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  1. Laboratory measurements of resonant contributions to Fe XXIV line emission

    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
  2. Measurement of the polarization of the K-shell resonance line emission of S{sup 13+} and S{sup 14+} at relativistic electron beam energies

    We have measured the polarization of the heliumlike sulfur resonance line 1s2p {sup 1}P{sub 1}{yields}1s{sup 2} {sup 1}S{sub 0} and of the blend of the lithiumlike sulfur resonance lines 1s2s2p {sup 2}P{sub 3/2}{yields}1s{sup 2}2s {sup 2}S{sub 1/2} and 1s2s2p {sup 2}P{sub 1/2}{yields}1s{sup 2}2s {sup 2}S{sub 1/2} as a function of electron beam energy from near threshold to 144 keV. These lines were excited with the LLNL high-energy electron beam ion trap and measured using a newly modified two-crystal technique. Our results test polarization predictions in an energy regime where few empirical results have been reported. We also present calculations ofmore » the polarization using two different methods, and good agreement is obtained.« less
  3. K-shell auger and radiative transitions in the carbon isoelectronic sequence, 6 {le} Z {le} 54

    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.
  4. Distorted-wave cross sections for electron-impact ionization of Ar{sup 7+}

    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
  5. Electron impact excitation cross section measurements of highly charged heliumlike and lithiumlike ions

    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
  6. Autoionizing resonances in electron-impact excitation of oxygenlike selenium

    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
  7. Resonance enhancement of electron collision rates for fluorinelike ions

    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
  8. Indirect processes in electron impact ionization of positive ions

    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.
  9. Coupling effects in electron-impact excitation of oxygen-like krypton

    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
  10. Resonant effects on electron-impact ionization of sodiumlike selenium

    We have calculated the resonant-excitation-double-autoionization (REDA) cross sections for sodiumlike selenium. The REDA process is treated as a three-step process (i.e., resonant electron capture followed by sequential double Auger emission). The necessary atomic data are computed using the multiconfiguration Dirac-Fock model. The effects of radiative damping can reduce the calculated REDA cross sections by a factor of 6, and the loss channels can further reduce these cross sections by a factor of 2. The REDA processes produce many strong narrow resonances and contribute 10--20 % to the average total ionization cross sections.

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"Reed, K.J."

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