Hyperfine Structure and Quadrupole Moment of Lanthanum-139

Abstract

We have studied the hyperfine structure of ${\mathrm{La}}^{139}$ in the ground level $5d6s^2{}_{}{}^2{}_{}{}^{}D$ by the atomic-beam magnetic resonance method. The beam of lanthanum atoms was detected by a hot tungsten filament operated at 2680±30°K. Of the eight hyperfine intervals resulting from the interaction of a nuclear spin of $\frac{7}{2}$, with the $J$ values of the ground state, seven have been measured. They are, for $J=\frac{5}{2}$, ${\nu }_1=W(F=6\mathrm{})-W(F=5\mathrm{})=1120.902\mathrm{}\pm 0.005 \mathrm{Mc}/sec,$ ${\nu }_2=W(F=5\mathrm{})-W(F=4\mathrm{})=912.793\mathrm{}\pm 0.005 \mathrm{Mc}/sec,$ ${\nu }_3=W(F=4\mathrm{})-W(F=3\mathrm{})=716.288\mathrm{}\pm 0.003 \mathrm{Mc}/sec,$ ${\nu }_4=W(F=3\mathrm{})-W(F=2\mathrm{})=529.090\mathrm{}\pm 0.010 \mathrm{Mc}/sec,$ and for $J=\frac{3}{2}$, ${\nu }_6=W(F=5\mathrm{})-W(F=4\mathrm{})=737.967\mathrm{}\pm 0.015 \mathrm{Mc}/sec,$ ${\nu }_7=W(F=4\mathrm{})-W(F=3\mathrm{})=551.987\mathrm{}\pm 0.005 \mathrm{Mc}/sec,$ ${\nu }_8=W(F=3\mathrm{})-W(F=2\mathrm{})=391.603\mathrm{}\pm 0.010 \mathrm{Mc}/sec.$ From these values the magnetic dipole interaction constants are found to be $A^{\prime }(J=\frac{5}{2})=182.1706\mathrm{}\pm 0.0006$ and $A^{\prime \prime }(J=\frac{3}{2})=141.1959\mathrm{}\pm 0.0016$ Mc/sec, the quadrupole interaction constants to be $B^{\prime }=54.213\mathrm{}\pm 0.014$, and $B^{\prime \prime }=44.781\mathrm{}\pm 0.014$ Mc/sec, while the octupole interaction constants are zero within the accuracy of the present experiment. The unobserved line of the ${}_{}{}^2{}_{}{}^{}D_{\frac{5}{2}}^{}{}_{}{}^{}$ hyperfine multiplet is calculated to be ${\nu }_5=W(F=2\mathrm{})-W(F=1\mathrm{})=348.843\mathrm{}\pm 0.014$ Mc/sec. A serious discrepancy was noted between the observed $A$ values, both relatively and absolutely, and those given by theory. The assumption of $s$-configuration mixing apparently resolved this difficulty. The values of the nuclear quadrupole moment calculated from $B^{\prime }$ and $B^{\prime \prime }$ are fairly consistent with each other. The average value, after application of the Sternheimer correction is $Q=(0.268\mathrm{}\pm 0.010)\times {10}^{-24\mathrm{}}$ ${\mathrm{cm}}^2$.