Abstract

Selective excitation of ²E → ⁴A₂ fluorescence from Cr³⁺ ions in MgO using a tunable CW dye laser has revealed optical transitions which are not observed in direct absorption or fluorescence measurements. These lines are assigned to a new centre involving an exchange-coupled pair of Cr³⁺ ions. Fluorescence measurements as a function of concentration show that the pair of lines at 698.9 nm and 703.5 nm arise from a tetragonally symmetric pair configuration. The assignment of these optical transitions to the system Cr³⁺ - VAC - Cr³⁺ has now been confirmed by an EPR study¹. Selective laser excitation in this spectral region indicates the presence of another optical spectrum which almost overlaps that of the Cr³⁺ - VAC - Cr³⁺ system, and which also exhibits tetragonal symmetry. A number of lines are observed from 698.7 nm up to the region of the cubic R-line at 698.1 nm due to the higher ²E_v orbital state of this new centre. The splitting due to the lower 2E transition ²E_v transition is not as readily observed partly because of the greater overlap with the Cr³⁺ - VAC - Cr³⁺ transitions. The data have been fitted to a configuration involving two Cr³⁺ ions in next nearest neighbour positions in a <100> crystal direction. The exchange coupling in the ground state is antiferromagnetic as is expected for Cr³⁺ ions.

© 1984 Optical Society of America