Free Exciton Luminescence in Mixed K_1-xRb_xI Crystals

Abstract

In K 1- x Rb x I single crystals, a sharp luminescence line due to free excitons has been observed throughout the entire composition range of the mixture. The peak position shifts almost linearly as a function of Rb concentration x , with a slightly downward convex curve. The quenching temperature of luminescence makes a considerable change as a small amount of RbI is added in KI, and it becomes constant for x >0.5. These results are explained satisfactorily in terms of the virtual crystal model. The E x (3.0 eV) luminescence observed in the KI-rich crystals exhibits the same thermal quenching as the free exciton luminescence. Such a coincidence strongly suggests that the E x luminescence of KI is ascribed to the radiative decay from a perturbed self-trapped exciton state which is formed as a result of the migration of free excitons to imperfection site.