Laser Excitation of the Th229 Nuclear Isomeric Transition in a Solid-State Host

LiSrAlF₆ crystals doped with ^229Th are used in a laser-based search for the nuclear isomeric transition. Two spectroscopic features near the nuclear transition energy are observed. The first is a broad excitation feature that produces redshifted fluorescence that decays with a timescale of a few seconds. The second is a narrow, laser-linewidth-limited spectral feature at 148. 382 19 (4) ₒₓ₀ₓ (20) ₒₘₒ nm 2020 407. 3 (5) ₒₓ₀ₓ (30) ₒₘₒ GHz that decays with a lifetime of 568 (13) ₒₓ₀ₓ (20) ₒₘₒ s. This feature is assigned to the excitation of the ^229Th nuclear isomeric state, whose energy is found to be 8. 355 733 (2) ₒₓ₀ₓ (10) ₒₘₒ eV in ^229Th: LiSrAlF₆.