Branching of exciton relaxation to the free and self-trapped exciton states

Abstract

The transient free-exciton luminescence and unrelaxed self-trapped exciton luminescence from pyrene have been studied. It is found that above 120 K, the structural phase-transition point, most of the photoproduced excitons relax directly towards 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 where excitons are produced close to the bottom of the free-exciton band. When free-exciton luminescence is absent, luminescence due to unrelaxed self-trapped excitons (transient self-trapped excitons) has been confirmed. The self-trapping barrier height at low temperatures is estimated to be about 360 cm −1, which should be compared to the height, 260 cm −1, at high temperatures.