Wigner Crystals of 229Th for Optical Excitation of the Nuclear Isomer

We have produced laser-cooled Wigner crystals of Th-229(3+) in a linear Paul trap. The magnetic dipole (A) and electric quadrupole (B) hyperfine constants for four low-lying electronic levels and the relative isotope shifts with respect to Th-232(3+) for three low-lying optical transitions are measured. Using the hyperfine B constants in conjunction with prior atomic structure calculations, a new value of the spectroscopic nuclear electric quadrupole moment Q = 3: 11(16) eb is deduced. These results are a step towards optical excitation of the low-lying isomer level in the Th-229 nucleus.