Nuclear Charge Radii of Sr Isotopes: Reevaluation based on Transition Frequency Measurements in the $5s-5p-4d$ manifold in Sr$^+$

Abstract

High-precision quasi-simultaneous collinear and anticollinear laser spectroscopy was performed on the $5s\,^2S_{1/2}\rightarrow 5p\,^2P_{1/2}$ (D1), $5s\,^2S_{1/2}\rightarrow 5p\,^2P_{3/2}$ (D2), and three $4d\rightarrow 5p$ transitions in the naturally abundant Sr$^+$ isotopes. Absolute transition frequencies were determined with uncertainties as low as $600$ kHz, while common-mode rejection enabled the extraction of isotope shifts with uncertainties down to $200$ kHz, one order of magnitude smaller than previously achieved. The uncertainties of the hyperfine-structure constants of the $5p$ states and the $4d\,^2D_{3/2}$ state in $^{87}$Sr were also improved. A King-plot analysis yielded a field-shift ratio of the D2 and D1 lines of $F_{\mathrm{D2}}/F_{\mathrm{D1}}=1.004(5)$, which lies within the theoretically allowed region and provides a benchmark for atomic-structure calculations. Using data from all stable isotopes and investigated transitions, we compare field- and mass-shift constants obtained using several established approaches, ranging from King plots based entirely on experimental input to state-of-the-art \textit{ab initio} atomic-structure calculations. We show that, above $N=50$, the extracted charge radii depend strongly on the approach used.