Optical spectroscopy of an atomic nucleus: Progress toward direct observation of the 229Th isomer transition

► Thorium-229 has a long-lived nuclear excited state. ► It is the only known nuclear isomer within the reach of optical spectroscopy. ► Thorium-229 doped fluoride crystals may offer sufficiently high luminescence rates. ► Luminescence excitation spectroscopy in the vacuum ultraviolet spectral region may enable the first direct observation of the exceptionally narrow isomer transition. ► Intrinsic excitation of thorium-229 via alpha-decay of uranium-233 is an alternative approach.