[en] Excited hydrogen atoms in the n = 3 state were formed in electron transfer collisions of 40-80 keV protons on helium inside a gas cell. An experimental measurement of the appropriate n = 3 density matrix provides a complete description of the excited hydrogen state and leads to a physical understanding of the charge transfer. The Balmer-α radiation due to decay of the n = 3 state was observed as a function of axial and transverse electric fields applied in a gas cell. The intensity and polarization fraction measurements contain a symmetric and an asymmetric part with respect to the direction of the applied electric field. The symmetric part depends upon the diagonal and off-diagonal even-parity elements of the density matrix. The asymmetric part depends on the off-diagonal elements that connect states of opposite parity. A linear least-squares fit of the data led to an experimental determination of both the diagonal and off-diagonal elements in the n = 3 density matrix. The total 3p and 3d cross sections normalized to 3s were obtained from the diagonal elements of the measured density matrix. The real part of the off-diagonal terms shows that the electron lags the proton after the collision forming a dipole. This observed dipole was large ranging from 4.2 ea₀ at 40 keV to 2.9 ea₀ at 80 keV. When interpreted in terms of a tilt operator, the imaginary part of the off-diagonal terms indicates that this dipole is tiled off-axis. The expectation value of the tilt operator ranged from 0.47 at 40 keV to -0.04 at 80 keV. By assuming an orientation to the circulation of the electrons in the dipole, the direction of the tilt with respect to the beam axis is found

[en] Low energy charge transfer is an important process in plasmas where multicharged ions and neutral atoms or molecules can exist at eV/amu energies and below. At eV/amu collision energies there are no scaling laws to characterize the charge transfer cross section and both theory and experiment are difficult. The capabilities of current experimental techniques used to measure low energy total and state-selective cross sections are discussed

[en] An ion-atom merged-beams technique is being used to measure total absolute electron-capture cross sections for multicharged ions in collisions with H (or D) in the energy range between 0.1 and 1000 eV/amu. Comparison between experiment and theory over such a large energy range constitutes a critical test for both experiment and theory. Total capture cross-section measurements for O³⁺ H(D) and O⁵⁺ + H(D) are presented and compared to state selective and differential cross section calculations. Landau-Zener calculations show that for O⁵⁺ the sharp increase in the measured cross section below 1 eV/amu is partly due to trajectory effects arising from the ion-induced dipole interaction between the reactants. 20 refs., 8 figs

[en] To quantitatively study electron capture during collisions of multiply charged ions with neutral atoms at near-thermal energies, keV-energy multicharged ion beams are merged with ground-state beams of H or D atoms of chosen velocity such that collisions in the relative energy range 1--1000 eV/amu result. Recent data for O³⁺, O⁴⁺ + H(D) are presented and compared with theoretical predictions. Recently completed modifications to the apparatus are described that will provide a significant improvement in signal-to-background and angular collection. These improvements will allow measurements to be extended to lower energies, where effects due to the ion-induced dipole attraction may be evident

[en] A multicharged ion-atom merged-beams apparatus has been used in conjunction with the ORNL-ECR ion source to measure accurate absolute electron-capture cross sections in the energy range from below 1 eV/amu to 1500 eV/amu. Measurements for N/sup 3+,4+,5+/ /plus/ H(D) collisions indicate good agreement with available theoretical calculations. However, measurements with O⁵⁺ /plus/ H(D) show an unexpected low-energy behavior which may be attributable to the ion-induced-dipole attraction between the reactants. Scaled Landau-Zener calculations presented here identify a transfer plus excitation channel which has the correct energy dependence at low energies. This finding suggest the need for a comprehensive coupled channel calculation which would include such product states. 25 refs., 8 figs

[en] At the Oak Ridge Multicharged Ion Source Facility, an experimental atomic collision physics program is centered around a recently upgraded ECR (electron cyclotron resonance) multicharged ion source. The 10 GHz CAPRICE source has been in operation since October 22, 1992, and has provided more intense, higher charged ion beams than the previous ECR ion source. Intense metallic beams have recently become available with the installation of a metallic oven on the source. In addition to measurements of electron-impact excitation, carried out in collaboration with the Joint Institute for Laboratory Astrophysics (JILA), experiments are presently on-line to study electron impact ionization, low-energy ion-atom collisions, and ion-surface interactions. A brief summary of the various activities with an emphasis on new capabilities will be presented

[en] 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-shell 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

[en] After a brief summary of the present capability and configuration of the ORNL Multicharged Ion Research Facility (MIRF), and of upcoming upgrades and expansions, the presently on-line atomic collisions experiments are described. In the process, the utility of intense, cw ion beams extracted from ECR ion sources for low-signal rate experiments is illustrated

[en] We have measured energy distributions of electrons produced during slow glancing collisions of H-like and He-like N and O projectiles with a clean Au(110) surface. For the case of the H-like incident projectiles, two peaks, at 250 and 350 eV, and 250 and 490 eV for incident N⁺⁶ and O⁺⁷, respectively, are observed. Both peaks are associated with Auger transitions to the K-shell vacancy brought into the collision by the incident H-like multicharged ions: from the higher lying shells of the projectile in the case of the high energy peaks, and from the target inner shells in the case of the low energy peaks. 10 refs., 3 figs

[en] High resolution measurements of the production of L₁L₂₃M Coster-Kronig and LMM-Auger electrons in slow collisions of C⁴⁺, N⁵⁺, O⁶⁺, and 0⁷⁺ with He and H₂ have been performed, using the method of 0⁰ Auger spectroscopy. For the latter three projectiles, strong Coster Kronig lines are observed, which are attributed to the configurations (core)2pnl, produced by double-electron capture. It is argued that production of these nonequivalent electron configurations must involve electron-electron correlation. From a comparison of the production cross sections for these Coster-Kronig electrons and the LMM-Auger electrons, it is further argued the correlation plays a significant role in two-electron transfer processes. 7 refs., 5 figs