Crossed beams of electrons and Al2+ ions combined with fluorescence detection were used to measure the absolute cross section for electron-impact excitation of the ions to yield 186.3 nm and 185.5 nm photons from the 3s-3p transition. The measured cross section near the 6.7 eV threshold energy is about 16x10(-16) cm(2), and some resonance structure near threshold is indicated. Total relative uncertainties (1sigma) are typically about 6%, to be combined with an 11% absolute calibration uncertainty. Total uncertainties at high confidence level (90%) are around 20%, using a coverage factor of 1.7. Comparisons made with various theoretical calculations, specifically distorted wave and nine-state close coupling, show better agreement of experimental values with distorted wave calculations, with the measured values being higher near threshold than the most recent close-coupling calculations.

Crossed beams of electrons and ions have been used to measure absolute cross sections for single ionization of Si6+ and Si7+ ions from below threshold to 1400 eV. The measurements for Si6+ and Si7+ agree with distorted-wave calculations for direct ionization within 20% and 5%, respectively, suggesting that this mechanism is the dominant contribution to the cross section. The bulk of the measurements were performed with the more abundant Si-28 isotope; however, due to beam impurities, measurements with Si-29 were used to correct the Si-28 data.

The electron-impact single-ionization cross section of Ar7+ has been measured from 18 to 1186 eV using a crossed-beams technique. The present results are higher than previous measurements at the higher energies and are in good agreement with recent distorted-wave calculations that include contributions from excitation autoionization. Below the threshold for direct ionization at 144 eV, the cross section contains contributions due to space-charge modulation and excitation of metastable ions in the target beam into autoionizing states. These transitions are predominantly spin allowed and dipole forbidden. Scaling laws for sodiumlike ions are discussed.

A new interference mechanism, analogous to "classic" double-slit electron scattering, has been identified in low-energy ion-atom collisions. This Coulomb "path" interference results from the existence of two trajectories, indistinguishable with respect to laboratory energy and emission angle, along which ejected autoionizing electrons may be scattered by the attractive Coulomb potential of the slowly receding spectator ion. A simple semiclassical description is presented. Calculated model line shapes are in excellent agreement with the strong angular dependence of the interference structure observed in the He-target 2s21S autoionizing line shape measured near 0-degrees following 10-keV He+ +He collisions.

For the first time absolute total cross sections for electron-impact excitation of a multiply-charged ion have been measured using an electron-energy- loss technique. Measurements were made near threshold for the process e + Si3+(3s2S1/2) → e + Si3+ (3p2P1/2,3/2) - 8.88 eV. The 10-15 cm2 measured cross section agrees with results of 7-state close coupling calculations to better than the ±20% (90% CL) total uncertainty of the measurements. Convoluting the theoretical curve with a Gaussian energy distribution indicates an energy width of 0.15 ≲ AE ≲ 0.20 eV.

Autoionizing electrons emitted following low energy ion-atom collisions may scatter significantly from the receding spectator ion's attractive Coulomb field. In such cases the observed electron intensity is focused'' in the direction of the scattering ion as a result of the effective compression of the emission solid angle. In addition, interference may occur between trajectories, corresponding to electrons scattering around opposite sides of the ion, which lead to the same final laboratory electron energy and emission angle. This Coulomb path'' interference mechanism manifests itself in the uncharacteristically rapid angular dependence of the He target 2s{sup 2} {sup 1}S autoionizing state measured near 0{degree} following low energy He{sup +} + He collisions. A classical trajectory model for Coulomb focusing is presented and a semi-classical approximation is used to model the Coulomb path'' interference mechanism. In this description we account for the evolution of the phase of the autoionizing state until its decay and the path dependence of the amplitude of the emitted electron following decay of the autoionizing state. Calculated model lineshapes, which include contributions from adjacent overlapping resonances, reproduce quite well the angular dependence observed in the data near 0{degree}. 14 refs., 7 figs.

Atomic data for the electron impact ionization of ions in the nickel isonuclear sequence are reviewed. Calculations have been performed for all charge states in the distorted-wave approximation. In 8 of the 28 ions of the nickel isonuclear sequence, experiment and theory are compared. In many of the ions, excitation-autoionization contributions to the total cross section are found to be quite important. For intermediate charge state ions a large fraction of the experimental cross section may be attributed to ionization from metastable levels of low-lying excited configurations. Maxwellian collisional rate coefficients are calculated from the cross section data and presented in parametrized form.

Coulomb "path" interference results from the existence of two trajectories, indistinguishable with respect to laboratory energy and emission angle, along which ejected autoionizing electrons may be scattered by the attractive Coulomb potential of the slowly receding spectator ion. Experimental evidence for this new phenomenon exists in the uncharacterestically rapid angular dependence of the postcollision broadened lineshape of the He target 2s21S autoionizing state measured near 0-degrees following 10 keV He+ + He collisions. We present a semiclassical model for this effect in which we account for the path dependence of the amplitude of the ejected electron following decay of the autoionizing state. Calculated model lineshape, which include contributions from adjacent overlapping resonances, are found to be in excellent agreement with the angular dependence observed in the data. Similar interference structure is observed in the calculated lineshape for a single isolated 2s21S resonance.

