Absorption Spectrum of Manganous Fluoride

Abstract

The absorption spectrum of single crystal MnF₂ has been measured in the range 2700–6000 A. In the room temperature axial spectrum peaks are observed which are identified with transitions from the ⁶A_1g(⁶S) ground level to various quartet levels. The wave numbers in units of 10³ cm^—1 corresponding to transitions to the various levels are: ⁴T_1g(⁴G), 19.44; ⁴T_2g(⁴G), 23.5; ⁴A_1g(⁴G), 25.19; ⁴E_g(⁴G), 25.50; ⁴T_2g(⁴D), 28.12 and 28.37; ⁴E_g(⁴D), 30.23; ⁴T_1g(⁴P), 33.06. The spectrum is interpreted in terms of a perturbation of the free ion energy levels by an octahedral ligand field including the diminution of the magnitude of the d_γ orbitals on the Mn⁺⁺ ion by a factor (1 — ε)^½ in calculating the coulomb and exchange integrals. ε is the ``covalency parameter'' of Koide and Pryce. Energy matrices in the strong field representation and the free‐ion representation are calculated. The spectra are fitted with ε = 0.064 and the octahedral ligand field parameter Dq = 0.78×10³ cm^—1. From measurements of the π and σ spectra with polarized light it is concluded that the transitions are electric dipole ones. The spectra were also photographed with a grating spectrograph at temperatures down to 20°K. At 20°K sharp lines are seen corresponding to the transitions to the ⁴A_1g(⁴G), ⁴T_2g(⁴D), and ⁴E_g(⁴D) levels. A shift of about 0.06×10³ cm^—1 in the transition to the ⁴A_1g level between 77° and 20°K is in agreement with that expected from the effect on the energy levels of the antiferromagnetic ordering energy.