Optical detection of magnetic resonance in MgO:Cr3+. I. Octahedral and orthorhombic site symmetries

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Abstract
Optically detected magnetic resonance (ODMR) of Cr3+ ions in magnesium oxide has been monitored by detecting the fluorescence from Cr3+ ions in cubic and orthorhombic symmetry sites. Resonance is detected as a change in the circular polarisation of the R-line emission (cubic centre) and 770 nm emission band (orthorhombic centres) during microwave pumping. The simultaneous observation of both 4A2 ground state resonance and 2E excited state resonance for cubic centres is discussed in terms of the selection rules for circularly polarised light and cross relaxation between excited and unexcited ions. Failure to observe a resonance from the excited fluorescent 4T2 of the orthorhombic centres is explained in terms of the large zero-field splitting implied from level crossing data in the circular polarisation of the luminescence as a function of magnetic field. That one does observe a ground state resonance from the cubic Cr3+ centres as a change in the circular polarisation of the 770 nm luminescence from orthorhombic centres is evidence of energy transfer from cubic to orthorhombic centres. The observed resonances can be explained by a simple, qualitative extension to the selection rules for cubic centres.