Optical Emission Spectrum of ${Cr}^{3 +}$ - ${Eu}^{3 +}$ Pairs in Europium Gallium Garnet

Measurements of the emission spectrum of pure and chromium-doped europium gallium garnet are presented. The emission from weakly coupled

{Cr}^{3 +}

{Eu}^{3 +}

pairs is of primary interest, and as was previously found for EuAl

O_{3}

{Cr}^{3 +}

, fluorescence originates on the chromium

{}^{2}E_{g}

level and terminates on the

{}^{7}F_{J}

levels of neighboring europium ions. Pair transitions to

J = 3

through 6 have been observed. The energy levels of the final states are in good agreement with the

{}^{7}F_{J}

levels of the pure crystal which are measured in absorption. Pair transitions to

J = 1 and 2

are obscured by strong vibronic transitions. A spin-dependent dipole moment resulting from the

{Cr}^{3 +}

{Eu}^{3 +}

exchange is invoked to explain the emission. This requires the three

t_{2 g}

chromium orbitals to be inequivalent. The term which is isotropic in the europium orbitals results in pair emission to

J = 1

. The transitions to

J > 1

occur from terms which contain the orbital dependence of the europium electrons. The experimental observation of pair transitions to levels with

J > 1

verifies the existence of europium electron anisotropy in the

{Cr}^{2 +}

{Eu}^{3 +}

exchange interaction.

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Optical Emission Spectrum ofCr3+-Eu3+Pairs in Europium Gallium Garnet