TIME RESOLUTION OF ELECTRONIC-TRANSITIONS OF PHOTOSYNTHETIC REACTION CENTERS IN THE INFRARED

Electronic transitions of the special pair excited state, P*, and of its positive ion, P+, have been identified by transient infrared spectroscopy in the 1700-2000-cm(-1) region. The P* transition is present immediately (ca. 300 fs) after light absorption and decays with time constant 3.4 ps. The pf transition appears with time constant 3.4 ps. The transition dipoles of these transitions are both measured to have a squared projection of 0.63 onto the direction of the ground state to Q(y)(-) (870 nm) transition, This is interpreted to imply that both P* and P+ transitions have dipoles along the line joining the centroids of charge of the two bacteriochlorophylls (BChl) composing the dimer. The P* transition is assigned as an interexciton transition brought about by mixing of exciton and charge-separated states. The P+ transition is assigned as a transition between the symmetric and antisymmetric combination of the localized hole states of the dimer. The results are compared with theoretical calculations, static FTIR, and Stark effect measurements on the reaction center. While the results are in qualitative agreement with recent theoretical calculations, better agreement requires a larger admixture of charge resonance states in the Q(y)(-) state than is found in most calculations.