Polarization sequences reveal the incoherent signals generated by multi-exciton photophysics in ultrafast 2D action spectroscopies

Action detected two-dimensional spectroscopy interrogates molecular systems and materials via the response of the system to the stimuli of the pulse sequence rather than measuring its optical response. Recent experiments revealed that, in addition to the coherent signal, action 2D spectra contain incoherent population mixing signals, which are created when two independent linear responses are mixed during the detection time. Disentangling coherent and incoherent contributions is essential for maximizing the informational content and interpretability of action 2D spectroscopy. We use simulations to propose a set of polarizations that selectively suppress the coherent signal and expose spectral signatures of multi-exciton processes occurring during the detection time. We showed that rephasing and non-rephasing spectra contain different incoherent responses. In addition, we showed that for molecular aggregates with efficient exciton-exciton annihilation, the ⟨0°, 60°, 120°, 30°⟩ polarization isolates electronic coherences, while ⟨0°, 30°, 90°, 150°⟩ or ⟨0°, 45°, 135°, 90°⟩ polarizations can be used to study energy transfer dynamics, both of which are otherwise obscured by incoherent signals. Our simulations provide a practical strategy for experimentally isolating incoherent contributions in action-detected 2D spectroscopies.