Specific features of the exciton+xciton annihilation process for slow diffusion systems are considered in terms of excimer luminescence in pyrene crystals. I n this case luminescence decay curves allow a direct determination of the diffusion coefficient and the mean free path of the excitation during its lifetime. Analogy between excimer excitations and the well-known slow excitons is discussed. The importance of the experimental separation of excinier and monomer excitons whose kinetic parameters are greatly different is emphasized.Die spezifischen Charakteristika des Exziton-Exziton-Vernichtungsprozesses fiir Systeme mit langsamer Diffusion werden anhand der Exzimer-Lumineszenz in Pyrenkristallen betrachtet. In diesem Fall erlauben die Lumineszenzabklingkurven eine direkte Bestimmuiig des Diffusionskoeffizienten und der mittleren freien Weglange der Anregung wilhrend ihrer Lebensdauer. Die Analogie zwischen Exzimer-Anregungen und den wohlbekannten langsamen Exzitonen wird diskutiert. Die Wichtigkeit der experimentellen Trennung von exzimeren und monomeren Exzitonen, deren kinetische Parameter sehr verschieden sind, wird betont.
In the present note we continue our study of the exciton-exciton annihilation process in pyrene crystals which was started in (1). The temperature dependence of noii;linear luminescence quenching in pyrene crystals in the temperature range from 77 to 373 were described in detail in (1). Nonlinear quenching was studied for five fixed temperatures: T = 77, 153, 223, 293, and 373 K , for which quenching kinetics and relatiire quantum yield of steady-state luminescence versus excitation intensity were determined. The third harmonic of an Nd l a s e r was used for sample excitation, with exciting pulse intensity changing in large limits. 0 K i s investigated. The experimental arrangement and techniques 0The experimental results a r e shown in Fig. 1 and 2. The experimental data show a number of surprising features in the temperature dependence of non-linear quenching which will be discussed here. On the one hand the quantum yield decreases practically without changes at all temperatures studied. Using the phenomenological equation for quantum yield versus excitation intensity we estimated the blmolecular quenching coefficient and obtained p= 10 cm s almost not varying with temperature. This certainly indicates that the microscopic mechanism responsible for exciton-exciton annihilation in pyrene crystals is insensitive to temperature. A small shift with temperature observed in our output curves is accounted for by an increase in exciton intrinsic lifetime at low temperatures and.is not related to the annihilation mechanism. On the other hand it is seen from kinetic curves that a greater power i s required for detecting the decay deviation from the exponent with temperature decrease; this power acquires its critical value at liquid nitrogen temperature and leads to crystal destruction. Thus excitonexciton annihilation vanishes from decay kinetics, but it is seen in quantum yield drop as before.-14 3 -1
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