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Creators/Authors contains: "Reed, K.J."
  1. Relativistic distorted-wave electron-collision strengths have been calculated for transitions from the ground state to the 36 n = 3 excited states of neonlike barium. These theoretical results are compared with recent experimental results obtained with the Electron Beam Ion Trap (EBIT) at Lawrence Livermore National Laboratory. Overall good agreement between our calculated results and the experimental results underscores the effectiveness of the new EBIT method for measuring electron-collision cross sections and supports confidence in the theoretical methods used to calculate these cross sections for highly charged ions.
  2. Electron-impact ionization cross sections have been calculated for magnesiumlike Kr[sup 24+] and sodiumlike Kr[sup 25+]. Electron-impact ionization is an important atomic process in hot dense plasmas. It can affect the ionization balance, electron temperature, electron density, and level population in the plasma. In the past decade, theoretical and experimental studies have revealed that indirect processes can make significant contributions to the cross sections for electron impact ionization of positive ions. The most important indirect process is excitation of an inner-shell electron followed by Auger emission. Higher-order processes such as resonant excitation followed by sequential double Auger emission, can also contributemore » significantly. The contributions of excitation-autoionization and resonant excitation double autoionization (REDA) were included, in addition to the cross sections for direct ionization of a 3s electron. The calculations were carried out using the relativistic distorted wave methods and the multiconfiguration Dirac-Fock model. For Kr[sup 25+], the total cross section is about 5 times the direct ionization cross section. For the Kr[sup 24+], the indirect contribution is about 2.5 times the direct ionization cross section. The REDA process produces many strong resonances and contributes about 20% to the average ionization cross section.« less
  3. Electron-impact ionization cross sections have been calculated for magnesiumlike Kr{sup 24+} and sodiumlike Kr{sup 25+}. Electron-impact ionization is an important atomic process in hot dense plasmas. It can affect the ionization balance, electron temperature, electron density, and level population in the plasma. In the past decade, theoretical and experimental studies have revealed that indirect processes can make significant contributions to the cross sections for electron impact ionization of positive ions. The most important indirect process is excitation of an inner-shell electron followed by Auger emission. Higher-order processes such as resonant excitation followed by sequential double Auger emission, can also contributemore » significantly. The contributions of excitation-autoionization and resonant excitation double autoionization (REDA) were included, in addition to the cross sections for direct ionization of a 3s electron. The calculations were carried out using the relativistic distorted wave methods and the multiconfiguration Dirac-Fock model. For Kr{sup 25+}, the total cross section is about 5 times the direct ionization cross section. For the Kr{sup 24+}, the indirect contribution is about 2.5 times the direct ionization cross section. The REDA process produces many strong resonances and contributes about 20% to the average ionization cross section.« less
  4. This report contains paper on the following topics: Evaporating Black Holes in an Inflationary Universe; Completely and Partially Polarized Signal Propagation; Is the Jet in Quasar 1038 + 064 Precessing Physics of the Anisotropic Ferrite Wakefield Accelerator; Absolute Multilayer Characterization at High Spatial Resolution via Real Time Soft X-Ray Imaging; Update on the Status of the Superconducting Supercollider (SSC) Laboratory; The Use of Charge Coupled Devices in Remote Sensing from Space; A Scientific Visit to the USSR; Migdal-Kadanoff Study of Z{sub 5} Symmetric Systems with Generalized Action -- Part 1; Migdal-Kadanoff Study of Z{sub 5} Symmetric Systems with Generalized Actionmore » -- Part 2; and The Voyager Mission Exploration of the Solar System with Robotic Spacecraft.« less
  5. Cross sections for electron-impact ionization of neonlike selenium have been calculated using close-coupling wave functions. We report cross sections for ionization of the 2s/sup 2/2p/sup 6/ /sup 1/S ground state and the 2s/sup 2/2p/sup 5/3s /sup 3/P excited state of the ion. We found numerous resonances in the ejected electron channels due to excitation of 2s/sup 2/2p/sup 4/3snl autoionizing states. The resonances are very narrow, but some have peaks that are several orders of magnitude above the nonresonant background cross section. Even with radiative decay taken into account, the resonances enhance the total integrated cross section by nearly a factormore » of 4.« less
