Hyperfine and Zeeman Studies of Low-Lying Atomic Levels ofLa139and the Nuclear Electric-Quadrupole Moment

The atomic-beam magnetic-resonance technique has been used to determine the hyperfine-interaction constants $A$, $B$, and $C$ and electron $g$ values $g_J$ for all previously unstudied atomic levels of ${\mathrm{La}}^{139}$ below 9000 ${\mathrm{cm}}^{-1\mathrm{}}$. The results are analyzed in detail in terms of a set of eigen-vectors spanning the three configurations $5d6s^2$, $5d^{2}6s$, and $5d^3$. Corrections for hyperfine and Zeeman interactions between all low-lying states are carried out in intermediate coupling. The value deduced from the observed hyperfine structure for $Q$, the ${\mathrm{La}}^{139}$ nuclear ground-state electric-quadrupole moment, is influenced strongly by taking account of (a) the large recently found configuration mixing between $5d6s^2$, $5d^{2}6s$, and $5d^3$ and (b) the rather substantial Sternheimer shielding effects. The effects (a) and (b), though both about 30%, are in opposite directions and partially cancel. The value obtained is $Q=(+0.22\mathrm{}\pm 0.03)b$. Although the order of consistency between the experimental results and theory is encouraging, a number of problems remain.