We excite the 717 nm electric quadrupole 6$D_{ 3/2} $ $\leftrightarrow$ 7$S_{ 1/2}$ transition in a laser-cooled $^{ 232}$Th$^{ 3+}$ ion crystal. The transition frequency and the lifetime of the metastable 7$S_{ 1/2}$ level are measured to be 417 845 964(30) MHz and 0.60(7) s, respectively. We subsequently employ the 7$S_{ 1/2}$ level to drive the ions with nanosecond-long 269 nm laser pulses into the 7$P_{ 1/2}$ level. The latter is connected to the 7$S_{ 1/2}$ electronic level within the $^{ 229}$Th nuclear isomer manifold by the strongest available electron-bridge transition, forming a basis for its laser excitation.
Categories: physics.atom-ph