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We present theoretical and experimental results showing the effects ofincoherent population mixing on two-dimensional (2D) coherent excitationspectra that are measured via a time-integrated population and phase-sensitivedetection. The technique uses four collinear ultrashort pulses and phasemodulation to acquire two-dimensional spectra by isolating specific nonlinearcontributions to the photoluminescence or photocurrent excitation signal. Wedemonstrate that an incoherent contribution to the measured lineshape, arisingfrom nonlinear population dynamics over the entire photoexcitation lifetime,generates a similar lineshape to the expected 2D coherent spectra incondensed-phase systems. In those systems, photoexcitations are mobile suchthat inter-particle interactions are important on any timescale, includingthose long compared to the 2D coherent experiment. Measurements on asemicrystalline polymeric semiconductor film at low temperature show that, insome conditions in which multi-exciton interactions are suppressed, thetechnique predominantly detects coherent signal and can be used, in ourexample, to extract homogeneous linewidths. The same method used on alead-halide perovskite photovoltaic cell shows that incoherent populationmixing of mobile photocarriers can dominate the measured signal sincecarrier-carrier bimolecular scattering is active even at low excitationdensities, which hides the coherent contribution to the spectral lineshape.

Incoherent population mixing contributions to phase-modulation two-dimensional coherent excitation spectra