Low-temperature investigation of electronic excitations in wide-gap materials doped with RE3+ or Cr3+ ions

Abstract

The processes of photon multiplication in insulators have been considered. The luminescence of Tb3+ ions (5 D 3 → 7 F J , 5 D 4 → 7 F J transitions) upon intracenter excitation, the optical excitation of oxyanions, or the formation of separated electrons and holes has been studied for CaSO4 doped with Tb3+ and Na+ ions at 6–9 K. An increase in Tb3+ concentration from 0.2 to 4 at % and transition from single Tb3+-Na+ states to centers that contain two or three terbium ions leads to the redistribution of the luminescence intensities in favor of the 5 D 4 → 7 F J transitions and increase in their efficiency due to the possibility of the cooperative 5 D 3 → 5 D 4 and 7 F 6 → 7 F J transitions and the 4f 75d 1 → 5 D 3 and 7 F 6 → 5 D 4 transitions in the two- and three-terbium centers. Based on the example of MgO single crystals with highly mobile excitons, holes, and electrons, the migration of free excitons and holes toward Cr3+ ions in the crystal bulk and their exit from the bulk to the surface have been revealed at 9 K. Surface losses limit the luminescence quantum yield of MgO:Cr3+, CaSO4:Tb3+, and many other materials.