Luminescence Lines Due to the n=2 Free Exciton State of Wannier Series in KI Crystals

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Abstract
A resonance luminescence line and its LO phonon side bands due to n =2 free excitons in KI are studied. Main results of measurements; (1) the resonance luminescence line is located at 6.186 eV which corresponds closely to the peak position of the zero phonon absorplion line due to n =2 excitons, (2) the shape of the line is symmetric Lorentzian with 1.6 meV width, (3) the quantum yield is 1 ×10 -5 , (4) the height of the potential barrier between the n =2 free exciton state and self-trapped exciton states is 3.0 meV, (5) the self-trapping probability by a tunnel process through the potential barrier is 2 ×10 11 sec -1 and (6) the scattering probability from the n =2 free exciton state to the n =1 free exciton state is 2 ×10 11 sec -1 . Furthermore, it is concluded that so called σ(4.12 eV) luminescence and π(3.31 eV) luminescence are emitted from the self-trapped exciton states due to the n =2 exciton state.