UCLA

LiSrAlF_{6} crystals doped with ^{229}Th 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)_{stat}(20)_{sys}  nm [2020 407.3(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 ^{229}Th nuclear isomeric state, whose energy is found to be 8.355 733(2)_{stat}(10)_{sys}  eV in ^{229}Th:LiSrAlF_{6}.