The luminescence of NaI:Tl for excitation in the vacuum ultraviolet (12-66 eV)

The excitation spectrum (luminescence intensity vs energy of exciting photon) of NaI:Tl is studied for excitation energies in the range 12-66 eV. When the exciting radiation is of low intensity only a prompt fluorescence is observed which shows a structured excitation spectrum. This is interpreted by the assumption of secondary excitations by energetic electrons which have been raised into the conduction band. The energy of these electrons depends on the level from which they were raised. For high illumination intensities an additional, strong, slowly rising luminescence is observed, the intensity and rise time of which depends on the energy as well as on the intensity of the exciting radiation. These results are interpreted by the assumption of trapping of electrons from the conduction band. These electrons may be raised again into the conduction band by interaction with energetic conduction band electrons. The rate for the thermal release of the trapped electrons is independent of the excitation energy. A detailed model is presented which permits the calculation of the various rate parameters from the experimental results.