Spectra of manganese(IV) hexafluoride ion (MnF6=) in environments of O h and D3d symmetry

The visible and near ultraviolet spectra of ${\mathrm{MnF}}_6^{=}$ have been observed in solid solutions in Cs₂GeF₆ and K₂GeF₆ in which the site symmetries of the hexafluoride ion are O_h and D_3d, respectively. At liquid helium temperatures these spectra show sharp line absorption corresponding to electric and magnetic dipole transitions. The electric dipole transitions are vibronically allowed and correspond to the excitation of the three ``u'' vibrations of an octahedral, XY₆ molecule. These are observed combined with the a_1g completely symmetrical vibration and, we have assumed, with certain of the lattice vibrations. The assignments given for the ⁴A_2g→²E_g and ²T_1g transitions are reasonably straightforward. For the first spin allowed transition, ⁴A_2g→⁴T_2g, it is necessary to assume a reduction in the spin‐orbit coupling parameter by a factor of about 10 to account for the observations. We have not observed the transition to the ²T_2g state, and the higher lying quartet ⁴T_1g appears as a poorly resolved progression corresponding to va_1g ≈ 460 cm⁻¹.