MULTIPLE EXCITATIONS AND THE YIELD OF CHLOROPHYLL a FLUORESCENCE IN PHOTOSYNTHETIC SYSTEMS

Abstract

Abstract. Analysis of the effect of multiple excitations on chlorophyll a fluorescence yields in the green alga Chlorella reveals several distinct reactions. The first excitation in dark‐z‐adapted units produces photochemistry and the high yield state with a rise time of 35 ns. It is ascribed to a change in coupling between the antenna pigments and the photochemical trap. The second hit produces with the same quantum yield a quenched state which changes to the high yield state with a rise time of 4 μs. This is ascribed to the formation and the decay of a particular carotenoid triplet state near the funnel or antenna‐trap junction. Further hits produce enhanced quenching assigned to mobile triplets with lifetimes in the order of 100 ns. The fluorescence yield decreases monotonically with increasing excitations during the 7 ns pulse. This effect can be adequately ascribed to annihilation of excitations with lifetimes longer than the trapping time, or by a unique model of a multi‐trapped unit. The latter model is favored by arguments based both on the absence of a local maximum in the graph of fluorescence yield vs excitation energy and on the fact that the high yield state shows a different behaviour on multiple excitation, fit by a single‐trapped unit. This analysis is related to that used in experiments with ps flashes and is applied to the qualitatively different bacterial system.