Doubly ExcitedCr3+Pairs in ZnGa2O4

Abstract

Optical transitions from the ground state (${}_{}{}^4{}_{}{}^{}A_2^{}{}_{}{}^{},{}_{}{}^4{}_{}{}^{}A_2^{}{}_{}{}^{}$) to the doubly excited (${}_{}{}^2{}_{}{}^{}E,{}_{}{}^2{}_{}{}^{}E$) state of nearest-neighbor ${\mathrm{Cr}}^{3+}$ pairs in the spinel Zn${\mathrm{Ga}}_2$ ${\mathrm{O}}_4$ have been studied. These transitions, which are situated in the ultraviolet part of the spectrum (at about 3400 ÅA), are observed in the excitation spectrum of the $({}_{}{}^4{}_{}{}^{}A_2^{}{}_{}{}^{},{}_{}{}^2{}_{}{}^{}E)\to ({}_{}{}^4{}_{}{}^{}A_2^{}{}_{}{}^{},{}_{}{}^4{}_{}{}^{}A_2^{}{}_{}{}^{})$ pair emission. Two possible interpretations of the spectrum are given. The energy-level diagram of the (${}_{}{}^2{}_{}{}^{}E,{}_{}{}^2{}_{}{}^{}E$) state is calculated using the exchange Hamiltonian ${\mathcal{H}}_{\mathrm{ex}}=-\Sigma {\mathrm{ij}}^{}{J}_{\mathrm{ij}}{\overrightarrow{\mathrm{s}}}_i·{\overrightarrow{\mathrm{s}}}_j$. When using the $J_{\mathrm{ij}}$ parameters as derived from the (${}_{}{}^4{}_{}{}^{}A_2^{}{}_{}{}^{},{}_{}{}^2{}_{}{}^{}E$) level diagram, it is found that that the theory is not in quantitative agreement with the experimental results. It is shown that the agreement is much better if we assume that the values of some of the parameters which determine the level energies (the ${}_{}{}^2{}_{}{}^{}E-{}_{}{}^4{}_{}{}^{}A_2^{}{}_{}{}^{}$ energy difference of the values of the $J_{\mathrm{ij}}$ parameters) are different in the (${}_{}{}^2{}_{}{}^{}E,{}_{}{}^2{}_{}{}^{}E$) state from their values in the (${}_{}{}^4{}_{}{}^{}A_2^{}{}_{}{}^{},{}_{}{}^4{}_{}{}^{}A_2^{}{}_{}{}^{}$) and (${}_{}{}^4{}_{}{}^{}A_2^{}{}_{}{}^{},{}_{}{}^2{}_{}{}^{}E$) states. It has been found that the ${\mathrm{Cr}}^{3+}$ pair emission can be excited selectively by irradiating in the uv pair-absorption band. An upper limit for the radiationless transition $({}_{}{}^2{}_{}{}^{}E,{}_{}{}^2{}_{}{}^{}E)\to ({}_{}{}^4{}_{}{}^{}T_1^{}{}_{}{}^{},{}_{}{}^4{}_{}{}^{}A_2^{}{}_{}{}^{})$ of 500 μsec has been established.