Browsing by Author "Cheung, Charles"
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Item Development of a relativistic atomic theory for accurate treatment of complex correlations(University of Delaware, 2021) Cheung, CharlesOne of the central objectives of atomic, molecular and optical physics is describing complex correlations in atomic structure. There has been an increasing need for high quality high-precision atomic data and software in several scientific communities, including atomic, plasma and astrophysics. Having an accurate atomic theory is indispensable for experiments involving studies of fundamental interactions, astrophysics, atomic clocks, plasma science, quantum degenerate gases, quantum information, precision measurements, and others. The need for high-precision calculations of atomic properties of systems in the middle columns of the periodic table has become increasingly urgent due to its relevance in new experiments probing physics beyond the Standard Model. ☐ The ultimate goal of the work described in this thesis is to develop methods and codes to enable accurate computation of atomic properties of atoms and ions with complex electronic structure. We apply our newly developed code package for reliable calculations of atomic properties of systems that were not possible before. This includes calculations correlating all 60 electrons in the highly charged Ir17+ ion, calculations predicting clock transitions and analyzing systemic effects that could affect the accuracy and stability of Cf15+ and Cf17+ ions, calculations predicting the 3C3D line intensity ratio in Fe16+, calculations detecting the lowest lying odd parity atomic levels in neutral Ac, and calculating properties of negative ions La- and Bi-. The new code package will also be used to produce large volumes of atomic data for a new online portal being developed at the University of Delaware.Item Natural-linewidth measurements of the 3𝐶 and 3𝐷 soft-x-ray transitions in Ni xix(Physical Review A, 2024-06-10) Shah, Chintan; Kühn, Steffen; Bernitt, Sonja; Steinbrügge, René; Togawa, Moto; Berger, Lukas; Buck, Jens; Hoesch, Moritz; Seltmann, Jörn; Kozlov, Mikhail G.; Porsev, Sergey G.; Gu, Ming Feng; Porter, F. Scott; Pfeifer, Thomas; Leutenegger, Maurice A.; Cheung, Charles; Safronova, Marianna S.; Crespo López-Urrutia, José R.We used the monochromatic soft-x-ray beamline P04 at the synchrotron-radiation facility PETRA III to resonantly excite the strongest 2𝑝−3𝑑 transitions in neonlike Nixix ions, [2𝑝6]𝐽=0→[(2𝑝5)1/23𝑑3/2]𝐽=1 and [2𝑝6]𝐽=0→[(2𝑝5)3/23𝑑5/2]𝐽=1, respectively dubbed 3𝐶 and 3𝐷, achieving a resolving power of 15 000 and signal-to-background ratio of 30. We obtain their natural linewidths, with an accuracy of better than 10%, as well as the oscillator-strength ratio 𝑓(3𝐶)/𝑓(3𝐷)=2.51(11) from analysis of the resonant fluorescence spectra. These results agree with those of previous experiments, earlier predictions, and our own advanced calculations.Item New Measurement Resolves Key Astrophysical Fe XVII Oscillator Strength Problem(Physical Review Letters, 2022-12-05) Kühn, Steffen; Cheung, Charles; Oreshkina, Natalia S.; Steinbrügge, René; Togawa, Moto; Bernitt, Sonja; Berger, Lukas; Buck, Jens; Hoesch, Moritz; Seltmann, Jörn; Trinter, Florian; Keitel, Christoph H.; Kozlov, Mikhail G.; Porsev, Sergey G.; Gu, Ming Feng; Porter, F. Scott; Pfeifer, Thomas; Leutenegger, Maurice A.; Harman, Zoltán; Safronova, Marianna S.; López-Urrutia, José R. Crespo; Shah, ChintanOne of the most enduring and intensively studied problems of x-ray astronomy is the disagreement of state-of-the art theory and observations for the intensity ratio of two Fe XVII transitions of crucial value for plasma diagnostics, dubbed 3C and 3D. We unravel this conundrum at the PETRA III synchrotron facility by increasing the resolving power 2.5 times and the signal-to-noise ratio thousandfold compared with our previous work. The Lorentzian wings had hitherto been indistinguishable from the background and were thus not modeled, resulting in a biased line-strength estimation. The present experimental oscillator-strength ratio Rexp=f3C/f3D=3.51(2)stat(7)sys agrees with our state-of-the-art calculation of Rth=3.55(2), as well as with some previous theoretical predictions. To further rule out any uncertainties associated with the measured ratio, we also determined the individual natural linewidths and oscillator strengths of 3C and 3D transitions, which also agree well with the theory. This finally resolves the decades-old mystery of Fe XVII oscillator strengths.