Extracting the excitonic Hamiltonian of a chlorophyll dimer from broadband two-dimensional electronic spectroscopy

We apply Frenkel exciton theory to model the entire Q-band of a tightly bound chlorophyll dimer inspired by the photosynthetic reaction center of photosystem II. The potential of broadband two-dimensional electronic spectroscopy experiment spanning the Q_x and Q_y regions to extract the parameters of the model dimer Hamiltonian is examined through theoretical simulations of the experiment. We find that the local nature of Q_x excitation enables identification of molecular properties of the delocalized Q_y excitons. Specifically, we demonstrate that the cross-peak region, where excitation energy is resonant with Q_y while detection is at Q_x, contains specific spectral signatures that can reveal the full real-space molecular Hamiltonian, a task that is impossible by considering the Q_y transitions alone. System-bath coupling and site energy disorder in realistic systems may limit the resolution of these spectral signatures due to spectral congestion.