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Observation of the 717 nm electric quadrupole transition in triply charged thorium

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.