Magnetic Circular Dichroism of Impurities in Solids: Vibrationally Inducedd→dTransitions in MgO: Ni

We report absorption spectra and magnetic circular dichroism (MCD) of $d\to d$ transitions in single-crystal MgO:Ni over the range 4-300°K. General theoretical expressions for the zeroth and first moments of the absorption and MCD of a vibration-induced band are derived. Their application to the MgO: Ni data leads to the conclusion that the intensities of the ${}_{}{}^3{}_{}{}^{}T_1^a{}_{}{}^{}$, ${}_{}{}^1{}_{}{}^{}T_2^{}{}_{}{}^{}$, and ${}_{}{}^3{}_{}{}^{}T_1^b{}_{}{}^{}$ bands are predominantly induced via $t_{1u}$ vibrations. A consistent qualitative analysis of observed structure in these bands is made. The main vibrational structure is assigned to the dispersion of the allowing $t_{1u}$ phonons. All spin-orbit components of the ${}_{}{}^3{}_{}{}^{}T_1^a{}_{}{}^{}$ band are identified. The analysis does not require Jahn-Teller effects to be invoked in any of these bands.