Subpicosecond Spectroscopy of Charge Separation in Rhodobacter capsulatus Reaction Centers

The time‐evolution of the near‐infrared absorption changes accompanying the conversion of the excited primary electron donor, P*, to the radical pair state, P⁺BPh_L⁻, has been examined in detail in Rhodobacter capsulatus reaction centers. In a series of spectra spanning the time interval from about 600 fs to 15 ps, two isosbestic points occurring at 765 run and 798 nm are maintained throughout. The finding of an isosbestic point at 798 nm in this series of spectra, which encompass the time during which P* decays and BPh_L⁻ forms, places severe constraints on the possible reduction of the 800‐nm‐absorbing monomeric BChl_L in the initial stage of the charge‐separation process. The near‐infrared P* spectrum, which is revealed clearly only when P is excited directly in its long‐wavelength absorption band, contains bleaching only of the 855‐nm band and a broad featureless transient absorption below 810 nm. This spectrum shows that P does not contribute any significant oscillator strength to the 800‐nm ground state absorption band.