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

Electronic transitions of the special pair excited state, P, and of i ts positive ion, P+, have been identified by transient infrared spectr oscopy in the 1700-2000-cm(-1) region. The P transition is present im mediately (ca. 300 fs) after light absorption and decays with time con stant 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 s quared 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 a nd P+ transitions have dipoles along the line joining the centroids of charge of the two bacteriochlorophylls (BChl) composing the dimer. Th e P transition is assigned as an interexciton transition brought abou t by mixing of exciton and charge-separated states. The P+ transition is assigned as a transition between the symmetric and antisymmetric co mbination of the localized hole states of the dimer. The results are c ompared with theoretical calculations, static FTIR, and Stark effect m easurements on the reaction center. While the results are in qualitati ve agreement with recent theoretical calculations, better agreement re quires a larger admixture of charge resonance states in the Q(y)(-) st ate than is found in most calculations.