Enhancement of $3d$ Impurity-Ion Spin-Forbidden Absorption by Color Centers in Mg${\mathrm{F}}_2$

The spin-forbidden optical transitions of ${\mathrm{Co}}^{2+}$, ${\mathrm{Ni}}^{2+}$, and ${\mathrm{Mn}}^{2+}$ have been observed in doped crystals of Mg${\mathrm{F}}_2$ following both $\gamma$ and electron irradiation. Radiation-induced $F$ centers trapped at impurity sites appear to reduce the forbiddenness significantly for many of the transitions. Oscillator strength increases of up to ${10}^3$ have been recorded.