Unifying Nonlinear Response and Incoherent Mixing in Action-2D Electronic Spectroscopy

Action-detection has expanded the scope and applicabilityof 2Delectronic spectroscopy, while posing new challenges for the unambiguousinterpretation of spectral features. In this context, identifyingthe origin of cross-peaks at early waiting times is not trivial, andincoherent mixing is often invoked as an unwanted contribution maskingthe nonlinear signal. In this work, we elaborate on the relation betweenthe nonlinear response and the incoherent mixing contribution by analyzingthe action signal in terms of one- and two-particle observables. Consideringa weakly interacting molecular dimer, we show how cross-peaks at earlywaiting times, reflecting exciton-exciton annihilation dynamics,can be equivalently interpreted as arising from incoherent mixing.This equivalence, on the one hand, highlights the information contentof spectral features related to incoherent mixing and, on the otherhand, provides an efficient numerical scheme to simulate the actionresponse of weakly interacting systems.