Three pulse photon echo peak shift study of the B800 band of the LH2 complex of Rps. acidophila at room temperature:: A coupled master equation and nonlinear optical response function approach

Three pulse photon echo peak shift (3PEPS) data were obtained for the B800 band of Rps, acidophila (strain 10050) at room temperature. The peak shift decays on two time scales: a sub 100 fs decay owing to the ultrafast solvation by the protein bath and a 500-600 fs decay to a value of 2.5 fs at 1.2 ps. Here we present a novel method that properly incorporates the effect of energy transfer on the nonlinear response functions to simulate the peak shift for weakly coupled, multi-chromophoric energy transfer systems. The method involves calculation of third-order response functions for energy transfer systems to account for the interaction of the laser pulses with the chromophores, whereas the energy transfer kinetics is incorporated by solving the master equation for the whole ring of B800 pigments. The effect of disorder on spectral overlaps is taken into account by a Monte Carlo sampling procedure that selects transition frequencies from a Gaussian distribution of site energies. The peak shift is then calculated by combining ensemble-averaged population kinetics with the appropriate response functions. We estimate the energy transfer time within the B800 band to be similar to 500-600 fs, acid the experiments are well described by Forster theory level calculations including energy disorder. By simultaneously modeling the peak shift data and the linear absorption spectrum, we suggest the presence of two levels of disorder (i.e., partially correlated disorder) in the B800 band. Strong beats are also found in our data with a frequency of 165 cm(-1).