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1. Magneto-optical trap of titanium atoms

   https://doi.org/10.1103/PhysRevResearch.7.023025  

   Eustice, Scott; Schrott, Jackson; Stöltzel, Anke; Wolf, Julian; Novoa, Diego; Cassella, Kayleigh; Stamper-Kurn, Dan M (April 2025, Physical Review Research)

   We realize a magneto-optical trap (MOT) of titanium (Ti) atoms, performing laser cooling on the 498 nm transition between the long-lived $3{\mathrm{d}}^3\left({}^4\mathrm{F}\right)4\mathrm{s}{\mathrm{a}}^5{\mathrm{F}}_5$metastable state and the $3{\mathrm{d}}^3\left({}^4\mathrm{F}\right)4\mathrm{p}{\mathrm{y}}^5{\mathrm{G}}_6^{\mathrm{o}}$excited state. Without the addition of any repumping light, we observe MOTs of the three stable, $I=0$bosonic isotopes, ${}^{46}\mathrm{Ti}, {}^{48}\mathrm{Ti}$, and ${}^{50}\mathrm{Ti}$. Up to $8.30\left(26\right)\times {10}^5{}^{48}\mathrm{Ti}$atoms are trapped at a maximum density of $1.3\left(4\right)\times {10}^{11}{\mathrm{cm}}^{-3}$and at a temperature of $90\left(15\right)\mu \mathrm{K}$. By measuring the decay of the MOT, we constrain the leakage branching ratio of the cooling transition ( $\le 2.5\times {10}^{-6}$) and the two-body loss coefficient ( $\le 2\times {10}^{-10}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$). Our approach to laser cooling Ti can be applied to other transition metals, enabling a significant expansion of the elements that can be laser cooled. Published by the American Physical Society2025 

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2. An optical atomic clock using $4D_J$ states of rubidium

   https://doi.org/10.1088/2058-9565/ad77ef  

   Duspayev, A.; Owens, C.; Dash, B.; Raithel, G. (September 2024, Quantum Science and Technology)

   Abstract We analyze an optical atomic clock using two-photon $5S_{1/2}\to 4D_J$transitions in rubidium. Four one- and two-color excitation schemes to probe the $4D_{3/2}$and $4D_{5/2}$fine-structure states are considered in detail. We compare key characteristics of Rb $4D_J$and $5D_{5/2}$two-photon clocks. The $4D_J$clock features a high signal-to-noise ratio due to two-photon decay at favorable wavelengths, low dc electric and magnetic susceptibilities, and minimal black-body shifts. Ac Stark shifts from the clock interrogation lasers are compensated by two-color Rabi-frequency matching. We identify a ‘magic’ wavelength near 1060 nm, which allows for in-trap, Doppler-free clock-transition interrogation with lattice-trapped cold atoms. From our analysis of clock statistics and systematics, we project a quantum-noise-limited relative clock stability at the ${10}^{-13}/\sqrt{\tau \left(s\right)}$-level, with integration timeτin seconds, and a relative accuracy of $\sim {10}^{-13}$. We describe a potential architecture for implementing the proposed clock using a single telecom clock laser at 1550 nm, which is conducive to optical communication and long-distance clock comparisons. Our work could be of interest in efforts to realize small and portable Rb clocks and in high-precision measurements of atomic properties of Rb $4D_J$-states. 

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3. Accuracy evaluation of primary frequency standard NIST-F4

   https://doi.org/10.1088/1681-7575/adc7bd  

   Gerginov, Vladislav; Hoth, Gregory_W; Heavner, Thomas_P; Parker, Thomas_E; Gibble, Kurt; Sherman, Jeff_A (April 2025, Metrologia)

   Abstract This work describes the apparatus for NIST-F4, an updated cesium atomic fountain at the National Institute of Standards and Technology (NIST), and presents an accuracy evaluation of the fountain as a primary frequency standard. The fountain uses optical molasses to laser cool a cloud of cesium atoms and launch it vertically in a fountain geometry. In high-density mode, the fractional frequency stability of NIST-F4 is ${\sigma }_y\left(\tau \right)=1.5\times {10}^{-13}/\sqrt{\tau }$, whereτis the measurement time in seconds. The short-term stability is limited by quantum projection noise and by phase noise from the local oscillator, an oven-controlled crystal oscillator operating at 5 MHz. Systematic frequency shifts and their uncertainties have been evaluated, resulting in a systematic (type B) fractional frequency uncertainty ${\sigma }_{\mathrm{B}}=2.2\times {10}^{-16}$. 

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4. Distinguishing the XUV-induced Coulomb explosion dynamics of iodobenzene using covariance analysis

   https://doi.org/10.1088/1361-6455/ad8799  

   Walmsley, Tiffany; Allum, Felix; Harries, James R; Kumagai, Yoshiaki; Lim, Suzanne; McManus, Joseph; Nagaya, Kiyonobu; Britton, Mathew; Brouard, Mark; Bucksbaum, Philip; et al (October 2024, Journal of Physics B: Atomic, Molecular and Optical Physics)

   Abstract The primary and secondary fragmentation dynamics of iodobenzene following its ionization at 120 eV were determined using three-dimensional velocity map imaging and covariance analysis. Site-selective iodine 4d ionization was used to populate a range of excited polycationic parent states, which primarily broke apart at the carbon-iodine bond to produce I⁺with phenyl or phenyl-like cations (C_nH ${{}_x}^{+}$or C_nH ${{}_x}^{2+}$, withn  =  1 – 6 andx  =  1 – 5). The molecular products were produced with varying degrees of internal excitation and dehydrogenation, leading to stable and unstable outcomes. This further allowed the secondary dynamics of ${\mathrm{C}}_6{\mathrm{H}}_x{}^{2+}$intermediates to be distinguished using native-frame covariance analysis, which isolated these processes in their own centre-of-mass reference frames. The mass resolution of the imaging mass spectrometer used for these measurements enabled the primary and secondary reaction channels to be specified at the level of individual hydrogen atoms, demonstrating the ability of covariance analysis to comprehensively measure the competing fragmentation channels of aryl cations, including those involving intermediate steps. 

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5. State selectivity in core ionization of methane

   https://doi.org/10.1103/PhysRevResearch.7.L022021  

   Rajput, J; Stindl, J; Cassimi, A; Gervais, B; Kircher, M; Kastirke, G; McGinnis, O D; Enoki, R; Williams, J B; Davis, V; et al (April 2025, Physical Review Research)

   A clear experimental signature of the population of the lowest triplet state ${}^{3}T_1$of the methane dication is identified in a photoionization experiment. This state is populated only in valence ionization and is absent when the dication is formed by core ionization followed by Auger-Meitner decay. For valence ionization, the total internal energy of the ${\mathrm{CH}}_3^{+}$fragment, formed during the deprotonation of ${\mathrm{CH}}_4{}^{2+}$, is evaluated. Notably, the distribution of this internal energy peaks at the same value regardless of the initially populated electronic state of ${\mathrm{CH}}_4{}^{2+}$. We find that excited electronic states of ${\mathrm{CH}}_3^{+}$are predominantly populated with significant rovibrational excitation. Published by the American Physical Society2025 

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