Nuclear Charge Radii of B 10 , 11

The first laser spectroscopic determination of the change in the nuclear charge radius for a five-electron system is reported. This is achieved by combining high-accuracy ab initio mass-shift calculations and a high-accuracy measurement of the isotope shift in the $2{s}^{2}2p\text{ }\text{ }{^{2}P}_{1/2}\ensuremath{\rightarrow}2{s}^{2}3s\text{ }\text{ }{^{2}S}_{1/2}$ ground state transition in boron atoms. Accuracy is increased by orders of magnitude for the stable isotopes $^{10,11}\mathrm{B}$ and the results are used to extract their difference in the mean-square charge radius $⟨{r}_{c}^{2}{⟩}^{11}\ensuremath{-}⟨{r}_{c}^{2}{⟩}^{10}=\ensuremath{-}0.49(12)\text{ }\text{ }{\mathrm{fm}}^{2}$. The result is qualitatively explained by a possible cluster structure of the boron nuclei and quantitatively used to benchmark new ab initio nuclear structure calculations using the no-core shell model and Green's function Monte Carlo approaches. These results are the foundation for a laser spectroscopic determination of the charge radius of the proton-halo candidate $^{8}\mathrm{B}$.