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1. Deep Minima in the Triply Differential Cross Section for Ionization of Atomic Hydrogen by Electron and Positron Impact

   https://doi.org/10.3390/atoms8020026  

   DeMars, C.; Ward, S.; Colgan, J.; Amami, S.; Madison, D. (June 2020, Atoms)

   We investigate ionization of atomic hydrogen by electron- and positron-impact. We apply the Coulomb–Born (CB1) approximation, various modified CB1 approximations, the three body distorted wave (3DW) approximation, and the time-dependent close-coupling (TDCC) method to electron-impact ionization of hydrogen. For electron-impact ionization of hydrogen for an incident energy of approximately 76.45 eV, we obtain a deep minimum in the CB1 triply differential cross section (TDCS). However, the TDCC for 74.45 eV and the 3DW for 74.46 eV gave a dip in the TDCS. For positron-hydrogen ionization (breakup) we apply the CB1 approximation and a modified CB1 approximation. We obtain a deep minimum in the TDCS and a zero in the CB1 transition matrix element for an incident energy of 100 eV with a gun angle of 56.13 ° . Corresponding to a zero in the CB1 transition matrix element, there is a vortex in the velocity field associated with this element. For both electron- and positron-impact ionization of hydrogen the velocity field rotates in the same direction, which is anticlockwise. All calculations are performed for a doubly symmetric geometry; the electron-impact ionization is in-plane and the positron-impact ionization is out-of-plane. 

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2. Quantum dynamics of positron-hydrogen scattering and three-body bound state formation with an assisting laser field: predictions of a reduced-dimensionality model

   https://doi.org/10.1088/1361-6455/ad1d33  

   Ji, Xiao Hu; Jiao, Li Guang; Liu, Aihua; Zhang, Yong Zhi; Thumm, Uwe; Ho, Yew Kam (January 2024, Journal of Physics B: Atomic, Molecular and Optical Physics)

   We investigate the quantum dynamics of target excitation and positronium formation in the positron-hydrogen atom scattering without and with an external assisting laser field within a reduced-dimensional quantum model. Strong interference fringes between the incident and reflected positron wave packets are observed in the reaction region. We further investigate the critical behavior of transition probabilities near the channel-opening thresholds for hydrogen excitation and positronium formation and find a strong competition between channels with similar threshold energies, but different parities. The transmission ratios of the incident positron in different reaction channels are calculated, and it is shown that only positronium formation in the ground state prefers forward scattering. Our simulation of the positron-hydrogen scattering with an assisting laser field indicates that the three-particle bound states can be formed during the collisions due to the photon emission induced by the external laser field. 

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3. Dipolar focusing in laser-assisted positronium formation

   https://doi.org/10.1088/1361-6455/ad3f5a  

   Ambalampitiya, H. B.; Matfunjwa, M. K.; Fabrikant, I. I. (April 2024, Journal of Physics B: Atomic, Molecular and Optical Physics)

   Abstract We consider positronium formation in collisions of positrons with excited hydrogen atoms H(n) in an infrared laser field theoretically. This process is assisted by the dipolar focusing effect: a positron moving in a superposition of a laser field and the dipolar field can approach the atomic target even if its trajectory starts with a very large impact parameter, leading to a significant enhancement of the Ps formation cross section. The classical trajectory Monte Carlo method, which is justified for $n\text{⩾}3$, allows efficient calculation of this enhancement. A similar effect can occur in collisions of positrons with other atoms in excited states, which can lead to improvements in the efficiency of positronium formation. 

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4. Simulation of the formation of antihydrogen via magnetobound positronium

   https://doi.org/10.1063/5.0060707  

   Aguirre, F_F; Ordonez, C_A (September 2021, AIP Advances)

   Antihydrogen formation involving magnetobound positronium is simulated by computing classical trajectories. Simulated collisions between electrons and positrons generate magnetobound positronium, which consists of electron–positron pairs that are not energetically bound but that have spatially correlated trajectories within a magnetic field. Simulations show that antihydrogen can form if such electron–positron pairs pass near antiprotons. In addition, the possibility of forming antihydrogen atomic ions or antihydrogen molecular ions via magnetobound positronium or magnetobound antihydrogen is discussed. 

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5. Electron-positronium scattering and Ps ^- photodetachment

   https://doi.org/10.1088/1361-6455/adbfe9  

   Mitchell, William; Ward_Quintanilla, Sandra (March 2025, Journal of Physics B: Atomic, Molecular and Optical Physics)

   Abstract We investigate the fundamental three-body Coulomb process of elastic electron-positronium (e^--Ps) scattering below the Ps(n = 2) threshold. Using the complex Kohn variational method and trial wave functions that contain highly correlated Hylleraas-type terms, we accurately compute^1,3S-,^1,3P-, and^1,3D-wave phase shifts, which may be considered as benchmark results. We explicitly investigate the effect of the mixed symmetry term in the short-range part of the^1,3D-wave trial wave function on the phase shifts and resonances. Using the complex Kohn phase shifts we compute, for e⁻-Ps scattering, the elastic differential, elastic integrated, momentum-transfer, and ortho-para conversion cross sections and determine the importance of the complex Kohn D-wave phase shifts on these cross sections. In addition, using the short-range part of the¹S-wave trial wave function for the bound state of the purely leptonic ion of Ps⁻, and the complex Kohn¹P trial wave function for the continuum state, we determine the Ps⁻photodetachment cross section in the length, velocity, and acceleration forms. 

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