K.J. Reed

X-ray measurement of the ionization balance in an electron beam ion trap

Abstract The charge-state abundances of bare, hydrogen-, helium-, lithium-, and beryllium-like iron in the Livermore electron beam ion trap have been measured with uncertainties ranging from 3 to 15% by monitoring the emitted X rays. The ionization balance is determined in two ways: (1) by observing radiative recombination photons with a solid-state detector, and (2) by observing line radiation with a Bragg crystal spectrometer. The results are in good agreement with an ionization balance model.

High–resolution measurements of the K-shell spectral lines of hydrogenlike and heliumlike xenon

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 the 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.

Electron collisions with very highly charged ions — Relativistic calculations

Abstract Cross sections for electron impact ionization of H-like U, Bi, Au, Dy, Ba, Mo, Kr and Fe ions out of the 1s groundstate for incident electron energies up to about six times the ionization energy have been calculated by the relativistic distorted wave method of Pindzola et al. (Phys. Rev. A (1989) 4911 [1]), including exchange and using the Moeller interaction. When Coulomb, magnetic, retardation and electron exchange are all included, the results are in good agreement with measurements using the Electron Beam Ion Trap reported by Marrs et al. (Phys. Rev. Lett. 72 (1992) 4052 [3]) for U at an incident electron energy of 198 keV.

Molecular orbital analysis of L X-ray cross sections measured for slow Ar-Cu collisions

Abstract X-ray production cross sections were determined from thick target analysis of the Ar and Cu L X-ray yields measured for 80–600 keV Ar ions impacting on solid Cu. The Cu cross sections are compared with calculated 3dδ + -3dπ + -3dσ rotational coupling cross sections in the ArCu quasimolecule, while the Ar cross sections are compared with 4fσ electron promotion along with 3dδ − -3dπ − and 3pπ + -3pσ rotational coupling cross sections.

Electron-scattered ion coincidence measurements of heavy ion-atom systems

Abstract Investigations of the energies of electrons emitted as a result of collisions between very heavy ions and atoms show that a significant number of the electrons have a continuous spectrum of energies that is difficult to associate with separated atom levels. This has caused speculation as to the origin of some of these electrons: ideas include the possibility of autoionization occurring during the collision while the molecular potentials are changing, autoionization involving more than two electrons and autoionization from a multiplicity of poorly defined states interacting at a specific distance of closest approach. The present measurements detect the continuum electrons in coincidence with ions scattered to a specific angle, thereby determining the impact parameter dependence of the underlying excitation mechanism.

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

The relative 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 line 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. After normalizing the relative results to theoretical radiative recombination cross sections, we find good agreement with results from a distorted‐wave calculation.

Ionization in 0.1–1.6 MeV Ar+-Kr and Kr+-Ar collisions

Abstract The final charge states of ions scattered to large angles (1° –18°) in Ar + -Kr and Kr + -Ar collisions have been measured for collision energies from 0.1 to 1.6 MeV. Both sets of data show stepwise increases in ionization beginning for those collisions having distances of closest approach of 0.7 and 0.4 a.u. For the smallest distances of closest approach reached (0.05 a.u.), the average number of electrons lost from the collision complex totals nearly twenty. Comparison with molecular orbital calculations shows possible electron promotions to exist for the 0.7 and 0.4 a.u. distances; however, comparison of the Ar-Kr and Kr-Ar data sets with the calculations shows that a variety of separated-atom states may participate in the promotions. This is in contrast to existing models for deeper-lying shells in which specific separated-atom-united-atom correlations are thought to exist.

Coupling effects in electron-impact excitation of oxygenlike 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 the 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.