Precision spectroscopy of ${\mathrm{Li}}^{+}$ is a promising testing ground for bound-state quantum electrodynamics (QED) and for measurements of nuclear properties such as the Zemach radius. We investigate the hyperfine and fine-structure splittings of the $2{}^{3}S_1$ and $2{}^3{P}_J$ states of ${}^7\mathrm{Li}^{+}$ using saturated fluorescence spectroscopy based on a $\sim 460\mathrm{eV}$ metastable ion beam. We measure in particular the $2{}^{3}S_1–2{}^{3}P_J$ transitions in ${}^7\mathrm{Li}^{+}$. With a triple nested loop scanning method, the long-term drift and systematic uncertainties are reduced or eliminated, resulting in a total uncertainty of less than 100 kHz. Our results are in good agreement with QED calculations. For the hyperfine splittings of $2{}^{3}S_1$, our measured values have a similar accuracy to previous measurements and theoretical calculations. For the $2{}^3{P}_J$ fine and hyperfine splittings, our measured results are one order of magnitude more accurate than those of previous measurements and have a similar accuracy to the theoretical values. The measurements lay the foundation for future work on the ${\mathrm{Li}}^{+}$ isotopes and their theoretical interpretation in terms of nuclear charge radii and the Zemach radii.

• Received 12 February 2020
• Revised 19 August 2020
• Accepted 20 August 2020

DOI:https://doi.org/10.1103/PhysRevA.102.030801