Absorption Spectrum of Manganous Fluoride

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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 6A1g(6S) ground level to various quartet levels. The wave numbers in units of 103 cm—1 corresponding to transitions to the various levels are: 4T1g(4G), 19.44; 4T2g(4G), 23.5; 4A1g(4G), 25.19; 4Eg(4G), 25.50; 4T2g(4D), 28.12 and 28.37; 4Eg(4D), 30.23; 4T1g(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 dγ 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 4A1g(4G), 4T2g(4D), and 4Eg(4D) levels. A shift of about 0.06×103 cm—1 in the transition to the 4A1g level between 77° and 20°K is in agreement with that expected from the effect on the energy levels of the antiferromagnetic ordering energy.