Absolute frequencies of the 6,7Li 2S 2S1/2 → 3S 2S1/2 transitions

The measurement of the absolute frequencies of the 2S -> 3S of atomic lithium is reported. To reduce systematic effects, we employed a frequency-comb-stabilized excitation laser, a weakly collimated atomic beam, and the cascading 2P -> 2S 670 nm fluorescence as the signal. The transition frequencies, including two isotopes (Li-6,Li-7), were measured to an accuracy of < 330 kHz. In comparison with the previous GSI Group experiment, the frequency of the 2S(1/2) -> 3S(1/2) transition of Li-7 is 815 618 181.45(9) MHz, which is improved by a factor of 2. The resultant hyperfine constants of the 3S state and the deduced difference of the nuclear charge radii delta < r(c)(2)> from the isotope shift are in good agreement with previous results. Since a more straightforward methodology is adopted, our measurement is less model dependent and serves as an independent investigation of the reported transitions.