Thorium-229 is a unique case in nuclear physics: it presents a metastable
first excited state Th-229m, just a few electronvolts above the nuclear ground
state. This so-called isomer is accessible by VUV lasers, which allows
transferring the amazing precision of atomic laser spectroscopy to nuclear
physics. Being able to manipulate the Th-229 nuclear states at will opens up a
multitude of prospects, from studies of the fundamental interactions in physics
to applications as a compact and robust nuclear clock. However, direct optical
excitation of the isomer or its radiative decay back to the ground state has
not yet been observed, and a series of key nuclear structure parameters such as
the exact energies and half-lives of the low-lying nuclear levels of Th-229 are
yet unknown. Here we present the first active optical pumping into Th-229m. Our
scheme employs narrow-band 29 keV synchrotron radiation to resonantly excite
the second excited state, which then predominantly decays into the isomer. We
determine the resonance energy with 0.07 eV accuracy, measure a half-life of
82.2 ps, an excitation linewidth of 1.70 neV, and extract the branching ratio
of the second excited state into the ground and isomeric state respectively.
These measurements allow us to re-evaluate gamma spectroscopy data that have
been collected over 40~years.