Exciton luminescence and polariton effects in alkali iodide crystals

An investigation of the edge luminescence due to free excitons in KI and RbI has been made on bulk crystals and thin crystal plates of thicknesses from 10 to 0.1 {μ}m at 11 K. In the thin samples, it is found that the edge luminescence line appears just around the absorption band attributed to the strain-induced spin-triplet states, both peaks showing the same thickness-dependent energy shift. From a close connection of these results with those obtained in unstrained bulk materials, the polariton model including the symmetry-breaking effect due to a finite exciton wave vector is invoked to account for the origin of the edge luminescence in alkali iodides. On the basis of the above arguments, the exchange energy {Δ} and the spin-orbit splitting {λ} of excitons are evaluated; {Δ}=20 meV and {λ}=0.89 eV for KI and {Δ}=26 meV and {λ}=0.78 eV for RbI.