Single-Crystal Electronic Absorption Spectra of MMnO₄ (M=K, Rb, Cs), K₂CrO₄, K₂MnO₄, and K₂FeO₄

Abstract

Electronic absorption spectra of single crystals of MMnO₄ (M=K, Rb, Cs), K₂CrO₄, K₂MnO₄, and K₂FeO₄ were measured by microspectrophotometry at room and liquid nitrogen temperatures in the region 260-700nm. Maxima of the absorption bands in single crystals were shifted to the blue compared to those in solution, and to the red compared to those in mixed crystals. For the permanganates, a blue shift was observed with an increase in the size of the cation: KMnO₄ < RbMnO₄ < CsMnO₄. The magnitude of these shifts was of the order of several hundred wavenumbers. In connection with nature of molecular orbitals, the observed shifts were explained mainly on the basis of the compression of the complex ions. It was found that the FeO₄^2- ion in alkaline aqueous solution showed a sharp absorption band at 9100cm^-1, which can probably be attributed to a transition (³A₂→¹E) within the ground-state (2e) ² configuration.