FEMTOSECOND INFRARED-SPECTROSCOPY OF REACTION CENTERS FROM RHODOBACTER-SPHAEROIDES BETWEEN 1000 AND 1800 CM(-1)

Time-resolved pump-and-probe experiments of reaction centers of the pu rple bacterium Rhodobacter sphaeroides (R26) in the mid-IR region betw een 1000 and 1800 cm(-1) are recorded with a time resolution of 300-40 0 fs. The difference spectra of the states P, P+HA- and P(+)Q(A)(-) w ith respect to the ground state P predominantly reflect changes of the special pair. They show positive and negative bands due to changes of distinct vibrational modes superimposed on a broad background of enha nced absorption. A number of certain bands can be assigned to the spec ial pair P, to the bacteriopheophytin H-A, and to the quinone Q(A). Th e temporal evolution of the IR absorbance changes is well described by the time constants known from femtosecond spectroscopy of the electro nic states. Differences occur only at very early times, which are indi cative of fast vibrational relaxation with a time constant of a few hu ndred femtoseconds.