Hyperfine Structure and Quadrupole Moment of Lanthanum-139

We have studied the hyperfine structure of La¹³⁹ in the ground level 5d6s²²D 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 7/2, with the J values of the ground state, seven have been measured. They are, for J=5/2, ν₁=W(F=6)-W(F=5)=1120.902±0.005 Mc/sec, ν₂=W(F=5)-W(F=4)=912.793±0.005 Mc/sec, ν₃=W(F=4)-W(F=3)=716.288±0.003 Mc/sec, ν₄=W(F=3)-W(F=2)=529.090±0.010 Mc/sec, and for J=3/2, ν₆=W(F=5)-W(F=4)=737.967±0.015 Mc/sec, ν₇=W(F=4)-W(F=3)=551.987±0.005 Mc/sec, ν₈=W(F=3)-W(F=2)=391.603±0.010 Mc/sec. From these values the magnetic dipole interaction constants are found to be A^'(J=5/2)=182.1706±0.0006 and A^''(J=3/2)=141.1959±0.0016 Mc/sec, the quadrupole interaction constants to be B^'=54.213±0.014, and B^''=44.781±0.014 Mc/sec, while the octupole interaction constants are zero within the accuracy of the present experiment. The unobserved line of the ²D5/2 hyperfine multiplet is calculated to be ν₅=W(F=2)-W(F=1)=348.843±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^' and B^'' are fairly consistent with each other. The average value, after application of the Sternheimer correction is Q=(0.268±0.010)×10^-24 cm².