In addition to the shallow state these impurity centers also possess a deep state.[@piekara; @dmochowski1; @dmochowski2] Application of UV and visible light at low temperatures results in disappearance of the absorption peak corresponding to the deep electronic state and an infrared absorption band associated with the occupation of the metastable shallow state. In other words, the electrons occupying the deep state are lifted due to photoexcitation to the shallow state. At low temperatures the electrons cannot be trapped back to the deep states due to a barrier induced by different atomic configurations for shallow and deep states. The change in the electronic occupation causes a large difference in the local refractive index, which can be used as means for holographic writing at nanoscale spatial resolution.[@ryskin] A general observation is that the formation energies of most defects are found to be very low.[@mattila] This suggests that the fluorite structure in $CdF_{2}$ often contains high defect concentrations in agreement to what is found experimentally.[@hayes] It is also essential that $CdF_{2}$ crystals of quite large size can be obtained at a relatively low cost.[@kalugin]

Optical properties of $CdF_{2}$ have been experimentally determined by different spectroscopic techniques since the seventies of the previous century[@berger; 