Title: Isotope Shifts in the 7s$\rightarrow$8s Transition of Francium: Measurements and Comparison to \textit{ab initio} Theory,
Abstract: We observe the electric-dipole forbidden $7s\rightarrow8s$ transition in the
francium isotopes $^{208-211}$Fr and $^{213}$Fr using a two-photon excitation
scheme. We collect the atoms online from an accelerator and confine them in a
magneto optical trap for the measurements. In combination with previous
measurements of the $7s\rightarrow7p_{1/2}$ transition we perform a King Plot
analysis. We compare the thus determined ratio of the field shift constants
(1.230 $\pm$ 0.019) to results obtained from new ab initio calculations (1.234
$\pm$ 0.010) and find excellent agreement.