Precision Spectroscopy and Nuclear Structure Parameters in ^6,7Li⁺ ions

The optical Ramsey technique was employed to explore the 2 ³S₁ − 2 ³P_J transitions in ^6,7Li⁺, resulting

in the most precise measurements of the hyperfine splittings for the 2 ³S₁ and 2 ³P_J states,

with uncertainties as small as 10 kHz [1]. These results markedly reduce the uncertainties of previous

experiments by a factor of 4-5 for the 2 ³S₁ states and a factor of 5 to 50 for the 2 ³P_J states, while

also demonstrating improved agreement with theoretical values. Combining the measured hyperfine

intervals of the 2 ³S₁ states with the latest quantum electrodynamic calculations, we determined, respectively, the

improved Zemach radii of the ^6,7Li nuclei to be 2.44(2) fm and 3.35(1) fm. Utilizing the determined Zemach radii and the hyperfine structure measurements

of the 2 ³P_J states, we derived the nuclear electric quadrupole moments Q_d of ^6,7Li as −0.39(5) fm² and −3.86(5) fm²,

deviating from the current recommended values by 6.2σ and 1.8σ [2], respectively. Furthermore, the ratio Q_d(⁶Li)/Q_d(⁷Li) = 0.101(13), exhibiting a

significant deviation from the measured value of 0.020161(13) by LiF molecular spectroscopy [3]. These discrepancies are perplexing, and we call for additional

experimental measurements in atomic and molecular spectroscopy, electron scattering, and related theoretical calculations pertaining to lithium.