Abstract
Lithium 2S1/2 → 3S1/2 transition has been observed by high-precision laser spectroscopy using two-photon Doppler-free excitation and fluorescence detection. The two-photon excitation is performed in a weakly collimated atomic beam using a titanium-sapphire (TIS) ring laser at 735 nm with 450 mW laser power. Four transition lines, including isotopes 6Li and 7Li, were observed and the linewidth is 10 MHz and SNR of the strongest line is ~100. Absolute frequencies of all hyperfine components have been measured to an uncertainty of 280 kHz using optical femtosecond comb. The resulting frequency of 7Li : F = 2→2 is 407808975.87(13) MHz and isotope shift is 11454.95(51) MHz. There are discrepancies with recent work, and this may be due to laser instability in our system. The results are compared with the theoretical works, including relativistic and QED energy contributions, and the accuracy is improved by an order of magnitude. Combined with theory, the squared nuclear radii difference between 6Li and 7Li is 1.2(3) fm2.