Frequency-comb based collinear laser spectroscopy of Be for nuclear structure investigations and many-body QED tests

TL;DR

This study used frequency-comb based collinear laser spectroscopy to measure transition frequencies in beryllium isotopes, enabling precise nuclear charge radius determination and testing of quantum electrodynamics in a three-electron system.

Contribution

It introduces a high-precision laser spectroscopy method using frequency combs for isotope shift and fine structure measurements in Be, improving accuracy and enabling fundamental tests.

Findings

Nuclear charge radii of Be isotopes were precisely determined.

The fine structure splitting was measured accurately, testing QED calculations.

Systematic uncertainties were thoroughly analyzed and minimized.

Abstract

Absolute transition frequencies of the $2s\,^2{\rm{S}}_{1/2}$ $\rightarrow$ $2p\,^2{\rm{P}}_{1/2,3/2}$ transitions in Be$^+$ were measured with a frequency comb in stable and short-lived isotopes at ISOLDE (CERN) using collinear laser spectroscopy. Quasi-simultaneous measurements in copropagating and counterpropagating geometry were performed to become independent from acceleration voltage determinations for Doppler-shift corrections of the fast ion beam. Isotope shifts and fine structure splittings were obtained from the absolute transition frequencies with accuracies better than 1\,MHz and led to a precise determination of the nuclear charge radii of $^{7,10-12}$Be relative to the stable isotope $^9$Be. Moreover, an accurate determination of the $2p$ fine structure splitting allowed a test of high-precision bound-state QED calculations in the three-electron system. Here, we describe the laser spectroscopic method in detail, including several tests that were carried out to determine or estimate systematic uncertainties. Final values from two experimental runs at ISOLDE are presented and the results are discussed.