Extrinsic self-trapping of excitons in TlBr(I)

Luminescence spectra have been measured in TlBr doped with I− ions under pulsed-light excitation. Two broad emission bands were observed at 2.2 and 2.45 eV.The emission band at 2.2 eV decayed non-exponentially with a t−1 dependence at long delay, and was attributed to radiative annihilation of a self-trapped exciton at the nearest neighbor of an I− ion. The exciton was formed by the tunneling recombination of a hole trapped at the nearest neighbor of an I− ion and an electron trapped at a distant lattice position. The emission band at 2.45 eV, which decayed with two time constants of 0.5 μs and less than 20 ns, was attributed to annihilation of another type of a self-trapped exciton in Br-rich region of the crystal. These emission bands are discussed with a theory on extrinsic self-trapping in a mixed crystal, and ascribed to the two types of extrinsic self-trapping of the excitons.