With a beam of unenriched lanthanum, the hyperfine structure (hfs) of ${}^{138,139}\mathrm{La}$ has been measured for the transition from the ${5d6s}^2{}^2{D}_{3/2}$ ground state to the $5d6s6p{}^4{F}_{5/2}$ level at 675.304 nm by means of atomic-beam diode-laser spectroscopy. Magnetic-dipole (A) and electric-quadrupole $\mathrm{}\left(B\right)\mathrm{}\mathrm{hfs}$ constants of the 0.090% abundant ${}^{138}\mathrm{La},$ as well as those of ${}^{139}\mathrm{La},$ have been determined for the ${5d6s}^2{}^2{D}_{3/2}$ and $5d6s6p{}^4{F}_{5/2}$ levels; this serves as a measurement of hyperfine structure constants A and B for ${}^{138}\mathrm{La}.$ The isotope shift between ${}^{139}\mathrm{La}$ and ${}^{138}\mathrm{La}$ has been obtained for the 675.304-nm transition and change in mean-square nuclear charge radii $\delta 〈r^2〉$ derived.

• Received 13 November 2000

DOI:https://doi.org/10.1103/PhysRevA.63.064501