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1. Photoinduced quenching of the ${}^{229}\mathrm{Th}$ isomer in a solid-state host

   https://doi.org/10.1103/glzr-thyw  

   Terhune, J_E S; Elwell, R; Tan, H_B Tran; Perera, U C; Morgan, H_W T; Alexandrova, A N; Derevianko, Andrei; Hudson, Eric R (June 2025, Physical Review Research)

   The population dynamics of the ${}^{229}\mathrm{Th}$isomeric state is studied in a solid-state host under laser illumination. A photoquenching process is observed, where off-resonant vacuum-ultraviolet (VUV) radiation leads to relaxation of the isomeric state. The cross-section for this photoquenching process is measured, and a model for the decay process, where photoexcitation of electronic states within the material band gap opens an internal conversion decay channel, is presented and appears to reproduce the measured cross-section. By engineering defects into ${}^{229}\mathrm{Th}$-doped solid-state hosts, this previously unrecognized photoquenching process may be used to reduce the clock transition readout time and thereby increase the stability of the nuclear clock. Published by the American Physical Society2025 

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2. Resonant X-ray excitation of the nuclear clock isomer 45Sc

   https://doi.org/10.1038/s41586-023-06491-w  

   Shvyd’ko, Yuri; Röhlsberger, Ralf; Kocharovskaya, Olga; Evers, Jörg; Geloni, Gianluca Aldo; Liu, Peifan; Shu, Deming; Miceli, Antonino; Stone, Brandon; Hippler, Willi; et al (October 2023, Nature)

   Abstract Resonant oscillators with stable frequencies and large quality factors help us to keep track of time with high precision. Examples range from quartz crystal oscillators in wristwatches to atomic oscillators in atomic clocks, which are, at present, our most precise time measurement devices¹. The search for more stable and convenient reference oscillators is continuing^2–6. Nuclear oscillators are better than atomic oscillators because of their naturally higher quality factors and higher resilience against external perturbations^7–9. One of the most promising cases is an ultra-narrow nuclear resonance transition in⁴⁵Sc between the ground state and the 12.4-keV isomeric state with a long lifetime of 0.47 s (ref. ¹⁰). The scientific potential of⁴⁵Sc was realized long ago, but applications require⁴⁵Sc resonant excitation, which in turn requires accelerator-driven, high-brightness X-ray sources¹¹that have become available only recently. Here we report on resonant X-ray excitation of the⁴⁵Sc isomeric state by irradiation of Sc-metal foil with 12.4-keV photon pulses from a state-of-the-art X-ray free-electron laser and subsequent detection of nuclear decay products. Simultaneously, the transition energy was determined as$${\mathrm{12,389.59}}_{+0.12\left({\rm{syst}}\right)}^{\pm 0.15\left({\rm{stat}}\right)}\,{\rm{eV}}$$ ${\mathrm{12,389.59}}_{+0.12\left(\mathrm{syst}\right)}^{\pm 0.15\left(\mathrm{stat}\right)}\mathrm{eV}$with an uncertainty that is two orders of magnitude smaller than the previously known values. These advancements enable the application of this isomer in extreme metrology, nuclear clock technology, ultra-high-precision spectroscopy and similar applications. 

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3. Tunable VUV frequency comb for ^229m Th nuclear spectroscopy

   https://doi.org/10.1364/ol.473006  

   Zhang, Chuankun; Li, Peng; Jiang, Jie; von der Wense, Lars; Doyle, John F.; Fermann, Martin E.; Ye, Jun (January 2022, Optics Letters)

   Laser spectroscopy of the 229m Th nuclear clock transition is necessary for the future construction of a nuclear-based optical clock. Precision laser sources with broad spectral coverage in the vacuum ultraviolet are needed for this task. Here, we present a tunable vacuum-ultraviolet frequency comb based on cavity-enhanced seventh-harmonic generation. Its tunable spectrum covers the current uncertainty range of the 229m Th nuclear clock transition. 

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4. Proposal and theoretical investigation of 229Th-doped nonlinear optical crystals for compact solid-state clocks

   https://doi.org/10.1063/5.0247867  

   Morgan, H_W T; Elwell, R; Terhune, J_E S; Tran_Tan, H B; Perera, U C; Derevianko, A; Alexandrova, A N; Hudson, E R (March 2025, Applied Physics Letters)

   The recent laser excitation of the 229Th isomeric transition in a solid-state host opens the door for a portable solid-state nuclear optical clock. However, at present, the vacuum-ultraviolet laser systems required for clock operation are not conducive to a fieldable form factor. Here, we propose a possible solution to this problem by using 229Th-doped nonlinear optical crystals, which would allow clock operation without a vacuum-ultraviolet laser system and without the need of maintaining the crystal under vacuum. We investigate electronic properties and thorium doping in BaMgF4 and BaZnF4 with density functional theory, predicting BaMgF4 to be the superior material, and evaluate the performance of a Th:BaMgF4 clock. 

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5. Influence of hydrogen and oxygen surface termination on the mechanism of sub-bandgap photoelectron emission from diamond(111) into vacuum and into water

   https://doi.org/10.1016/j.diamond.2025.112011  

   Saucedo, Cesar; Rieders, Nathaniel; Hamers, Robert J (January 2025, Diamond and Related Materials)

   Wide-bandgap semiconductors have unique electron emission properties by virtue of having high-lying conduction bands. Among these, diamond stands out because of its chemical stability, allowing it to serve as a solid-state electron source in vacuum and non-vacuum environments, including water. However, the underlying mechanisms of electron emission are not well understood. Here, we report investigations of the mechanisms of electron emission from H-terminated and oxidized surfaces of single-crystal boron-doped diamond(111) in vacuum and in water using both sub-bandgap (4.75 eV and 3.05 eV) and above-bandgap (21.2 eV) excitation. Energy-resolved photoemission spectra in vacuum using different incident photon energies reveal two distinct energy distributions, reflecting different emission pathways. While oxidation greatly reduces electron emission into vacuum using both sub-bandgap and above-bandgap sources, facile electron emission into water persists on the oxidized samples using sub-bandgap excitation and is directly observed through transient optical absorption measurements using sub-bandgap excitation. Low-energy inverse photoemission spectroscopy shows that oxidation leads to broad distribution of surface states throughout the diamond bandgap. Our studies highlight 

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