Absolute frequencies of the ${}^{6,7}$Li 2$S$ ${}^2{S}_{1/2}\to \mathbf{3}S$ ${}^2{S}_{1/2}$ transitions

The measurement of the absolute frequencies of the $2S\to 3S$ 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 $2P\to 2S$ 670 nm fluorescence as the signal. The transition frequencies, including two isotopes (${}^{6,7}\mathrm{Li}$), were measured to an accuracy of $<330\text{kHz}$. In comparison with the previous GSI Group experiment, the frequency of the $2S_{1/2}\to 3S_{1/2}$ transition of ${}^7\mathrm{Li}$ is 815 618 181.45(9) MHz, which is improved by a factor of 2. The resultant hyperfine constants of the 3$S$ state and the deduced difference of the nuclear charge radii $\delta \langle r_c^2\rangle$ 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.

Figure 1

Partial level diagram of atomic lithium (not to scale).

Figure 2

Experimental apparatus.

Figure 3

Typical spectra of ${}^7\mathrm{Li}$ (upper) and ${}^6\mathrm{Li}$ (lower) hyperfine transitions. The horizontal axes are the laser frequency. The solid lines show the fitting residuals.

Figure 4

ac Stark shifts of the four transitions. The data points were fitted into a function of $\nu ={\nu }_0+k{P}_{\text{laser}}$. $P_{\text{laser}}$ is the total radiation power in the interaction region.

Figure 5

Chronological distribution of the ${}^7\mathrm{Li}$ $F=2–2$ absolute frequency measurements. The horizontal red line represents the resulting absolute frequency.

Figure 6

Allan deviation of the locked Ti:sapphire laser referenced to the OFC. The deviation is the widths of the beat-note frequency distribution measured with various integration times (1 ms to 1 s), while the laser is locked to the OFC. The inset is the frequency distribution fitted to a Gaussian profile with a 0.1 s integration time.