Simulations of pump probe spectra of a molecular complex at high excitation intensity

The most wide spread theoretical treatment of molecular non-linear spectroscopy and quantum dynamical processes employs a perturbative expansion in powers of the excitation field. However, its direct application requires knowing multi-excitonic state energies and their relaxation pathways. This is not necessary if real space propagation is used. In this paper, the standard Nonlinear Exciton Equations (NEE) for molecular excitations are extended to include fifth order processes. The coherent part of it corresponds to saturation of molecular transition dipoles, while the incoherent part is constructed to reflect the exciton-exciton annihilation (EEA) processes. We show that numerical NEE-based simulations of pump probe spectra of a molecular dimer quantitatively describe both coherent and incoherent processes as a function of excitation intensity.