## Abstract

Endor experiments have been performed on $^{173}$Yb at tetragonal sites in CaF$_{2}$. The spin-Hamiltonian describing the frequencies of the endor lines contains many small terms, which arise in second order from matrix elements of Zeeman and hyperfine interactions that couple the ground doublet to excited states of the J = $\frac{7}{2}$ manifold. From the experimental values of these small terms the energies of the excited states of this manifold are deduced. This information is used together with g values of the lowest doublet for both J = $\frac{7}{2}$ and J = $\frac{5}{2}$ manifolds, and the energy of the lowest doublet of the J = $\frac{5}{2}$ manifold, to calculate the following crystal field parameters (all in cm$^{-1}$): B$_{2}^{0}$ = +8.4 (1.3); B$_{4}^{0}$ = +0.19 (0.03); B$_{6}^{0}$ = +0.054 (0.005); B$_{4}^{4}$ = +2.8 (0.4); B$_{6}^{4}$ = -0.735 (0.07). The measured value of the nuclear electric quadrupole interaction for $^{137}$Yb(I = $\frac{5}{2})$ is separated into a component due to the field gradient from the 4f electrons and one due to the field gradient from the surrounding lattice. Comparison of the latter component with the value of B$_{2}^{0}$ gives (1 - $\gamma _{\infty}$)/(1 - $\sigma _{2}$) = 75 $\pm $ 18, considerably smaller than values of this ratio predicted theoretically or found in earlier experimental investigations. The most likely cause of this low value is the considerable contribution of covalent effects to B$_{2}^{0}$.