Potential pitfalls of the early-time dynamics in two-dimensional electronic spectroscopy

Two-dimensional electronic spectroscopy, and especially the polarization-controlled version of it, is the cutting edge technique for disentangling various types of coherences in molecules and molecular aggregates. In order to evaluate the electronic coherences, which often decay on a 100 fs time scale, the early population times have to be included in the analysis. However, signals in this region are typically plagued by several artifacts, especially in the unavoidable pulse overlap region. In this paper, we show that, in the case of polarization-controlled two-dimensional spectroscopy experiment, the early-time dynamics can be dominated by the "incorrect" pulse ordering signals. These signals can affect kinetics at positive times well beyond the pulse overlap region, especially when the "correct" pulse ordering signals are much weaker. Moreover, the "incorrect" pulse ordering contributions are oscillatory and overlap with the spectral signatures of energy transfer, which may lead to misinterpretation of "incorrect" pulse ordering signals for fast-decaying coherences.