The luminescence spectra and the degrees of magnetic circular polarization of luminescence (MCPL) have been studied in paramagnetic garnet Тb 0.2 Y 2.8 Al 5 O 12 (YAG : Тb

The energy spectrum of the 7F3 multiplet of a Tb3+ ion in one of the non‐equivalent positions (of D2 symmetry) occupied by a rare‐earth ion in the Y3Al5O12 crystallographic garnet structure is calculated. The energy spectrum of the 7F3 state is characterized by the presence of quasi‐doublet states formed by closely located Stark singlet states, that are effectively “mixed” in an external magnet field H. This inference is seriously confirmed by results of experimental studies of magnetopolarized luminescence on the radiative 5D4 → 7F3 4f–4f transition in Y3Al5O12:Tb3+. The optical transitions are identified, and the components of the g‐factor of the 7F3(Γ2, Γ4) quasi‐doublets in the Y3Al5O12 structure are determined.

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The Zeeman effect is investigated in the green luminescence band corresponding to the 4f→4f radiative transition 5 D 4 → 7 F 5 of Tb 3+ ions in Tb 0.2 Y 2.8 A l5 O 12 terbium-yttrium-aluminum garnet at the temperature Т = 85 K. It is demonstrated that an external field causes not only the Zeeman shifts of the resonant frequencies of radiative transitions in the luminescence spectra but also changes in the emission line intensities caused by modification of the optical transition probabilities in the σ -and σ + opposite circular polarization states. Special features of field dependences of the Zeeman effect for the emission lines are qualitatively explained based on the results of numerical energy spectrum calculations for the 5 D 4 and 7 F 5 multiplets of the Tb 3+ ion in yttrium-aluminum garnet in the geometry of the longitudinal and transverse Zeeman effects. The possibility of nonequilibrium population of sublevels of the (Г 1 , Г 4 ) quasi-doublet state of the 5 D 4 multiplet with energy of ∼20585 cm -1 is demonstrated in an external magnetic field.