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1. State-Selective Double Photoionization of Atomic Carbon and Neon

   https://doi.org/10.3390/atoms12120070  

   Yip, Frank L (December 2024, Atoms)

   Double photoionization (DPI) allows for a sensitive and direct probe of electron correlation, which governs the structure of all matter. For atoms, much of the work in theory and experiment that informs our fullest understanding of this process has been conducted on helium, and efforts continue to explore many-electron targets with the same level of detail to understand the angular distributions of the ejected electrons in full dimensionality. Expanding on previous results, we consider here the double photoionization of two 2p valence electrons of atomic carbon and neon and explore the possible continuum states that are connected by dipole selection rules to the coupling of the outgoing electrons in 3P, 1D, and 1S initial states of the target atoms. Carbon and neon share these possible symmetries for the coupling of their valence electrons. Results are presented for the energy-sharing single differential cross section (SDCS) and triple differential cross section (TDCS), further elucidating the impact of the initial state symmetry in determining the angular distributions that are impacted by the correlation that drives the DPI process. 

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2. Double photoionization of atomic carbon and neon

   https://doi.org/10.1088/1742-6596/1412/1/011035  

   Yip, F L; Rescigno, T N; McCurdy, C W (June 2020, Journal of Physics: Conference Series ICPEAC XXXI)

   We consider the double photoionization of the 2p2 valence electrons of atomic carbon, which provides for distinct final-state symmetries depending on the three possible angular momentum couplings (3P,1D, and 1S) of the initially-bound p2 electrons that are ejected into the continuum by a single photon. Comparison of this process with neon provides an analogous case for the resulting final states within the treatment of the double photoionization proceeding with the ejected electrons influenced by the remaining bound electrons. 

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3. Observation of radiative double electron capture for

   https://doi.org/10.1002/xrs.3063  

   Kumara, Nuwan; La_Mantia, David_S; Buglione, Stephanie_L; McCoy, Craig_P; Taylor, Charles_J; White, Jacob_S; Kayani, Asghar; Tanis, John_A (June 2019, X-Ray Spectrometry)

   Radiative double electron capture (RDEC), occurring when two electrons are captured to a projectile ion with the simultaneous emission of a single photon, has been investigated. RDEC can be considered as the time inverse process of double photoionization. Strong evidence for RDEC is found in F⁹⁺+ N₂collisions and additionally for one‐electron F⁸⁺for which the probability for the process is expected to be considerably smaller. Preliminary values for the cross sections for RDEC have been determined. A significant advantage of the gas target is that multiple‐collision effects seen for a solid target are avoided due to the single‐collision conditions that prevail for gas targets. 

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4. Radiative double-electron capture by fully stripped and one-electron ions in gas and thin-foil targets

   Tanis, John A (September 2021, Physical review)

   null (Ed.)

   Radiative double-electron capture (RDEC), in which two-electron capture is accompanied by simultaneousemission of a single photon, was investigated for fully stripped and one-electron projectiles colliding withgaseous and thin-foil targets. RDEC can be considered the inverse of double photoionization by a single photon.For the gaseous targets, measurements were done for 2.11 MeV/uF9+and F8+ions interacting with N2and Ne,while for the thin-foil target the measurements were done for 2.11 MeV/uF9+and F8+and 2.19 MeV/uO8+andO7+ions striking thin C targets. Reports on this work were already published separately in shorter accounts by LaMantiaet al.[Phys. Rev. Lett.124, 133401 (2020)for the gas targets andPhys.Rev.A102, 060801(R) (2020)forthe thin-foil targets]. The gas targets were studied under single-collision conditions, while the foil targets sufferedunavoidable multiple collisions. The measurements were carried out by detecting x-ray emission from the targetat 90◦to the beam direction in coincidence with outgoing ions undergoing double, single, and, in the caseof the foil targets, no charge change inside the target. Striking differences between the gaseous and foil targetswere found from these measurements, with RDEC for the gaseous targets occurring only in coincidence with q-2outgoing projectiles as expected, while RDEC for the foil targets was seen in each of the outgoing q-2, q-1, and nocharge-change states. The no charge-change result was totally unexpected. The cross sections for RDEC for thefully stripped ions on gas targets were found to be about six times larger than those for the one-electron projec-tiles. For the foil targets, the RDEC cross sections for the fully stripped and one-electron projectiles differ some-what from one another but not to the the extent they did for the gas targets. In this work the cross sections for allof the projectiles for the foil targets were adjusted due to the target contaminant background from potassium andcalcium atoms that existed in the spectra. Also, the cross sections for the incident one-electron projectiles weremodified due to a correction for the fraction of these ions that becomes fully stripped in passage through the foil.These differences are attributed to the effects of the multiple collisions that occur for the foil targets. The differ-ential cross sections at 90◦determined for each of the projectiles interacting with each of the targets are comparedwith each other and with the previous measurements. To the extent that the cross sections follow a sin2θdepen-dence, the total cross sections are compared with theoretical calculations [E. A. Mistonova and O. Yu. Andreev,Phys. Rev. A87, 034702 (2013)], for which the agreement is poor, with the measured cross section exceedingthe predicted ones by about an order of magnitude. Possible reasons for this discrepancy will be discussed. 

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5. Partial Photoionization Cross Sections of Chromium from the Ground and Excited States

   https://doi.org/10.3390/atoms8030051  

   Zatsarinny, Oleg; Tayal, Swaraj (September 2020, Atoms)

   null (Ed.)

   Partial and total photoionization cross sections of iron-peak elements are important for the determination of abundances in late-type stars and nebular objects. We have investigated photoionization of neutral chromium from the ground and excited states in the low energy region from the first ionization threshold at 6.77 eV to 30 eV. Accurate descriptions of the initial bound states of Cr I and the final residual Cr II ionic states have been obtained in the multiconfiguration Hartree-Fock method together with adjustable configuration expansions and term-dependent non-orthogonal orbitals. The B-spline R-matrix method has been used for the calculation of photoionization cross sections. The 194 LS final ionic states of Cr II 3d44s, 3d34s2, 3d5, 3d44p, and 3d34s4p principal configurations have been included in the close-coupling expansion. The inclusion of all terms of these configurations has significant impact on the near-threshold resonance structures as well as on the nonresonant background cross sections. Total photoionization cross sections from the ground 3d54sa7S and excited 3d54sa5S, 3d44s2a5D, 3d54pz5P, and 3d44s4py5P states of Cr I have been compared with other available R-matrix calculation to estimate the likely uncertainties in photoionization cross sections. We analyzed the partial photoionization cross sections for leaving the residual ion in various states to identify the important scattering channels, and noted that 3d electron ionization channel becomes dominant at higher energies. 

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