Exchange effects on the electronic spectra of antiferromagnetic salts of Mn2+

Abstract

The electronic absorption spectra of several antiferromagnetic Mn²⁺ salts are compared. The strongly interacting bridged compounds such as CsMnCl₃·2H₂O and MnCl₂·2H₂O absorb about an order of magnitude more strongly than the isolated systems such as Cs₂MnCl₄·2H₂O and MnSO₄·4H₂O. The intensity of the bridged compounds is less temperature dependent than that of the isolated system, whose intensities show the dependence expected of a vibronic coupling mechanism. External magnetic field effects on the spectra of CsMnCl₂·2H₂O, MnCl₂·2H₂O and MnSO₄·4H₂O are compared. MnSO₄·4H₂O shows a normal Zeeman splitting of its absorption bands. The lack of such splitting in the bridged compounds both below and above their ordering temperatures is interpreted as resulting from a cooperative intensity mechanism in these crystals. The effects of magnetic symmetry on the spectrum of CsMnCl₃·2H₂O below T_N are discussed. Polarizations found in this spectrum are shown to be anomalous.