This study reports the successful laser excitation of the isomeric transition in thorium-229 (²²⁹Th) within LiSrAlF₆ crystals. The research team observed two spectroscopic features near the nuclear transition energy. The first is a broad excitation feature with red-shifted fluorescence decaying over seconds. The second is a narrow, laser-linewidth-limited spectral feature at 148.38219(4) nm (2020407.3(5) GHz) with a decay lifetime of 568(13) s, assigned to the excitation of the ²²⁹Th isomeric state. The energy of this state was measured as 8.355733(2) eV in ²²⁹Th:LiSrAlF₆. The ²²⁹Th isomeric state, described by Nilsson quantum numbers (3/2)+, has the lowest energy of all known nuclear excited states and a long lifetime, making it suitable for applications like an optical nuclear clock. Recent work has improved the knowledge of the isomeric state energy, but precise measurement of the isomeric transition is needed for these applications. The study used a vacuum-ultraviolet (VUV) laser system to search for the transition between 147.43 nm and 182.52 nm. The observed narrow spectral feature at 148.38219(4) nm is assigned to the ²²⁹Th isomeric transition, as it does not appear in ²³²Th-doped crystals. The results were interpreted using ab initio crystal structure calculations, suggesting that the observed short-time fluorescence may result from coupling between the ²²⁹Th nucleus and the electronic and phononic degrees of freedom of the crystal. This coupling may explain why only a fraction of the ²²⁹Th nuclei contribute to the narrow isomeric transition. The experiments involved monitoring fluorescence from crystals after illumination with a VUV laser. Four different LiSrAlF₆ crystals were used, with varying ²²⁹Th doping densities. The VUV laser produced radiation via resonance-enhanced four-wave mixing in Xe gas. The crystal chamber was maintained in an Ar atmosphere to ensure high VUV transmission. The study found a narrow spectral feature at 148.38219(4) nm with a lifetime of 568(13) s, consistent with the isomeric transition. The measured isomeric decay lifetime was 568(13) s, with a vacuum lifetime of approximately 1860(43) s. The results suggest that the isomeric transition is now measured with laser spectroscopic precision, completing the journey started by KroThis study reports the successful laser excitation of the isomeric transition in thorium-229 (²²⁹Th) within LiSrAlF₆ crystals. The research team observed two spectroscopic features near the nuclear transition energy. The first is a broad excitation feature with red-shifted fluorescence decaying over seconds. The second is a narrow, laser-linewidth-limited spectral feature at 148.38219(4) nm (2020407.3(5) GHz) with a decay lifetime of 568(13) s, assigned to the excitation of the ²²⁹Th isomeric state. The energy of this state was measured as 8.355733(2) eV in ²²⁹Th:LiSrAlF₆. The ²²⁹Th isomeric state, described by Nilsson quantum numbers (3/2)+, has the lowest energy of all known nuclear excited states and a long lifetime, making it suitable for applications like an optical nuclear clock. Recent work has improved the knowledge of the isomeric state energy, but precise measurement of the isomeric transition is needed for these applications. The study used a vacuum-ultraviolet (VUV) laser system to search for the transition between 147.43 nm and 182.52 nm. The observed narrow spectral feature at 148.38219(4) nm is assigned to the ²²⁹Th isomeric transition, as it does not appear in ²³²Th-doped crystals. The results were interpreted using ab initio crystal structure calculations, suggesting that the observed short-time fluorescence may result from coupling between the ²²⁹Th nucleus and the electronic and phononic degrees of freedom of the crystal. This coupling may explain why only a fraction of the ²²⁹Th nuclei contribute to the narrow isomeric transition. The experiments involved monitoring fluorescence from crystals after illumination with a VUV laser. Four different LiSrAlF₆ crystals were used, with varying ²²⁹Th doping densities. The VUV laser produced radiation via resonance-enhanced four-wave mixing in Xe gas. The crystal chamber was maintained in an Ar atmosphere to ensure high VUV transmission. The study found a narrow spectral feature at 148.38219(4) nm with a lifetime of 568(13) s, consistent with the isomeric transition. The measured isomeric decay lifetime was 568(13) s, with a vacuum lifetime of approximately 1860(43) s. The results suggest that the isomeric transition is now measured with laser spectroscopic precision, completing the journey started by Kro