Laser Excitation of the Th-229 Nucleus

The low-energy nuclear isomer state in Th-229 is resonantly excited in Th-doped calciumfluoride crystals using a tabletop tunable laser system at 148 nm wavelength. A resonance fluorescence signal is observed in two crystals with different Th-229 dopant concentrations, while it is absent in a control experiment using Th-232. The nuclear resonance frequency for the Th4+ ions in Th:CaF2 is measured with an uncertainty of 7 GHz. The fluorescence lifetime in the crystal is 630(15) s, corresponding to an isomer half-life of 1740(50) s for a nucleus isolated in vacuum. These results pave the way towards high-resolution nuclear laser spectroscopy of Th-229 and an optical nuclear clock with high sensitivity in fundamental tests.