Fig. 1
Double-sided Feynman diagrams for EEI2D spectroscopy. Top right: Energy scheme of a non-interacting pair of two-level systems (black) with an idealized annihilation channel in the case of interaction (red). Symbols E1 and E2 label the excitation energies of individual subsystems and symbols μ1 and μ2 their respective transition dipole moments. Left two columns: Feynman diagrams of non-interacting molecules attributed to ground-state bleach (GSB) or stimulated emission (SE) signals of the individual subsystems (denoted by the numerical index). The diagrams perfectly compensate each other. In the case of interaction, the annihilation process replaces the two lowest diagrams (red frame) with a new pair (bottom right) leading to a non-zero EEI signal. The indicated diagram signs are given by the parity of number of interactions acting on the right side (odd: negative, even: positive) as we neglect the additional overall −1 factor coming from the fifth order of the perturbation theory. Each diagram is present four times (as indicated) since each of the double interactions allows for two different time orderings
