Chromium-Europium Pair Emission in EuAlO3:Cr3+

A detailed discussion of the emission spectrum of trivalent chromium-europium pairs is given. Emission originating on the chromium ${}_{}{}^2{}_{}{}^{}E_g^{}{}_{}{}^{}$ level and terminating on the ${}_{}{}^7{}_{}{}^{}F_J^{}{}_{}{}^{}$ levels of neighboring europium ions has been observed for $J=1\mathrm{} \mathrm{through} 6$. Good agreement is obtained with the ${}_{}{}^5{}_{}{}^{}D_0^{}{}_{}{}^{}$ to ${}_{}{}^7{}_{}{}^{}F_J^{}{}_{}{}^{}$ emission of pure EuAl${\mathrm{O}}_3$. The relative intensities of the lines enable us to identify the exchange interaction as the most probable coupling mechanism. Because of the directional character of the chromium-europium exchange, a matrix element of the chromium-ion electron spin exists between the ${}_{}{}^4{}_{}{}^{}A_{2g}^{}{}_{}{}^{}$ and ${}_{}{}^3{}_{}{}^{}E_g^{}{}_{}{}^{}$ even in the absence of spin-orbit interaction. Additional contributions which do not depend on the anisotropy of the chromium orbitals occur when the spin-orbit interaction is included.