We present the isotope shifts of the $7s_{1/2}$ to $7p_{1/2}$ transition for francium isotopes ${}^{206-213}\mathrm{Fr}$ with reference to ${}^{221}\mathrm{Fr}$ collected from two experimental periods. The shifts are measured on a sample of atoms prepared within a magneto-optical trap by a fast sweep of radio-frequency sidebands applied to a carrier laser. King plot analysis, which includes literature values for $7s_{1/2}$ to $7p_{3/2}$ isotope shifts, provides a field shift constant ratio of 1.0520(10) and a difference between the specific mass shift constants of 170(100) GHz amu between the $D_1$ and $D_2$ transitions, of sufficient precision to differentiate between ab initio calculations.

• Received 4 June 2014

DOI:https://doi.org/10.1103/PhysRevA.90.052502