  The isotope $^{229}$Th is the only nucleus known to possess an excited state
$^{229m}$Th in the energy range of a few electron volts, a transition energy
typical for electrons in the valence shell of atoms, but about four orders of
magnitude lower than common nuclear excitation energies. A number of
applications of this unique nuclear system, which is accessible by optical
methods, have been proposed. Most promising among them appears a highly precise
nuclear clock that outperforms existing atomic timekeepers. Here we present the
laser spectroscopic investigation of the hyperfine structure of
$^{229m}$Th$^{2+}$, yielding values of fundamental nuclear properties, namely
the magnetic dipole and electric quadrupole moments as well as the nuclear
charge radius. After the recent direct detection of this long-searched-for
isomer, our results now provide detailed insight into its nuclear structure and
present a method for its non-destructive optical detection.
