Accurately describing nuclear interactions within atomic nuclei remains a challenge, which hinders our exploration of new physics beyond the Standard Model. However, these nuclear interactions can be characterized by nuclear parameters such as the Zemach radius and the electric quadrupole moment, which are reflected in atomic spectra. Our work achieves high-precision measurements of lithium-ion hyperfine splittings at the level of 10 kHz, and directly extracts these important nuclear structure parameters. We observe significant discrepancies between our results and both nuclear theory and molecular spectra regarding the electric quadrupole moment. The result for ${}^7\mathrm{Li}$ deviates by $2.3\sigma$ from the currently recommended value, whereas the result for ${}^6\mathrm{Li}$ deviates by up to $6.2\sigma$ from the recommended value determined by molecular spectroscopy. We supplement the measurements with independent calculations based on nuclear structure theory. They provide support for the results obtained from the present ion spectroscopy measurements. Our results provide valuable information for characterizing nuclear forces, serve as sensitive benchmarks for testing nuclear structure theories, and enable critical comparisons with both electron-nuclear scattering and molecular spectroscopy.

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• Atomic spectra
• Fine & hyperfine structure
• Nuclear charge distribution
• Relativistic & quantum electrodynamic effects in atoms, molecules,& ions
• Zeeman effect