In Eq. 5a and Eq. 5b, we arrange the terms on the right-hand side in the order of SE, GSB, and ESA, respectively. Note that the formalism we present below can be used to calculate all possible non-linear spectroscopy experiments and is not limited to just Eq. 5a and Eq. 5b. Usually, 2D spectra are represented as the imaginary part of a t 2 varying series of Fourier transformed t 1 and t 3 axes. Because of the rephasing and non-rephasing, the signals are generated in different quadrants of the frequency domain. As such, the 2D Fourier transform is calculated by