Femtosecond Infrared Spectroscopy of Low-Lying Excited States in Reaction Centers of Rhodobacter sphaeroides

The mid- to near-IR difference spectra of optically pumped and unpumped reaction centers were recorded over a time range circa 0.1−20 ps allowing us to isolate kinetically the transient electronic spectra of the excited special pair P* and its charge-separated form P + H - . The spectrum of P* between 1 and 5.2 μm exhibits two main bands, one at 5300 cm -1 and another at 2710 cm -1, while P + H - exhibits two bands, one at 2600 cm -1 and another at 8000 cm -1 . These new states do not appear as transitions of unexcited reaction centers at E (P*) + 5300 cm -1 or E (P*) + 2710 cm -1 (although the former is close to the Q X transition region of the special pair), suggesting that they derive almost exclusively from charge transfer (P L + P M - or P L - P M + ) states. This conclusion is strengthened by the anisotropies obtained by pumping P* and probing these transitions with polarized light. The transition of P + H - at 2600 cm -1 corresponds to the known hole transfer transition, and this assignment is confirmed by kinetic and anisotropy measurements: the transition dipole is along the direction of charge transfer between P L and P M . The anisotropy for the transition at 8000 cm -1 is consistent with a transition dipole along the y -axis of P M, in agreement with theoretical predictions for a trip−doublet transition.