Excitonic energy transport in wide-band inorganic scintillators

The known experimental data on the energy transport in alkali-halide scintillators and rare gas crystals with a similar electronic structure can be explained only by taking into account the vibrational levels of a two-site self-trapped exciton (excimer quasi-molecule). In a crystal, these vibrational levels turn to narrow excitonic subbands with a very large effective mass which provides a possibility for a two-site exciton to be localized in shallow potential wells produced by impurity centers and lattice defects. A very high rate of the excitonic energy transfer, observed for alkali-halide at a low temperature, is explained by a coherent directional motion of two-site excitons with a velocity close to the sound velocity in the crystal. These two mechanisms, in their combination, provide an efficient energy transfer from the host crystal to a weak impurity or radiation defects by self-trapped two-site excitons formed after thermal relaxation of photoproduced electronic excitations.