Collective Excitation Dynamics in Molecular Aggregates

Abstract: The aim of this thesis is to study collective excitation dynamics in molecular aggregates. Half of the work presented here is method development and the remainder is application of these methods to several di.erent molecular aggregates. In the first two papers, the pump-probe signal was derived using the Green function approach of the optical response functions and formulated in the Doorway-Window representation. This method was applied to the B850 aggregate of the purple bacteria, Rhodobacter sphaeroides and compared with recent experimental results. The steady-state absorption as well as the thermalized pump-probe spectra were in good agreement with the experimental data for a given parameter set. Furthermore, the temporal evolution of the transient absorption spectra was also well described for the same parameter set. The concept of exciton delocalization was addressed using this method and several theoretical estimations were discussed and compared with each other. Furthermore, the time evolution and dependency of the initial preparation were studied and compared with different experimental techniques. In the second part, the excitation dynamics in molecular aggregates was studied using the surface hopping technique. This method incorporates coupling to the nuclear coordinates explicitly and treats the nuclear system classically. The third and the forth paper included in this thesis, develops the surface hopping method and the simulations were made for model aggregates. The fifth paper introduces an analytical version of the previously presented surface hopping model. In the last included paper, this method was applied to the B850 antenna complex of Rhodobacter sphaeroides with parameters extracted from a realistic spectral density distribution. A number of explicit harmonic oscillators were used to describe the molecular vibrations. Special attention was directed to the nonlinear dependence of the excitonic potential energy surfaces and the exciton-vibrational feedback term. Polaron formation, migration and self-trapping was shown to occur for certain parameter sets.