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.