We evaluated the third order non linear response in the pump probe scheme. Two beams are used to accomplish the 3 interacitons, excluding two photon processes and using RWA the number of fiagrams is reduced to 6, corresponding to 3 different processes: excited state absorption, stimulated emission, ground state bleaching. Bleaching and stimulated response give the same response, unless the system is evolving onto the excited state, for example due to vibrational relaxation.
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