Laser excitation of the ²²⁹Th 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 red-shifted fluorescence that decays with a timescale of a few seconds. The second is a narrow, laser-linewidth-limited spectral feature at 148.38219(4)ₛₜₐₜ(20)sys nm (2020407.3(5)ₛₜₐₜ(30)sys GHz) that decays with a lifetime of 568(13)ₛₜₐₜ(20)sys s. This feature is assigned to the excitation of the ²²⁹Th nuclear isomeric state, whose energy is found to be 8.355733(2)ₛₜₐₜ(10)sys eV in ²²⁹Th::LiSrAlF₆.