We study the $2S-3S$ transition of ${}^{6,7}\mathrm{L}\mathrm{i}$ by high-precision laser spectroscopy using two-photon Doppler-free excitation and photoionization detection. Interferometric cross referencing to metrologic Rb $3S-5D$ two-photon transitions allowed measurement of the transition isotope shift and hyperfine splitting in the $3S$ state with precision at the 30 kHz level. The results are $\mathrm{I}\mathrm{S}=11453.734(30)\mathrm{M}\mathrm{H}\mathrm{z}$, $A_{3S}({}^6\mathrm{L}\mathrm{i})=35.263(15)\mathrm{M}\mathrm{H}\mathrm{z}$, and $A_{3S}({}^7\mathrm{L}\mathrm{i})=93.106(11)\mathrm{M}\mathrm{H}\mathrm{z}$. Combined with recent theoretical work, the isotope shift yields a new value for the change in squared nuclear charge radii $\Delta R^2=0.47(5){\mathrm{f}\mathrm{m}}^2$. This is compared with other work and some existing discrepancies are resolved.

• Received 13 March 2003

DOI:https://doi.org/10.1103/PhysRevLett.91.043004