Laser Mössbauer spectroscopy of ²²⁹Th in CaF₂

Mössbauer spectroscopy is widely used in chemistry, geology, and solid-state physics to probe the local physical and chemical environment of nuclei in materials. Here, we extended this technique into the optical range using a vacuum ultraviolet laser to probe the low-energy nuclear transitions of thorium-229 (²²⁹Th) doped in calcium fluoride (CaF₂) crystals. We discovered four distinct doping sites for the thorium ions, determined the characteristic electric field gradients emerging from the interaction with the host crystal, and identified the microscopic structure of the two dominant configurations. Site-selective laser excitation enabled the study of the isomeric state lifetime and laser-induced quenching for all sites. This technique provides a powerful probe of the nuclear environment, yielding foundational data for designing future solid-state nuclear clocks.