A crossed-electron-ion-beam experiment was used to measure the total ionization cross section for Ar8+ in the energy range from 50 to 600 eV. The present results are roughly in agreement with the previous measurements of Defrance et Defrance et al. [Nucl. Instrum. Methods Phys. Res. B 23, 256 (1987)] above 450 eV. However, the present measurements show a significant contribution below the threshold for direct ionization. The present measurements agree with the distorted-wave calculations of Pindzola et al. [Phys. Rev. A 41, 1375 (1990)] when 3% of the incident ion beam is assumed to be in metastable states and excitation autoionization from metastable states is included.

The available data for electron collision processes of most importance to the fusion community are evaluated and referenced in this summary for collisions involving the primary metallic impurity elements found in present and near-future fusion devices. Recommendations are made concerning work needed in the future to fill in gaps and to complete our basic understanding of the important processes involving these impurity elements.

For the first time absolute total cross sections for electron-impact excitation of a multiply-charged ion have been measured using an electron-energy-loss technique. Measurements were made near threshold for the process e + Si3+ (3s2S1/2) → e + Si3+ (3p2P1/2,3/2) - 8.88 eV. The 10−15 cm2 measured cross section agrees with results of 7-state close coupling calculations to better than the ±20% (90% CL) total uncertainty of the measurements. Convoluting the theoretical curve with a Gaussian energy distribution indicates an energy width of 0.15 ≲ ΔE ≲ 0.20 eV.

Absolute cross sections for electron-impact ionization of ${\mathrm{Ti}}^{5+}$ have been measured from below threshold to 1500-eV collision energies. Distorted-wave calculations including direct ionization and excitation autoionization from the 3${s}^{2}$3${p}^{5}$ ground state and from the 3${s}^{2}$3${p}^{4}$3d excited configurations have been performed. Comparison of experiment and theory indicates that about 33% of the incident ions were in metastable configurations. Ionization rate coefficients and fitting parameters derived from the experimental results are also presented.

Absolute cross sections for ionization of the Na-like ions ${\mathrm{Ti}}^{11+}$ and ${\mathrm{Cr}}^{13+}$ due to electron impact have been measured over the energy ranges 300--800 eV and 500--1500 eV, respectively. These measurements confirm predictions by Griffin, Pindzola, and Bottcher [Phys. Rev. A 36, 3642 (1987)] concerning the magnitude of indirect processes for these ions.

Measured cross sections for single ionisation of Li-like N4+, O5+ and Ne7+ by electron impact are reported for energies from threshold to 2500 eV. The animated crossed beams method has been employed. In addition to direct ionisation, the 1s22s to 1s2l2l' excitation followed by autoionisation is shown to give a significant contribution to the total cross section, and its magnitude has been estimated for all three ions. Comparisons are made with other experimental results and different theoretical predictions. Cross sections for N4+ and O5+ are in a good agreement with previous results and the first result on Ne7+ ionisation is well reproduced by the Coulomb-Born calculations.

Absolute measurements are presented for electron-impact single ionization of U{sup 10+}, U{sup 13+}, and U{sup 16+}, and for double ionization of U{sup 10+} and U{sup 13+} from below the thresholds to 1500 eV. Metastable components are observed in all three incident-ion beams. The single-ionization cross sections are enhanced due to indirect ionization processes by factors of 4--20 near their peaks, and the double-ionization cross sections are apparently also dominated by indirect ionization. Good agreement is found with distorted-wave calculations for total ionization of U{sup 16+}, the only case where detailed calculations are available for comparison. Individual or closely spaced resonances are clearly resolved in the single-ionization channels for U{sup 13+} and U{sup 16+}.

Absolute measurements are presented for electron-impact single ionization of ${\mathrm{U}}^{10+}$, ${\mathrm{U}}^{13+}$, and ${\mathrm{U}}^{16+}$, and for double ionization of ${\mathrm{U}}^{10+}$ and ${\mathrm{U}}^{13+}$ from below the thresholds to 1500 eV. Metastable components are observed in all three incident-ion beams. The single-ionization cross sections are enhanced due to indirect ionization processes by factors of 4--20 near their peaks, and the double-ionization cross sections are apparently also dominated by indirect ionization. Good agreement is found with distorted-wave calculations for total ionization of ${\mathrm{U}}^{16+}$, the only case where detailed calculations are available for comparison. Individual or closely spaced resonances are clearly resolved in the single-ionization channels for ${\mathrm{U}}^{13+}$ and ${\mathrm{U}}^{16+}$.