Luminescence of Eu ²-v _c ^- dipoles and their associates in CsI: Eu crystals

Spectral-kinetics properties of photo-scintillation excited with single light pulses of a nitrogen laser (λ=337.1 nm, t _1/2=5 ns, Q=1 mJ) have been studied in CsI:Eu crystals at temperature within 80-300 K. It is found that the exponential decay of 463 nm emission band has a time constant which grows from 0.85 μs at 78 K to 1.6 μs at 380 K. Such an anomalous temperature behavior of 463 nm emission decay kinetics is discussed in terms of the crystal thermal expansion. It has been proposed that 463 nm emission is caused by a cluster center consisting of three dipoles Eu ²v _c ^- bounded with each other in a hexagon. Owing to the exchange resonance in the cluster, the energy passes from an excited dipole to a non-excited one and the distance between them gets longer due to thermal expansion of the crystal.