G. Zitzer

The hyperfine structure of $^{229}$Th$^{3+}$ ions in the nuclear ground state is investigated via laser spectroscopy of trapped Th$^{3+}$ ions that are sympathetically cooled by laser-cooled $^{88}$Sr$^+$ ions in a linear Paul trap. The isotope shift to $^{230}$Th$^{3+}$ and the hyperfine constants for the magnetic dipole (A) and electric quadrupole (B) interactions for the 5F$_{5/2}$ and 6D$_{5/2}$ electronic states of $^{229}$Th$^{3+}$ are determined. These measurements provide nuclear moments of $^{229}$Th with reduced uncertainty and serve as a preparation for improved hyperfine spectroscopy of the 8.4 eV nuclear isomeric state in $^{229}$Th$^{3+}$ ions.

Sympathetic cooling of Th$^{3+}$ ions is demonstrated in an experiment where $^{229}$Th and $^{230}$Th are extracted from uranium recoil ion sources and are confined in a linear Paul trap together with laser-cooled $^{88}$Sr$^+$ ions. Because of their similar charge-to-mass ratios the ions are closely coupled and arrange themselves in two-species Coulomb crystals, containing up to a few tens of Th$^{3+}$ ions. To show the suitability of the sympathetically cooled Th$^{3+}$ ions for high-resolution laser spectroscopy, the absolute frequencies and isotope shifts of 5F$_{5/2}$\,$\rightarrow$\,6D$_{5/2}$ and 5F$_{7/2}$\,$\rightarrow$\,6D$_{5/2}$ transitions of $^{230}$Th$^{3+}$ have been measured. The system is developed for hyperfine spectroscopy of electronic transitions of nuclear ground and isomeric states in $^{229}$Th$^{3+}$.