  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.
