Third-order pump-probe spectroscopy applied to molecular dimers: characterization of relaxation dynamics and exciton-exciton annihilation.

We investigate theoretically the excitonic dynamics in molecular dimers which is monitored by two time-delayed femtosecond laser pulses. A two-photon absorption leads to a wave packet dynamics in the manifold of second excited states. This opens up the channel for exciton-exciton annihilation (EEA) which involves non-radiative electronic transitions. It is shown that the time interval during which EEA takes place can be monitored by the detection of third-order signals which can be interpreted as originating from a pump-probe scheme. In the case of transient absorption, the spectra directly map intraband relaxation processes.