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

The hyperfine structure of the ground state 4f7 6s28S7/2 of 151Eu and 153Eu has been measured by the method of magnetic resonance in an atomic beam. The sign of the hyperfine structure and the value of gJ have also been measured. The results are: gJ = 1·9935 ± 0·0003; A(151) = -20·0523 ± 0·0002 Mc/s, A(153) = -8·8532 ± 0·0002 Mc/s, B(151) = - 0·7012 ± 0·0035 Mc/s, B(153) = - 1·7852 ± 0·0035 Mc/s; A(151)/ A(153) = 2·26498 ± 0·00008, B(151)/ B(153) = 0·393 + 0·003. These results disagree with the values expected for the pure 8S7/2 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 gJ but do not explain the values of the hyperfine structure constants A and B. Further theoretical work is in progress.