Figure 2 Schematic representation of a 2D spectrum (at a fixed value of the waiting time T) showing cross peaks. In general, both ground-state bleaching and stimulated emission (positive) and excited-state absorption (negative) features appear. Negative features can partially or even wholly cancel positive features. Partial cancellation leads to distortions in the line shapes, as seen in the highest frequency diagonal peak. Note that the 2D spectrum is not symmetric around the diagonal. Cross (off-diagonal) peaks appear (for T = 0) only when coupling between chromophores is present. Cross peaks can also be generated by energy transfer, coherence transfer, chemical exchange, physical transformation, and so on for larger values of the waiting time T. Note that the orientation of the cross peaks is controlled by whether the fluctuations of two different transition frequencies are positively or negatively correlated with each other. Modulation of coupling strength by bath or intramolecular degrees of freedom produces an antidiagonally elongated cross peak, whereas any modulation of site energies (monomeric transition frequency) makes the peak diagonally elongated at short time T.
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