J.R. Mowat

Dielectronic recombination in He+ ions

Abstract Dielectronic recombination involving 1s + e− → nln′l′ transitions has been investigated for He+ ions. This work was done using the ion storage ring and electron cooler at the Indiana University Cyclotron Facility. Resonant maxima from DR were observed, but the energy resolution was insufficient to identify individual transitions. The magnitude of the measured cross sections appears to be about a factor of two lower than theory.

Proposed measurements of dielectronic recombination using the Indiana cooler facility

Abstract It is proposed to measure dielectronic recombination (DR) using the IUCF Cooler. This process, which involves the electron-electron interaction, takes place when electron capture is accompanied by simultaneous excitation of the interacting ion resulting in the formation of a doubly-excited intermediate state. The IUCF Cooler, with its high electron beam intensity and high electron-energy resolution, provides a unique opportunity for investigating this fundamental atomic collision process. Specifically, it is proposed to measure DR for collisions of He + ions with electrons in the Cooler at relative energies (in the rest frame of the ion) of about 30–40 eV. The DR cross sections obtained will be used to test theoretical calculations of this correlated two-electron process, which apart from their fundamental interest, have applications to astrophysical studies and to the development of nuclear fusion plasmas.

Dielectronic recombination from the ground and excited states of Li

Abstract Dielectronic recombination (DR) was investigated for Li + + e − collisions using the electron cooler at the Indiana University Cyclotron Facility (IUCF). For Li + (1s 2 ) ions, DR is expected to occur for relative energies E rel between 50–60 eV, and for metastable Li + (1s2s) DR should occur for E rel E rel near zero, radiative recombination (RR) (inverse photoelectric effect) is expected. This work is an extension of our previous work for He + ions. These light He + and Li + ions pose stringent tests of DR theory because the electron-electron interaction, which mediates DR, is stronger compared to the electron-nucleus interaction than is it for heavier ions. Additionally, the electron coupling in two-electron Li + gives rise to angular momentum configurations different from those in He + . Preliminary results of recent measurements at IUCF, where DR was observed for both ground-state and metastable Li + ions, are presented.

K- and l-shell resonant transfer and excitation in ion-atom collisions

Recent experimental studies of resonant transfer and excitation (RTE) in ion-atom collisions are reviewed. In the RTE process correlated electron capture and projectile excitation occur together in a single encounter with a target atom. Measurements of Ca/sup q+/ + H/sub 2/ (q = 10 to 19) from 100 to 370 MeV establish the projectile charge-state dependence of K-shell RTE and provide a detailed test of the theory. Structure due to RTE is observed in the energy dependence of the total electron-capture cross sections for this collision system. A comparison of the Ca/sup 17 +/ + H/sub 2/ data with previous results for Ca/sup 17 +/ + He demonstrates the effect of the target-electron momentum distribution on the RTE process. Studies of 230 to 610 MeV Nb/sup 31 +/ + H/sub 2/ provide information about RTE involving agreement with theoretical calculations. 15 refs., 4 figs.

L-SHELL RESONANT TRANSFER AND EXCITATION FOR NE-LIKE NIOBIUM IONS

Projectile excitation and charge transfer (capture) can occur simultaneously in a single encounter with a target atom through the electron-electron interaction between a projectile electron and a weakly bound target electron. This process is referred to as resonant transfer and excitation (RTE). L-shell RTE has been investigated for 230 to 610 MeV Nb/sup 31 +/ (neonlike) ions incident on H/sub 2/. 11 refs., 1 fig. (WRF)

Recent Results on Radiative Electron Capture

We have three new results concerning REC since our last publication.1–3 (1) The measured REC cross-sections per K-vacancy scale according to the number of “free” electrons in the target atom at least for the cases of CI projectiles incident on C and Cu. “Free” electrons are those bound target electrons with average velocity ≪ than the projectile velocity. Excellent agreement with the Bethe-Salpeter free-electron theory4 is obtained if it is assumed that each “free” target electron contributes equally to the REC cross-section, and the effective Cl K-shell binding energy needed to compute the cross-section is taken as the measured thin target e.g. 10 µg/cm2 REC centroid energy less the mean kinetic energy of the captured electrons in the projectile reference frame. Our methods of determining the experimental and theoretical REC cross-section per K-vacancy may explain some discrepancies found by Lindskog et al5.

Charge transfer and excitation in high-energy ion-atom collisions☆

Abstract Coincidence measurements of charge transfer and simultaneous projectile electron excitation provide insight into correlated two-electron processes in energetic ion-atom collisions. Projectile excitation and electron capture can occur simultaneously in a collision of a highly charged ion with a target atom; this process is called resonant transfer and excitation (RTE). The intermediate excited state which is thus formed can subsequently decay by photon emission or by Auger-electron emission. Results are shown for RTE in both the K-shell of Ca ions and the L-shell of Nb ions, for simultaneous projectile electron loss and excitation, and for the effect of RTE on electron capture.

Three-Component Model Analysis of C1 Projectile Hypersatellite K X-Ray Intensities vs C Foil Thickness

The production of ions with two K vacancies is of fundamental interest in ion-atom collisions1–3. Previously, double K-vacancy production has been inferred from the ratio R = σ x h /σx of hyper-satellite-to-single K-vacancy x-ray yields, assuming the fluorescence yields for single and double K-vacancies are the same1. This ratio has been used to determine double K-vacancy sharing ratios between target and projectile in near symmetric collisions3 and to determine branching ratios for two-electron-one-photon transitions4,5. In these works, it was tacitly assumed that R does not depend upon the target thickness.

Dielectronic and Radiative Recombination of Highly Stripped Sulfur Ions Incident on Argon

We have measured sulfur K x-ray emission following electron capture for 70 MeV Sq+(q=13-16) ions incident on Ar. K radiation resulting from capture was isolated by detecting coincidences between S K x rays and electron capture events. Two recombination processes which result in the emission of a K x ray were identified: (1) dielectronic recombination, which is the inverse Auger effect and (2) radiative recombination, which results from capture into a high n state (n¿2). The cross section for K radiation following capture increases by more than three orders of magnitude for q = 13+ to 16+. The fraction of capture events which result in K radiation increases from 0.0004 for q = 13+ to 0.3 for q = 16+, indicating that radiative recombination becomes relatively more important for high charge states, i.e. as q ¿ Z. Possible applications to radiative loss processes in a plasma are discussed.