The $2s\to 3s$ transition of ${}^{6,7,8,9}\mathrm{L}\mathrm{i}$ was studied by high-resolution laser spectroscopy using two-photon Doppler-free excitation and resonance-ionization detection. Hyperfine structure splittings and isotope shifts were determined with precision at the 100 kHz level. Combined with recent theoretical work, the changes in the nuclear-charge radii of ${}^{8,9}\mathrm{L}\mathrm{i}$ were determined. These are now the lightest short-lived isotopes for which the charge radii have been measured. It is found that the charge radii monotonically decrease with increasing neutron number from ${}^6\mathrm{L}\mathrm{i}$ to ${}^9\mathrm{L}\mathrm{i}$.

• Received 20 May 2004

DOI:https://doi.org/10.1103/PhysRevLett.93.113002