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Summary form only given. Femtosecond spectroscopy has been used to reveal details of excitation energy (exciton) relaxation and transfer in various types of chromophore complexes. One of the most fascinating objects in this context is the light harvesting complex located in the photosynthetic unit of bacteria and higher plants. But of actual interest are also dendrimers forming artificial antennae systems where similar definite spatial arrangement of chromophores can be achieved as in biological antennae. It is a common approach to vary the intensity of the laser pulse used to excite the chromophore complex. This results in higher excited states beyond the presence of a single Frenkel exciton and leads to the opening of new relaxation channels. A particular relaxation channel, the exciton annihilation process is known for decades. At higher excitation conditions two or more excitons may be excited simultaneously in the same complex. Then, the excitation energy belonging to two excitons can be transferred into a single higher intra-chromophore level. A fast internal conversion results in a transfer down to the first excited state, and one of the two excitons has been annihilated. It is the aim of this work to present a unique density matrix theory which (a) is valid for a variable number of excitons, which (b) accounts for electronic (excitonic) coherences, and (c) which includes different channels of exciton relaxation. High frequency intra-chromophore vibrations are used to incorporate the internal conversion process which is a part of the exciton annihilation. Relaxation within a given manifold of (multi-)exciton levels is described via low frequency vibrations. Different types of pump-probe spectra are presented including discussion of their dependence on some unknown system parameter and on the pump beam intensity. Furthermore, the approach is used to indicate in detail the experimental signature of exciton annihilation in photosynthetic antennae as well as in - rtificial dendrimeric nanostructures.