Fluorine Electron Nuclear Double Resonance (ENDOR) in Calcium Fluoride

The method of electron nuclear double resonance (ENDOR) has been used to measure the interaction between a Eu²⁺ ion in CaF₂ and its ligand fluorine nuclei. Expressing the interaction in the form A_sS. I + A_p(3S_zI_z -S. I), with z along the bond axis, the parameters are found to be A_s = -2.23±0.01 Mc/s and A_p = +4.01±0.01 Mc/s. The value of A_s is of the same sign as, but a factor of 4 smaller than, the value predicted theoretically by Freeman and Watson in 1961. Interactions with next nearest neighbour fluorine nuclei are found to be nearly dipolar. An appendix discusses the possible line shift and line broadening of both fluorine and europium ENDOR due to high order effects in the fluorine and europium hyperfine interactions.