Anomalous Quadrupole Coupling in Europium Ethylsulfate

Hyperfine structure, apparently of pure quadrupolar form, has been observed in the Eu³⁺ 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 4fⁿ. Arguments are presented for ascribing the anomaly to the admixing, through the crystal-field potential, of the state 5p⁵6p¹D₂ into the closed shell state 5p⁶¹S₀. For Eu¹⁵², we find P₁₅₂=-(6.7±0.5)×10^-4 cm^-1 and for Eu¹⁵⁴, P₁₅₄=-(8.3±0.7)×10^-4 cm^-1. The spin and parity assignments of 2- for the 1531-keV state in Sm¹⁵² and the 1400- and 1723-keV states in Gd¹⁵⁴, as well as the electric-dipole multipolarities of the radiations depopulating these states, are confirmed. The quadrupole moment of Eu¹⁵⁴ is found to be 3.29±{}0.37 b.