Figure 4.

Schematic representation of 2D spectrum for a multilevel system. (a) At early population times, the GSB and SE features of two levels with frequency ω_a and ω_b appear as signals elongated on the diagonal. (b) As the population time evolves, the spectral diffusion produces a broadening of the features, the Stokes shift produces a redshift of the signals along the emission axis, and a cross-peak may appear in the lower part of the map as a consequence of the relaxation from the high energy level to the low energy one. Negative ESA signals can also appear.