Mass-separated samples of the unstable nuclides ${}_{}{}^{128}{}_{}{}^{}\mathrm{Ba}$ ($T_{\frac{1}{2}}=2.4\mathrm{}$ d) and ${}_{}{}^{131}{}_{}{}^{}\mathrm{Ba}$ ($T_{\frac{1}{2}}=11.5\mathrm{}$ d) have been investigated by means of Doppler-free laser spectroscopy, using cw dye lasers and a well-collimated atomic beam. From isotope shifts and hyperfine structure of the ${}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}-{}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{}$ transition ($\lambda =553.6\mathrm{}$ nm) we obtained the differences of mean-square charge radii $\delta 〈r^2〉({}_{}{}^{131}{}_{}{}^{}\mathrm{Ba}-{}_{}{}^{130}{}_{}{}^{}\mathrm{Ba})=-0.006\mathrm{}\left(3\right)\mathrm{}$ ${\mathrm{fm}}^2$ and $\delta 〈r^2〉({}_{}{}^{130}{}_{}{}^{}\mathrm{Ba}-{}_{}{}^{128}{}_{}{}^{}\mathrm{Ba})=-0.022\mathrm{}\left(3\right)\mathrm{}$ ${\mathrm{fm}}^2$ and the magnetic moment of ${}_{}{}^{131}{}_{}{}^{}\mathrm{Ba}$, $\mu =-0.714\mathrm{}\left(6\right)\mathrm{}{\mu }_N$.

• Received 26 May 1977

DOI:https://doi.org/10.1103/PhysRevLett.39.332