Peak shape analysis of diagonal and off-diagonal features in the two-dimensional electronic spectra of the Fenna–Matthews–Olson complex

We have recorded a series of two-dimensional electronic spectra of the Fenna-Matthews-Olson (FMO) complex from Prosthecochloris aestuarii, with several crosspeaks sufficiently resolved to permit a quantitative analysis of both the amplitude and the two-dimensional peak shape. The exponential growth and/or decay of peaks on and off the main diagonal provides information on population transfer rates between pairs of excitons. Quantum beats observed in the amplitudes and shapes of these peaks persist throughout the relaxation process, indicating that energy transfer in FMO involves both incoherent and coherent dynamics. By comparing the oscillations in the amplitude and shape of crosspeaks, we confirm theoretical predictions regarding their correlation and identify previously indistinguishable combinations of nonlinear response pathways that contribute to the signal at particular positions in the spectra. Such analysis is crucial to understanding the enormous amount of information contained in two-dimensional electronic spectra and offers a new route to uncovering a complete description of the energy transfer kinetics in photosynthetic antennae.