The hyperfine structure of the four lowest atomic levels of ${\mathrm{Ni}}^{61}$ has been studied by the atomic-beam magnetic-resonance technique. Agreement between the results and the theory of the hyperfine interaction is good. The importance of taking account of the substantial departure of the atom from the $\mathrm{LS}$ limit in evaluating the electric quadrupole moment $Q$ of the ${\mathrm{Ni}}^{61}$ nuclear ground state is discussed. Values of $Q$ are determined from two different electron configurations; the values obtained differ by only 3%. The result is in agreement with the previous theoretical prediction of Cohen et al.

• Received 31 January 1968

DOI:https://doi.org/10.1103/PhysRev.170.136