Laser excitation of the ${}^{229}$Th nuclear isomeric transition in a solid-state host

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 red-shifted fluorescence that decays with a timescale of a few seconds. The second is a narrow, laser-linewidth-limited spectral feature at $148.38219(4{)}_{\text{stat}}(20{)}_{\text{sys}}$ nm ($2020407.3(5{)}_{\text{stat}}(30{)}_{\text{sys}}$ GHz) that decays with a lifetime of $568(13{)}_{\text{stat}}(20{)}_{\text{sys}}$ s. This feature is assigned to the excitation of the ${}^{229}$Th nuclear isomeric state, whose energy is found to be $8.355733(2{)}_{\text{stat}}(10{)}_{\text{sys}}$ eV in ${}^{229}$Th:\thor:LiSrAlF${}_6$.