ENDOR of 171Yb3+ and 173Yb3+ on cubic sites in ThO2

ENDOR of Yb3+ in ThO2 has given similar results to those in CaF2. The measurements have allowed us to determine the energies of the excited states of the lowest J=7/2 manifold of Yb3+ showing that the crystal field in ThO2 is almost identical to that in CaF2. This is in excellent agreement with the predictions of the superposition model of Newman (1971), where the crystal field is dominated by covalent contributions from the ligands. In particular the energy of the Gamma 8 state is 605(3) cm-1, which becomes 650 cm-1 if a similar correction is applied to that necessary in CaF2. The ENDOR linewidth of approximately 60 kHz is significantly narrower than that in CaF2, which has allowed the determination of a somewhat more precise value of the hyperfine structure anomaly, Delta =-0.007(8)%. This leads to a ratio of s to f electron contributions to the hyperfine structure, As/Af=+1.9+or-2.2%.