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

We excite the 717-nm electric quadrupole 6D3/2 ↔ 7S1/2 transition in a laser-cooled 232Th3+ ion crystal. The transition frequency and the lifetime of the metastable 7S1/2 level are measured to be 417 845 964(30) MHz and 0.60(7) s, respectively. We subsequently employ the 7S1/2 level to drive the ions with nanosecond-long 269-nm laser pulses into the 7P1/2 level. The latter is connected to the 7S1/2 electronic level within the 229Th nuclear isomer manifold by the strongest available electron-bridge transition, forming a basis for its laser excitation.