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Ultrafast exciton dynamics in one- and two-dimensional 
para
-sexiphenyl clusters

Laser-pulse-induced ultrafast electronic excitation energy transfer in molecular systems is studied theoretically. As a particular excitation channel, we consider the simultaneous transition of different molecules into their first excited state. Accordingly, a restricted picture of the total nonlinear response of the molecular system on laser pulse excitation is obtained, but complete insight is gained on the contribution of the first excited states. To describe the temporal evolution of this multiexciton state, a direct computation of the related density matrix is circumvented. Instead, we derive equations of motion for expectation values of interest in using the quantum master equation governing the respective density operator. The subsequently generated hierarchy of kinetic equations is truncated following a general scheme. The whole approach is applied to a chain and a disk of para-sexiphenyl molecules. Different signatures of multiexciton effects are discussed.