Anomalous Quadrupole Coupling in Europium Ethylsulfate

Hyperfine structure, apparently of pure quadrupolar form, has been observed in the ${\mathrm{Eu}}^{3+}$ ion in a neodymium ethylsulfate lattice. It has twice the predicted magnitude and opposite sign to that calculated on the basis of a pure ground electronic configuration of the type $4f^n$. Arguments are presented for ascribing the anomaly to the admixing, through the crystal-field potential, of the state $5p^{5}6p{}_{}{}^1{}_{}{}^{}D_2^{}{}_{}{}^{}$ into the closed shell state $5p^6{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$. For ${\mathrm{Eu}}^{152}$, we find $P_{152}=-(6.7\mathrm{}\pm 0.5)\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$ and for ${\mathrm{Eu}}^{154}$, $P_{154}=-(8.3\mathrm{}\pm 0.7)\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$. The spin and parity assignments of 2- for the 1531-keV state in ${\mathrm{Sm}}^{152}$ and the 1400- and 1723-keV states in ${\mathrm{Gd}}^{154}$, as well as the electric-dipole multipolarities of the radiations depopulating these states, are confirmed. The quadrupole moment of ${\mathrm{Eu}}^{154}$ is found to be 3.29±0.37 b.