Pump–probe spectroscopy of dissipative energy transfer dynamics in photosynthetic antenna complexes: A density matrix approach

Abstract

The photoinduced ultrafast dynamics of singlet excitons in light-harvesting antennae is investigated using multilevel Redfield theory. Formulating the equations of motion for the reduced exciton density operator in terms of one- and two-exciton eigenstates we focus attention on the influence of dynamic exciton-vibrational coupling and static diagonal disorder on transient absorption spectra of peripheral antennae in photosynthetic purple bacteria. The simulations are discussed in view of 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 emphasis on the dynamic character of exciton localization. For the B850 pigment pool we find that at room temperature the pump–pulse initially prepares a coherent superposition of one-exciton eigenstates which can be delocalized over the whole aggregate. With increasing delay time the exciton coherence domain shrinks to cover about four pigments in the asymptotic limit.