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International audience; In this paper we present the first high-resolution laser spectroscopy results obtained at the GISELE laser laboratory of the GANIL-SPIRAL2 facility, in preparation for the first experiments with the S$^3$-Low Energy Branch. Studies of neutron-deficient radioactive isotopes of erbium and tin represent the first physics cases to be studied at S$^3$. The measured isotope-shift and hyperfine structure data are presented for stable isotopes of these elements. The erbium isotopes were studied using the $4f^{12}6s^2$$^3H_6 \rightarrow 4f^{12}(^3 H)6s6p$$J = 5$ atomic transition (415 nm) and the tin isotopes were studied by the $5s^25p^2 (^3P_0) \rightarrow 5s^25p6s (^3P_1)$ atomic transition (286.4 nm), and are used as a benchmark of the laser setup. Additionally, the tin isotopes were studied by the $5s^25p6s (^3P_1) \rightarrow 5s^25p6p (^3P_2)$ atomic transition (811.6 nm), for which new isotope-shift data was obtained and the corresponding field-shift $F_{812}$ and mass-shift $M_{812}$ factors are presented.

The radionuclide thorium-229 features an isomer with an exceptionally low excitation energy that enables direct laser manipulation of nuclear states. It constitutes one of the leading candidates for use in next-generation optical clocks$^{1–3}$. This nuclear clock will be a unique tool for precise tests of fundamental physics$^{4–9}$. Whereas indirect experimental evidence for the existence of such an extraordinary nuclear state is substantially older$^{10}$, the proof of existence has been delivered only recently by observing the isomer’s electron conversion decay$^{11}$. The isomer’s excitation energy, nuclear spin and electromagnetic moments, the electron conversion lifetime and a refined energy of the isomer have been measured$^{12–16}$. In spite of recent progress, the isomer’s radiative decay, a key ingredient for the development of a nuclear clock, remained unobserved. Here, we report the detection of the radiative decay of this low-energy isomer in thorium-229 ($^{229m}$Th). By performing vacuum-ultraviolet spectroscopy of $^{229m}$Th incorporated into large-bandgap CaF$_{2}$ and MgF$_{2}$ crystals at the ISOLDE facility at CERN, photons of 8.338(24) eV are measured, in agreement with recent measurements$^{14–16}$ and the uncertainty is decreased by a factor of seven. The half-life of $^{229m}$Th embedded in MgF$_{2}$ is determined to be 670(102) s. The observation of the radiative decay in a large-bandgap crystal has important consequences for the design of a future nuclear clock and the improved uncertainty of the energy eases the search for direct laser excitation of the atomic nucleus. The nucleus of the radioisotope thorium-229 (${}^{229}$Th) features an isomer with an exceptionally low excitation energy that enables direct laser manipulation of nuclear states. For this reason, it is a leading candidate for use in next-generation optical clocks. This nuclear clock will be a unique tool, amongst others, for tests of fundamental physics. While first indirect experimental evidence for the existence of such an extraordinary nuclear state is significantly older, the proof of existence has been delivered only recently by observing the isomer's electron conversion decay and its hyperfine structure in a laser spectroscopy study, revealing information on the isomer's excitation energy, nuclear spin and electromagnetic moments. Further studies reported the electron conversion lifetime and refined the isomer's energy. In spite of recent progress, the isomer's radiative decay, a key ingredient for the development of a nuclear clock, remained unobserved. In this Letter, we report the detection of the radiative decay of this low-energy isomer in thorium-229 (${}^{229\mathrm{m}}$Th). By performing vacuum-ultraviolet spectroscopy of ${}^{229\mathrm{m}}$Th incorporated into large-bandgap CaF${}_2$ and MgF${}_2$ crystals at the ISOLDE facility at CERN, the photon vacuum wavelength of the isomer's decay is measured as 148.71(42) nm, corresponding to an excitation energy of 8.338(24) eV. This value is in agreement with recent measurements, and decreases the uncertainty by a factor of seven. The half-life of ${}^{229\mathrm{m}}$Th embedded in MgF${}_2$ is determined to be 670(102) s. The observation of the radiative decay in a large-bandgap crystal has important consequences for the design of a future nuclear clock and the improved uncertainty of the energy eases the search for direct laser excitation of the atomic nucleus.