Optical Absorption Induced by Ca Impurities in MnF2

We have measured the absorption spectrum of Mn${\mathrm{F}}_2$ doped with 0.1% of Ca in the 18 100-to 18 400-${\mathrm{cm}}^{-1\mathrm{}}$ spectral region at 2K. There is considerable weak structure in this region induced by the presence of the impurity. We concentrate upon two lines, $\mathrm{Ca} E1\mathrm{} \left(\mathrm{I}\right)$ at 18 122 ${\mathrm{cm}}^{-1\mathrm{}}$ and $\mathrm{Ca} E2\mathrm{} \left(\mathrm{I}\right)$ at 18 322 ${\mathrm{cm}}^{-1\mathrm{}}$, which we identify on the basis of their polarization as pure electronic transitions of ${\mathrm{Mn}}^{2+}$ ions perturbed by first-nearest-neighbor ${\mathrm{Ca}}^{2+}$ impurities. These two lines appear in $\pi$ and $\sigma$, but not $\alpha$ polarization. Thus, the lines have both electric and magnetic dipole character and are consistent with a $C_2$ site symmetry for the absorbing ion. Stress results for the impurity transitions for [001]- and [110]-directed uniaxial stress are presented. It is found that the stress behavior of $\mathrm{Ca} E1\mathrm{} \left(\mathrm{I}\right)$ and $\mathrm{Ca}E2\left(\mathrm{I}\right)$ is closely related to that of the intrinsic excitons $E1\mathrm{}$ and $E2\mathrm{}$, respectively.