Observation of Interatomic Radiative Transition of Valence Electrons to Outermost-Core-Hole States in Alkali Halides

Excitation spectra of visible luminescence in CsF (2.5-5.6 eV), CsCl (3.9-6.0 eV), CsBr (4.4-6.2 eV), and RbF (2.6-6.2 eV) have been measured at room temperature with use of synchrotron radiation for the energy range up to 40 eV. It is shown that the excitation thresholds of these luminescence bands are located at the transition energies from the outermost-core band to the conduction band. This fact provides direct evidence for the radiative decay of valence electrons into outermost core holes in which the Auger decay process is energetically impossible in these materials.