Isotope shift measurements on the D1 line in francium isotopes at TRIUMF*

Francium is the heaviest alkali and has no stable isotopes. The longest-lived among them, with half-lives from seconds to a few minutes, are now available in the new Francium Trapping Facility at TRIUMF, Canada, for future weak interaction studies. We present isotope shift measurements on the $7S_{1/2} \rightarrow 7P_{1/2}$ ($D1$) transition on three isotopes, 206, 207 and 213 in a magneto-optical trap. The shifts are measured using a c.w. Ti:sapphire laser locked to a stabilized cavity at the mid-point between two hyperfine transitions of the reference isotope $^{209}$Fr. Scanning tunable microwave sidebands locate transitions in the other isotopes. In combination with the $D2$ isotope shifts, analysis can provide a separation of the field shift, due to a changing nuclear charge radius, and specific mass shift, due to changing electron correlations, in these isotopes.