Mechanisms of Energy Transfer Involving Trivalent Er and Tb or Tm in Sodium Rare-Earth Tungstates

Abstract

Emission is observed from six manifolds of Er in the tungstates at 77°K. Their signatures and intrinsic lifetimes in microseconds are: Er²H_9/2, 4S_3/2, 25; Er⁴F_9/2, ∼5; Er⁴I_9/2, ∼13; Er⁴I_11/2, 120; and Er⁴I_13/2, ∼4500. A seventh manifold (Er²H_11/2) emits when thermally populated from Er⁴S_3/2 at 295°K. The principal transitions from all of these manifolds are to the ground manifold (Er⁴I_15/2); as a result, radiation reabsorption is strong for each manifold and random migration of excitation between Er⁴I_13/2 manifolds is possible at low Er concentrations. Further, due to the small spacings between the various manifolds of Er, relaxation by internal conversion processes is prevalent. Taking these factors and self‐quenching interactions of Tb⁵D₃ and Er into account, the nonradiative transfer of energy from Tb⁵D₃ or Tb⁵D₄ to Er, from Er²H_11/2, Er⁴S_3/2, Er⁴I_9/2, Er⁴I_11/2, or Er⁴I_13/2 to Tb, and the equivalent self‐quenching interactions of Er are all indicated to be by dipole—dipole in character.