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1. Precision Rydberg state spectroscopy with slow electrons and the proton-radius puzzle

   https://doi.org/10.1103/PhysRevA.108.062822  

   Jentschura, Ulrich D; Yost, Dylan C (December 2023, Physical Review A)

   The so-called proton-radius puzzle (the current discrepancy of proton radii determined from spectroscopic measurements in ordinary versus muonic hydrogen) could be addressed via an accurate measurement of the Rydberg constant because the proton radius and the Rydberg constant values are linked through high-precision optical spectroscopy. We argue that, with manageable additional experimental effort, it might be possible to improve circular Rydberg state spectroscopy, potentially leading to an important contribution to the clarification of the puzzle. Our proposal involves circular and near-circular Rydberg states of hydrogen with a principal quantum number around n = 18, whose classical velocity on a Bohr orbit is slower than that of the fastest macroscopic man-made object, the Parker Solar Probe. We obtain improved estimates for the quality factor of pertinent transitions and illustrate a few recent improvements in instrumentation which facilitate pertinent experiments. 

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2. The MUSE Beam Line Calorimeter

   https://doi.org/10.1051/epjconf/202532000031  

   Lin, Win (January 2025, EPJ Web of Conferences)

   Chujo, T; Ootani, W (Ed.)

   The MUon proton Scattering Experiment (MUSE) at the PiM1 beam line of the Paul Scherrer Institute is simultaneously measuring the elastic scattering of electrons and muons from a liquid hydrogen target to extract the charge radius of the proton with both positive and negative beam polarities. In addition to providing precise data for addressing the proton radius puzzle, comparing the four scattering cross sections directly tests lepton universality, radiative corrections, and two-photon exchange effects for electrons and muons. In order to study radiative correction and get more precise incident lepton energy at the scattering vertex for the cross section measurements, MUSE uses a lead-glass calorimeter located at the downstream end of the beam line. This proceeding discusses the specifications and calibration process of the calorimeter detector. Data are compared to simulation to demonstrate the performance of the detector. 

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3. Phase transitions in ε-FeOOH at high pressure and ambient temperature

   https://doi.org/10.2138/am-2020-7468  

   Thompson, Elizabeth C.; Davis, Anne H.; Brauser, Nigel M.; Liu, Zhenxian; Prakapenka, Vitali B.; Campbell, Andrew J. (December 2020, American Mineralogist)

   null (Ed.)

   Abstract Constraining the accommodation, distribution, and circulation of hydrogen in the Earth's interior is vital to our broader understanding of the deep Earth due to the significant influence of hydrogen on the material and rheological properties of minerals. Recently, a great deal of attention has been paid to the high-pressure polymorphs of FeOOH (space groups P21nm and Pnnm). These structures potentially form a hydrogen-bearing solid solution with AlOOH and phase H (MgSiO4H2) that may transport water (OH–) deep into the Earth's lower mantle. Additionally, the pyrite-type polymorph (space group Pa3 of FeOOH), and its potential dehydration have been linked to phenomena as diverse as the introduction of hydrogen into the outer core (Nishi et al. 2017), the formation of ultralow-velocity zones (ULVZs) (Liu et al. 2017), and the Great Oxidation Event (Hu et al. 2016). In this study, the high-pressure evolution of FeOOH was re-evaluated up to ~75 GPa using a combination of synchrotron-based X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and optical absorption spectroscopy. Based on these measurements, we report three principal findings: (1) pressure-induced changes in hydrogen bonding (proton disordering or hydrogen bond symmetrization) occur at substantially lower pressures in ε-FeOOH than previously reported and are unlikely to be linked to the high-spin to low-spin transition; (2) ε-FeOOH undergoes a 10% volume collapse coincident with an isostructural Pnnm → Pnnm transition at approximately 45 GPa; and (3) a pressure-induced band gap reduction is observed in FeOOH at pressures consistent with the previously reported spin transition (40 to 50 GPa). 

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4. Determination of nuclear charge radius by extreme-ultraviolet spectroscopy of Na-like ions

   https://doi.org/10.1103/PhysRevResearch.7.L012024  

   Hosier, A; Dipti; Blundell, S A; Lapierre, A; Silwal, R; Gwinner, G; Tan, J N; Naing, A; Gillaspy, J D; Yang, Y; et al (February 2025, Physical Review Research)

   We report on a method for determining the absolute nuclear charge radius of high- $Z$elements using extreme-ultraviolet spectroscopy of highly charged Na-like ions in tandem with highly accurate atomic structure calculations of transition energy differences. The application of this method has reduced the nuclear charge radius uncertainty of ${}^{191}\mathrm{Ir}$by a factor of 8 from the currently accepted literature value, with a recently reported charge radius of 5.435(12) fm. The result reduces the charge radius uncertainty along the full Ir isotopic chain when combined with prior optical isotope shift measurements. The technique utilizes only a few million ions stored in an ion trap, which should apply to measurements with small quantities of radioactive nuclei. Published by the American Physical Society2025 

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5. Optical Properties and Photocarrier Dynamics of Bi ₂ O ₂ Se Monolayer and Nanoplates

   https://doi.org/10.1002/adom.201901567  

   Liu, Shuangyan; Tan, Congwei; He, Dawei; Wang, Yongsheng; Peng, Hailin; Zhao, Hui (January 2020, Advanced Optical Materials)

   Abstract A comprehensive experimental study on optical properties and photocarrier dynamics in Bi₂O₂Se monolayers and nanoplates is presented. Large and uniform Bi₂O₂Se nanoplates with various thicknesses down to the monolayer limit are fabricated. In nanoplates, a direct optical transition near 720 nm is identified by optical transmission, photoluminescence, and transient absorption spectroscopic measurements and is attributed to the transition between the valence and conduction bands in the Γ valley. Time‐resolved differential reflection measurements reveal ultrafast carrier thermalization and energy relaxation processes and a photocarrier recombination lifetime of about 200 ps in nanoplates. Furthermore, by spatially resolving the differential reflection signal, a photocarrier diffusion coefficient of about 4.8 cm²s⁻¹is obtained, corresponding to a mobility of about 180 cm²V⁻¹s⁻¹. A similar direct transition is also observed in monolayer Bi₂O₂Se, suggesting that the states in the Γ valley do not change significantly with the thickness. The temporal dynamics of the excitons in the monolayer is quite different from the nanoplates, with a strong saturation effect and fast exciton–exciton annihilation at high densities. Spatially and temporally resolved measurements yield an exciton diffusion coefficient of about 20 cm²s⁻¹. 

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