PUMP-PROBE SPECTROSCOPY OF DISSIPATIVE ENERGY-TRANSFER DYNAMICS IN PHOTOSYNTHETIC ANTENNA COMPLEXES - A DENSITY-MATRIX APPROACH

The photoinduced ultrafast dynamics of singlet excitons in light-harve sting antennae is investigated using multilevel Redfield theory. Formu lating the equations of motion for the reduced exciton density operato r in terms of one-and two-exciton eigenstates we focus attention on th e influence of dynamic exciton-vibrational coupling and static diagona l disorder on transient absorption spectra of peripheral antennae in p hotosynthetic purple bacteria. The simulations are discussed in view o f recent experimental results obtained for the B850 absorption band of Rhodobacter sphaeroides. Further, we suggest a new way of estimating the size of the exciton coherence domain in these systems which puts e mphasis on the dynamic character of exciton localization. For the B850 pigment pool we find that at room temperature the pump-pulse initiall y prepares a coherent superposition of one-exciton eigenstates which c an be delocalized over the whole aggregate. With increasing delay time the exciton coherence domain shrinks to cover about four pigments in the asymptotic limit. (C) 1997 American Institute of Physics.