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.