Intrinsic Luminescences and Exciton Diffusion in RbI Crystals

Luminescence and excitation spectra of intrinsic and extrinsic luminescences of RbI crystals are studied in a temperature range of 2.5∼80 K. Under the excitation in the first exciton absorption band, only one luminescence at 3.03 eV is observed in an undoped crystal. This luminescence is ascribed to the self-trapping of a free exciton to a lattice site. In Tl + doped crystals under the excitation in the exciton absorption band, the 3.03 eV luminescence decreases and conversely the Tl + luminescence increases with the increase of the Tl + concentration, while the total yield of both luminescences remains constant. It is concluded that the induced Tl + luminescence is the result of the exciton diffusion. The diffusion coefficient of the free exciton is found to be proportional to \(T-1/2\) below 20 K and above that temperature to \(T1/2\ [exp(ω/kT)-1]\) in which \(ω\) is the energy of the longitudinal optical phonon.