Electronic Absorption Spectrum of MnO4− in the Far-Red Region

Polarized absorption spectra of single crystals of ${\mathrm{LiClO}}_4\text{· 3}{\mathrm{H}}_2{\mathrm{O/LiMnO}}_4\text{· 3}{\mathrm{H}}_2\mathrm{O}$ and ${\mathrm{Ba(ClO}}_4{)}_2\text{· 3}{\mathrm{H}}_2{\mathrm{O/Ba(MnO}}_4{)}_2\text{· 3}{\mathrm{H}}_2\mathrm{O}$ have been measured at 4.2°K in the region $\text{λ>5800 Å}$ . The transition whose $\perp$ components possess zero‐phonon origins at 13774 cm⁻¹ in the lithium salt and at 14247 cm⁻¹ in the barium salt is unequivocally identified as an electric‐dipole forbidden ${}^1{T}_1{\leftarrow }^1{A}_1$ transition of the parent tetrahedral point group. In the C_3v site symmetry of the lithium salt the $\perp$ component of this transition is shown to be electric‐dipole allowed; in the C₃ site symmetry of the barium salt the crystallographic site‐splitting of the $\perp$ and $\Vert$ components is 152 cm⁻¹, and both component transitions are shown to be electric‐dipole allowed. It is deduced that both the far‐red transition and the 5200‐Å absorption band of permangante ion arise dominantly from ${\mathrm{e\leftrightarrow \; t}}_1$ or ${\mathrm{e\leftrightarrow \; t}}_2$ single‐configurational excitations. A discussion of the extensive phonon coupling and the anomalous Franck—Condon pattern of this transition is also provided.