Hyperfine structure in the ground state of the stable isotopes of europium

The hyperfine structure of the ground state $4f^76s^{28}S_{\frac{7}{2}}$ of $^{151}$Eu and $^{153}$Eu has been measured by the method of magnetic resonance in an atomic beam. The sign of the hyperfine structure and the value of g$_J$ have also been measured. The results are: $g_J = 1.9935 \pm 0.0003;$ $A(151) = -20.0523 \pm 0.0002 Mc/s, A(153) = -8.8532 \pm 0.0002 Mc/s,$ $B(151) = -0.7012 \pm 0.0035 Mc/s, B(153) = -1.7852 \pm 0.0035 Mc/s;$ $\frac{A(151)}{A(153)} = 2.26498 \pm 0.00008,$ $\frac{B(151)}{B(153)} = 0.393 \pm 0.003.$ These results disagree with the values expected for the pure $^8$S$_\frac{7}{2}$ state formed from a half-filled shell of f-electrons. Calculations on the basis of admixture of higher states of the configuration by spin-orbit coupling account for the discrepancy in g$_J$, but do not explain the values of the hyperfine structure constants A and B. Further theoretical work is in progress.