PICOSECOND SPECTROSCOPY OF EXCITON RELAXATION IN ORGANIC-CRYSTALS

This paper is a review of already published work. Exciton-relaxation processes in organic solids are discussed. Pyrene and poly(phenylenevinylene) (PPV) crystals are investigated as two examples of strong exciton-phonon coupled systems. Free-exciton luminescence is observed simultaneously with self-trapped exciton luminescence in these crystals. Since the presence of a self-trapping (a barrier that separates the free-exciton state from the self-trapped exciton state) directly affects the exciton-relaxation process and excimer (self-trapped exciton) -formation time, attention is focused to reveal the presence of the barrier. The intensity and the decay time of transient free-exciton luminescence are discussed, as is the rise of self-trapped exciton luminescence. The exciton-relaxation path within the free-exciton band is also discussed. In pyrene above 120 K, the structural phase-transition point, most of the photoproduced excitons relax directly toward the self-trapped state, and only a small fraction of them relax to the bottom of the free-exciton band. At temperatures below 60 K no exciton relaxes to the bottom of the free-exciton band, except in the case when excitons are produced close to the bottom of the free-exciton band. In PPV all the photoproduced excitons relax to the bottom of the free-exciton band. © 1990 Optical Society of America.