picosecond timescale were investigated with highly enriched reaction center particles of photosystem I (PS I) prepared from spinach chloroplasts. Spectral changes were observable even under conditions of single photon excitation for each reaction center. Immediately after excitation, bleaching was observed at 676 nm, the absorption peak of the antenna chlorophylls. Several effects were observed which depended upon the number of photons absorbed by a reaction center. Upon multiple photon excitation the intensity of bleaching decayed quickly. Single photon excitation resulted in no obvious decay of the main bleaching band, but produced a remarkable variation of the band feature. The most characteristic change was the appearance of a band around 700 nm which had not decayed even after 8 ns. In an oxidized sample, P700+, the main bleaching decayed without any spectral change. These observations indicate that the excitation transferred from antenna chlorophylls to P700. Subsequently P700 was oxidized resulting in the long‐lived P700+. The overall process apparently proceeds with t1/e∼ 25 ps upon single photon excitation.
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