Paramagnetic resonance of S -state ions in calcium fluoride

Paramagnetic resonance has been observed in Mn$^{2+}$, Eu$^{2+}$ and Gd$^{3+}$ ions in single crystals of calcium fluoride grown from the melt. The Mn$^{2+}$ ion has a very small cubic-field Stark splitting (a = + 0$\cdot $6 $\pm $ 0$\cdot $4 $\times $ 10$^{-4}$ cm$^{-1}$), and a fluorine hyperfine structure of overall splitting of about 60 G due to some covalent bonding with the fluorine ligands. The Eu$^{2+}$ spectrum has cubic symmetry with splitting parameters b$_{4}$ = 57$\cdot $9 + 0$\cdot $2, b$_{6}$ = 0$\cdot $5 + 0$\cdot $2 ($\times $ 10$^{-4}$ cm$^{-1}$), but the Gd$^{3+}$ spectrum has tetragonal symmetry with much larger Stark splittings; neither ion shows a resolved fluorine structure. The manganese and europium hyperfine structures are closely the same as in other salts with small covalent bonding.