Non-perturbative calculation of 2D spectra in heterogeneous systems: Exciton relaxation in the FMO complex

Experimental 2D frequency resolved spectra showing the relaxation of excitons in the photosynthetic antenna system FMO are reproduced by a multi-exciton density matrix approach explicitly including the external fields. The interference of the polarizations of a number of randomly placed, rotated and energetically inhomogeneous complexes is used to extract the signal in the phase matching directions. It is shown that varying the laser intensity has only a minor impact, but changing the phase within the single pulses makes higher order contributions possible which lead to a fundamental change of the 2D spectra.