  6. The effects of rotational coupling are examined in two low energy charge exchange collisions. A justification for a semi-classical treatment of the collisions is presented and it is then shown that for low incident velocity the impact parameter treatment reduces to the Perturbed Stationary State method. Thus the ion-atom system is treated as a quasi-molecule. During the collision the internuclear axis rotates and this rotation is treated as a perturbation which couples certain states in the quasi-molecule. This approach is used to derive expressions for the scattering amplitudes of the ..sigma.., PI, and ..delta.. states of the He + -Hemore » system. This derivation suggests that the phase difference between the ..sigma.. and PI states exhibits small amplitude oscillations about ninety degrees. The molecular states for the H/sup +/ + Xe system are determined and the scattering amplitudes for low energy proton-Xenon collisions are calculated. It is shown that a molecular model leads to the prediction of spin polarized hydrogen in accordance with the earlier work of Shakeshaft and Macek.« less
  7. We report high-resolution measurements of electron impact excitation and ionization cross sections for the Na-like ions Kr{sup 25+} and Xe{sup 43+}. Ions with ionization states centered on the Na-like configurations were produced in an electron beam ion trap (EBIT) using electrons with energies below the L shell ionization thresholds. The Na-like ions were exposed to an electron beam with an energy between 3 and 7 keV. The Na- and Ne-like ions were then extracted and their intensities measured as a function of the electron beam energy. Theoretical ionization cross sections were calculated using relativistic distorted wave methods. Complex resonance structuresmore » that appear in the computed cross sections are observed in the experimental results. These results are the first experimental observation of resonant-excitation-double-autoionization (REDA) in highly charged high-Z ions.« less
  8. With the implementation of a transmission-type curved crystal spectrometer at the Livermore high-energy electron beam ion trap (SuperEBIT) the window on sub-eV level measurements of the ground-state quantum electrodynamics and the two-electron quantum electrodynamics of high-Z ions has been opened. High-resolution spectroscopic measurements of the K{alpha} spectra of hydrogenlike Xe{sup 53+} and heliumlike Xe{sup 52+} are presented. The electron-impact excitation cross sections have been determined relative to the radiative recombination cross sections. The electron-impact energy was 112 keV which is about 3.7 times the excitation threshold for the n = 2 {yields} 1 transitions. Although the relative uncertainties of themore » measured electron-impact excitation cross sections range from about 20% to 50%, significant disagreement between the measured and calculated cross section values has been found for one of the heliumlike xenon lines. Overall, the comparison between experiment and theory shows that already for xenon (Z=54) the Breit interaction plays a significant part in the collisional excitation process. The measured cross sections for the hydrogenlike transitions are in good agreement with theoretical predictions. Additionally, the Xe{sup 53+} Ly-{alpha}{sub 1} transition energy has been measured utilizing the K{alpha} emission of neutral cesium and barium for calibration. Surprisingly, the experimental result, (31279.2 {+-} 1.5) eV, disagrees with the widely accepted theoretically predicted value of (31283.77 {+-} 0.09) eV. However, this disagreement does not (yet) call for any correction in respect to the theoretical values for the transition energies of the hydrogenlike isoelectronic sequence. It rather emphasizes the need for a reevaluation of the commonly used x-ray wavelengths table for atomic inner-shell transitions, in particular, for the cesium K{alpha} lines.« less
  9. We have investigated the contributions of doubly excited, autoionizing resonances to the rate coefficients for electron-impact excitation of the n = 2 to n = 2 transitions in oxygenlike selenium. The cross sections for direct excitation were calculated using a relativistic distorted-wave approximation. The detailed Auger and radiative rates required for evaluating the resonance contributions were calculated using a multiconfigurational Dirac-Fock model. The largest effect is on the electric-dipole-forbidden transitions. For some J = 0 to J = 0 transitions involving two-electron excitations, the resonances enhance the collisional excitation rates by factors up to 10/sup 4/ at electron temperatures relevantmore » for laboratory plasmas.« less
  10. The distorted-wave approximation and similar weak-coupling methods are frequently used to calculate electron collision cross sections for studies of astrophysical and laboratory plasmas. Autoionizing resonances neglected in these approximations can significantly enhance the calculated rates for collisional excitation of some transitions in highly charged ions. We investigated the effects of such resonances on the excitation rates for n = 2 to n = 2 transitions in fluorinelike Fe, Se, Mo, Ag, Xe, and Eu. The background excitation rates were calculated using a relativistic distorted-wave approximation, and the detailed Auger and radiative rates were calculated using a multiconfiguration Dirac-Fock model. Themore » largest effect is on the electric-dipole-forbidden transitions. The enhancement diminishes with increasing atomic number for all transitions.« less
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