Magnetic circular and linear dichroism of cubic or tetragonally distorted high spin d⁵ systems

Abstract

The low temperature, single crystal and polymer matrix absorption, magnetic circular dichroism (MCD) and magnetic linear dichroism (MLD) of the d-d electronic transitions in the tetrahalomanganates (II), [(C₂H₅)₄N]₂MnX₄ (where X = Cl, Br and I), have been investigated. The effect of crystal rotation about the incident radiation propagation direction is theoretically examined and shown to yield anisotropic MLD terms for ⁴ T ₁ and ⁴ E states and isotropic ones for ⁴ A ₂ and ⁴ T ₂ states. The combination of the sign of the MCD zeroth order moment and the MLD crystal rotational dependence is used to assign the observed bands with a high degree of certainty. It is shown that although conventional crystal field theory correctly predicts the d-d state ordering in the lowest energy range (⁴ T ₁ < ⁴ T ₂ < ⁴ E) it fails badly in the higher energy region (⁴ T ₁ < ⁴ T ₂ < ⁴ A ₂). An improved model including both charge-transfer and d electron configurations is proposed to explain the discrepancy. Spin-orbit and tetragonal field calculations are shown to play a very effective role in explaining fine details in our experimental spectra. Finally, evidence for Jahn-Teller activity in several bands is noted.