The hyperfine structure and $g_J$ values of the 17 lowest-lying atomic levels of ${\mathrm{Tb}}^{159}$ have been measured by the atomic-beam magnetic-resonance technique. Two of the three levels studied in the $4f^{9}6s^2$ configuration had not been seen before, and the present work in $4f^{8}5d6s^2$ provides the first hyperfine-structure information obtained for 11 of the 14 levels studied. The results are analyzed in detail by use of the best eigenvectors available and the effective-operator formalism. After making suitable corrections for hfs and Zeeman interactions with neighboring atomic states, values for a number of radial integrals are deduced by treating them as adjustable parameters. The value obtained for the electric-quadrupole moment of the ${\mathrm{Tb}}^{159}$ nuclear ground state is $Q\left({\mathrm{Tb}}^{159}\right)=+1.34\mathrm{}\pm 0.11$ b, in good agreement with earlier results. This value of $Q$ has been corrected for Sternheimer shielding effects in both the $4f$ and $5d$ electron shells; it results from least-squares fits to the quadrupole hfs measurements in all 17 atomic states studied in both configurations.

• Received 5 March 1970

DOI:https://doi.org/10.1103/PhysRevA.2.316