Probing the spatial distribution of nuclear magnetism in francium by optical spectroscopy

The recently commissioned Francium Trapping Facility at TRIUMF in Vancouver, Canada will enable experiments to study weak interactions in francium atoms. We have successfully trapped and cooled $^{206,207,209,213,221}$Fr isotopes in large quantities (10$^4$ to 10$^5$) with trap lifetimes comparable to the radioactive lifetimes of the shortest lived trapped isotope (t$_{1/2}$ = 14.8s). We use a combination of radio-frequency and optical spectroscopy to determine the hyperfine splittings of the $7P_{1/2}$ level of isotopes $^{206,207,209,213}$Fr to the 100 ppm level. These measurements, in combination with the known hyperfine ground state splittings, can be used to study the hyperfine anomaly in these isotopes. Our results extend previous work on the neutron distribution\footnote{Grossman {\it et al.}, {\it Phys. Rev. Lett.} {\textbf 83}, 935 (1999).} to a closed neutron shell isotope (213) and to neutron deficient isotopes (206, 207). These spectroscopic measurements also allow us to extract the isotope shifts to study changes in the charge radius.