Singlet-singlet annihilation experiments have been performed on trimeric and aggregated light-harvesting complex II (LHCII) using picosecond spectroscopy to study spatial equilibration times in LHCII preparations, complementing the large amount of data on spectral equilibration available in literature. The annihilation kinetics for trimers can well be described by a statistical approach, and an annihilation rate of (24 ps)(-1) is obtained. In contrast, the annihilation kinetics for aggregates can well be described by a kinetic approach over many hundreds of picoseconds, and it is shown that there is no clear distinction between inter- and intratrimer transfer of excitation energy. With this approach, an annihilation rate of (16 ps)(-1) is obtained after normalization of the annihilation rate per trimer. It is shown that the spatial equilibration in trimeric LHCII between chlorophyll a molecules occurs on a time scale that is an order of magnitude longer than in Photosystem I-core, after correcting for the different number of chlorophyll a molecules in both systems. The slow transfer in LHCII is possibly an important factor in determining excitation trapping in Photosystem II, because it contributes significantly to the overall trapping time.

The excited state mixing effect is taken into account considering the difference spectra of dimers. Both the degenerate ͑homo͒ dimer as well as the nondegenerate ͑hetero͒ dimer are considered. Due to the higher excited state mixing with the two-exciton states in the homodimer, the excited state absorption ͑or the difference spectrum͒ can be strongly affected in comparison with the results obtained in the Heitler-London approximation. The difference spectrum of the heterodimer is influenced by two resonance effects ͑i͒ mixing of the ground state optical transitions of both monomers in the dimer and ͑ii͒ mixing of the excited state absorption of the excited monomer with the ground state optical transition in the nonexcited monomer. These effects have been tested by simulating the difference absorption spectra of the light-harvesting complex of photosystem II ͑LHC II͒ experimentally obtained with the 60 fs excitation pulses at zero delay times and various excitation wavelengths. The pairs of coupled chlorophylls a and b for simulations have been taken from the best LHC II assignment model obtained by simulating the steady-state spectra and the transient absorption at various excitation wavelengths. Qualitatively the spectral peculiarities of the difference spectra are explained by means of the resonance interpigment interactions, which are responsible for the excited state mixing.

Despite the fact that fluorescence induction is a very complicated process, the technique is used to obtain information regarding connectivity in photosynthetic systems. The models generally used for the analysis are oversimplified, which in some cases has led to questionable interpretations. Here we describe the effects of nonlinear loss processes in (pulsed) induction experiments and how they obscure the features attributed to large-scale connectivity in chloroplasts. We simulate the fluorescence induction process for finite size domains (1-4 reaction centers per domain) and describe both the trapping process and the generation of triplets by a discrete state model. From our numerical calculations it is demonstrated that singlet-triplet annihilation is unavoidable when using microsecond pulses for actinic illumination.

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