  Electronic energy transfer in photosynthesis occurs over a range of time
scales and under a variety of intermolecular coupling conditions. Recent work
has shown that electronic coupling between chromophores can lead to coherent
oscillations in two-dimensional electronic spectroscopy measurements of
pigment-protein complexes measured with femtosecond laser pulses. A persistent
issue in the field is to reconcile the results of measurements performed using
femtosecond laser pulses with physiological illumination conditions.
Noisy-light spectroscopy can begin to address this question. In this work we
present the theoretical analysis of incoherent two-dimensional electronic
spectroscopy, I(4) 2D ES. Simulations reveal diagonal peaks, cross peaks, and
coherent oscillations similar to those observed in femtosecond two-dimensional
electronic spectroscopy experiments. The results also expose fundamental
differences between the femtosecond-pulse and noisy-light techniques; the
differences lead to new challenges and new opportunities.
