The relaxed excited state responsible for the luminescence associated with the forbidden transition in KI:Sn²⁺crystals

The lifetime of luminescence in KI:Sn 2+ crystals has been investigated using a streak camera. By excitation in the absorption band B due to a dipole-forbidden 1 S o to 3 P 2 transition in the Sn 2+ centre, an emission band with a peak at 446 nm is produced by the reverse transition 3 P 2 to 1 S 0 . The emission B has a lifetime of 27.4 ns at 15 K and its lifetime decreases with increasing temperature. Such a short lifetime, which was unexpected for the originally forbidden band, is explained by a displacement of the vacancies around Sn 2+ ions in the relaxed excited state which gives rise to a mixing of the forbidden 3 P 2 states with the allowed 1 P 1 state.