Impurity-Induced Optical Fluorescence in MnF2

Abstract

The optical fluorescence of nominally pure Mn${\mathrm{F}}_2$ has been studied at low temperatures. It is found that all of the fluorescence, except the very weak intrinsic fluorescence, is induced by small concentrations of cation impurities in the samples. By selectively doping crystals, the characteristic structure for ${\mathrm{Mg}}^{2+}$, ${\mathrm{Zn}}^{2+}$, and ${\mathrm{Ca}}^{2+}$ impurities has been identified. In each case, the fluorescence consists of one or two sharp electronic transitions (exciton lines) shifted to a lower energy relative to the intrinsic exciton line, a magnon sideband associated with each exciton line, and a broad band. The exciton lines are identified by polarization and Zeeman experiments. The magnon sidebands are identified by their positions, shapes, and Zeeman behaviors. Each sideband is associated with a particular exciton line by comparing the temperature-dependent lifetimes and intensities of the individual lines. All of these experiments, especially the lifetime and intensity results, strongly suggest that the fluorescence originates at ${\mathrm{Mn}}^{2+}$ ions which are perturbed by nearby impurity ions. The excited energy level of the perturbed ${\mathrm{Mn}}^{2+}$ ion can be shifted to a lower energy, thereby creating a trap for the freely propagating intrinsic excitons. As a result, the intensities and inverse lifetimes of the fluorescence exhibit an activation behavior; that is, they vary as $e^{\frac{\Delta }{\mathrm{kT}}}$, where $\Delta$ is the depth of the trap. The data are accounted for quantitatively by this model, and values are obtained for the various parameters that describe the energy transfer between the perturbed and unperturbed ions. Finally, it is shown that this model provides an alternative interpretation for the band shift previously discussed by other workers.