Optical spectra and emission characteristics of terbium-doped potassium–lead double chloride crystals (KPb2Cl5:Tb3+)

Abstract

Optical transitions in KPb₂Cl₅:Tb³⁺ crystals are studied experimentally and theoretically. The absorption cross-section spectra are plotted and the oscillator strengths of transitions from the ground terbium state to excited multiplets are determined. Intensity parameters Ω_t for KPC:Tb³⁺ are determined by the Judd–Ofelt method to be Ω₂ = 2.70 × 10^–20 cm², Ω₄ = 7.0 × 10^–20 cm², and Ω₆ = 0.72 × 10^–20 cm². These values were used to calculate such characteristics of spontaneous radiative transitions as oscillator strengths, probabilities of radiative transitions, and radiative lifetimes. The emission spectra of KPb₂Cl₅:Tb³⁺ crystals upon UV excitation and the decay kinetics of luminescence from the excited ⁵D₃ and ⁵D₄ levels are studied experimentally, the lifetimes of these levels are determined, and the dependences of the rates of nonradiative relaxation from the excited ⁷F_j (j = 0–5), ⁵D₄, and ⁵D₃ levels to lower-lying terbium levels are calculated. It is shown that the population of the ⁵D₄ level in KPC:Tb³⁺ crystals occurs according to a cascade scheme, which leads to quenching of the ⁵D₃ level. The calculated data agree well with the known experimental rates of multiphonon nonradiative transitions for Dy:KPC, Nd:KPC, Er:KPC, Tb:KPB, and Nd:KPB crystals. It is shown that transitions in the near-IR (3–6 μm) region in double halide crystals (MPb₂Hal₅) are almost unquenched and the rates of nonradiative relaxation of excited levels spaced by energy gaps ΔE_ji > 1000 cm^–1 are W_ji^NR < 10³s^–1. This circumstance suggests that it is possible to obtain stimulated emission in KPb₂Cl₅:RE³⁺ crystals in the IR spectral region up to 6 μm.