Luminescence Due to Radiative Transitions between Valence Band and Upper Core Band in Ionic Crystals (Crossluminescence)

In some wide band gap halides of heavy alkali and alkaline‐earth metals (BaF₂, CsCl, RbF etc.) the energy separation between the valence band involving mainly the halogen electronic states and the upper cation core band is smaller than the band gap. Under the excitation generating holes in the core band radiative electronic transitions from the valence band to the core band take place giving rise to a specific kind of intrinsic emission which is proposed to call crossluminescence. Its decay time is about 1 ns and the thermal quenching starts at temperatures well above room temperature. The width of the emission spectrum practically coincides with the width of the valence band deduced from ultraviolet photoelectron spectroscopy and the shape of the spectrum depicts the electronic structure of the valence band.