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Creators/Authors contains: "Reed, K.J."
  1. The cross sections for electron impact excitation of the heliumlike forbidden line z, intercombination lines y and x, and the resonance line w, together with the associated lithiumlike satellites lines q, r, t, u, and v have been measured for Ti, V, Cr, Mn, and Fe on the Livermore electron beam ion trap. The measurements were made near the threshold for excitation of the heliumlike transitions where no other processes can contribute to the excitation of the lines. The uncertainties in the measurements are up to 15% for the heliumlike transitions and between 14 and 27% for the lithiumlike transitions.more » We find good agreement with results from a distorted-wave calculation.« less
  2. A number of X-ray astronomy satellites are scheduled for launch in the next few years. The Advanced X-ray Astrophysics Facility (AXAF) is scheduled for launch in 1998, and the X-Ray Multi-mirror Mission (XMM) and Astro-E in 1999. These satellites will carry spectrometers with resolving powers in the Fe L-shell emission region over an order of magnitude greater than the spectrometers aboard A CA. Interpreting AXAF, XMM, Astro-E spectra will require atomic data at an accuracy significantly greater than the data presently used in the standard emission codes. To address some of the existing and upcoming needs of X-ray astrophysics, wemore » have continued our studies of Fe XXIV line emission. In this work, we measured Fe XXIV 3{yields}2 line emission at energies around threshold, using EBIT to examine the resonance contributions to the line emissivity. Here we present relative cross sections, at electron energies between 700 and 1500 eV, for producing line emission at wavelength A = 11.18 of the Fe XXIV 3d{sub 5/2}{yields}2P{sub 3} transition. Various processes can contribute to line emission observed from a collisional plasma. Direct excitation (DE) is the most important one at energies above the EIE threshold. Below threshold, Dielectronic recombination (DR) produces high n satellites which cannot be resolved from the EIE line. Resonant excitation (RE) can populate the same levels as DE via dielectronic capture followed by autoionization to the level of interest.« less
  3. Coupling effects among the channels for collisional excitation of 2s/sup 2/2p/sup 4/, 2s2p/sup 5/, and 2p/sup 6/ configurations of oxygen-like krypton have been investigated by comparing six-, five-, four-, three-, and two-state close-coupling (CC) cross sections for the electron collision energy range 26.2 Ryless than or equal toEless than or equal to500 Ry. We discovered the surprising result that the calculated three-state CC (3CC) cross sections for the transition from the ground state to the 2p/sup 6/ /sup 1/S/sup e/ and the two-state CC cross section to the other excited states are reduced (considerably (45%) for the former) when themore » 2s2p/sup 5/ /sup 3/P/sup o/ channel is added to the 5CC approximation. In addition, the 3CC cross section sigma(2p/sup 4/ /sup 3/P/sup e/--2p/sup 6/ /sup 1/S/sup e/) is diminished by a factor of 100 at 26.2 Ry by the addition of the 2s2p/sup 5/ /sup 3/P/sup o/ state. Approximations which omit the 2s2p/sup 5/ /sup 3/P/sup o/ state overestimate the cross sections.« less
  4. The high-energy electron beam ion trap SuperEBIT at the Lawrence Livermore National Laboratory allows the study of the x-ray emission from highly charged ions interacting with electrons with energy in excess of 200 keV. Radiation from ions as highly charged as Cf96+ has been produced this way. The facility is being used to investigate the contributions from quantum electrodynamics in heavy ions. Here the focus is lithiumlike ions, especially U89+, which provide the opportunity for the most accurate test of QED in highly charged ions. We have also used the facility to measure the degree of x-ray line polarization asmore » a function of the energy of the electron collision energy. For example, we have studied the linear polarization of the K-shell emission lines of Fe24+ for electron-impact energies high as 120 keV. A new area of research is the investigation of nuclear excitation by electronic transitions. This is the inverse process of internal conversion, where an atomic x ray is absorbed by the nucleus resulting in an excited nuclear state. We are planning to study this process in 189Os using 217 keV atomic x rays generated in the interaction with a 196 keV electron beam.« less
  5. K-shell energies and transition rates for the Auger and radiative decays in carbon-like ions with 1s2s{sup m}2p{sup n} (m + n = 5) configurations have been calculated using the multiconfiguration Dirac-Fock method including Breit interaction and quantum electrodynamic corrections. Seventeen ions with atomic numbers 6 {le} Z {le} 54 were considered. Transition rates were evaluated in intermediate coupling with configuration interaction within the same complex. Line fluorescence yields were also computed. 14 refs., 2 tabs.
  6. Distorted-wave methods have been used to calculate cross sections for electron-impact ionization of sodiumlike Ar{sup 7+} for collision energies ranging from near threshold to 1 keV. We included contributions due to excitation-autoionization and resonant excitation{endash}double autoionization (REDA), as well as the contributions from direct ionization of the 3{ital s} electron and the {ital n}=2 inner-shell electrons. We find that the effects of radiative damping and loss channels reduce the REDA contributions by a factor of 4. We compare our calculated results to experimental results and to theoretical cross sections obtained in a close-coupling approximation. {copyright} {ital 1996 The American Physicalmore » Society.}« less
  7. The report explores the indirect processes in electron input ionization of positive ions. The topics concerned in this report are: Direct Ionization; Excitation-Autoionization; Resonant-Excitation (Capture) Process; Ionization-Autoionization; Excitation; Autoionization; Resonant Capture Processes; Multiple Ionization; Method of Close-Coupled Target States; and Comparison of Theoretical and Experimental Data. 10 refs., 151 figs., 7 tabs.
  8. The observation of the decay of inner-shell vacancies can serve as an excellent probe of the neutralization of multicharged ions during their approach to a metal surface. Several recent experiments that have measured electrons emitted during this neutralization are discussed. Measurements of the total electron yield for incident ions with inner-shell vacancies first showed marked differences from the yield observed for lower charge states and indicated the need for further investigations. Measurements of the emitted electron energy distributions have led to a qualitative understanding of the timescales of the neutralization process. For incident ions with high enough energies, projectile inner-shellmore » vacancies have been observed to survive the neutralization process above the surface and then to be transferred to target atoms in close collisions. The inner-shell reactions occurring in such close collisions are analogous to those that have been observed in ion-atom and ion-foil collisions. Recent measurements of angular distributions of the electrons emitted due to the decay of target vacancies created during the interaction show evidence of the projectile penetrating several layers below the surface. 27 refs., 7 figs.« less
  9. Electron-impact excitation cross sections have been calculated for transitions from the ground state 2s/sup 2/2p/sup 4/ /sup 3/P to the n = 2 and n = 3 excited states of oxygenlike krypton. Configuration-interaction-type wave functions were employed, and the cross sections were calculated in a close-coupling approximation for impact energies ranging from near threshold to 10 keV. Coupling between the n = 2 and n = 3 excitation levels was generally found to be weak, and a two-state close-coupling approximation should be adequate for calculating reliable cross sections for most transitions.
  10. Radiation emitted from ions excited by a directed beam of electrons can be strongly linearly polarized. According to Itikawa et al. the degree of polarization should be independent of Z in an isoelectronic sequence when expressed as a function of incident electron energy in threshold units. We calculated cross sections for excitation to specific magnetic sublevels, and polarization of the resulting radiation, for H- and He- like ions with Z=13,18,22,42,56,79 and 92. The polarization is indeed independent of Z in the non-relativistic limit. However, when relativistic effects are properly included, the polarization is markedly Z-dependent. Also, for each specific ion,more » the difference between relativistic and non-relativistic results increases with increasing incident electron energy.« less
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