Pump-probe polarization anisotropy study of femtosecond energy transfer within the photosynthetic reaction center of Rhodobacter sphaeroides R26
- Univ. of Chicago, IL (United States)
The energy transfer from the accessory bacteriochlorophylls (B) to the special pair (P) in the photosynthetic reaction center has been time resolved with pump-probe polarization anisotropy measurements using 20-25 fs duration pulses near 800 nm. The pump excitation corresponds to 1.4 x 10{sup 6} photons/{mu}m{sup 2}: the `saturation intensity` for the charge separation quantum yield is 3 x 10{sup 7} photons/{mu}m{sup 2}. The initial pump-probe anisotropy is 0.4 and decays with a nearly 80 fs time constant, which we attribute to dipole reorientation by electronic energy transfer. Simultaneous kinetic modeling of the parallel, perpendicular, and magic angle pump-probe transients using the reaction center structure and dipole orientations is consistent with energy transfer proceeding in two steps: nearly 80 fs electronic energy transfer from the accessory bacteriochlorophylls to the upper exciton component of the special pair (B {yields} P{sub +}) followed by a nearly 150 fs internal conversion from the upper exciton component to the lower exciton component of the special pair (P{sub +} {yields} P{sub -}). Finally, charge separation after electron transfer from P{sub -} to H causes an electrochromic (Stark) shift of B and produces the 2.8 ps bleach rise. The two-step energy transfer model is supported by the observation of weak quantum beat oscillations (125 cm{sup -1} and 227 cm{sup -1}) with near-zero anisotropy in the pump-probe signals. 86 refs., 13 figs., 4 tabs.
- OSTI ID:
- 380798
- Journal Information:
- Journal of Physical Chemistry, Vol. 100, Issue 30; Other Information: PBD: 25 Jul 1996
- Country of Publication:
- United States
- Language:
- English
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