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 mu m 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 (PL+PM- or PL-PM+) 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.