A. C. H. Smith

Absolute cross sections for near-threshold electron-impact excitation of Be-like C2+, N3+, and O4+ ions

Abstract Absolute total cross sections for electron-impact excitation of the Be-like C2+, N3+, and O4+ ions have been measured near threshold using the merged electron-ion beams energy-loss (MEIBEL) technique and calculated using a close-coupling R-matrix (CCR) approach. The 2s2 1S → 2s2p 1P0 dipole-allowed transition from the ground state was investigated for all three ions and satisfactory agreement between experiment and theory is found. In addition, calculations and measurements for the 2s2p 3P0 → 2p2 3P allowed transition from the metastable level of C2+ are in excellent agreement. The sum of the cross sections for the 2s2 1S → 2s2p 3P0 and 2s2p 3P0 → 2s2p 1P0 spin-forbidden transitions, not completely resolved by the MEIBEL technique, are measured for C2+ and O4+ and compared to CCR calculations scaled to account for the ground state and metastable fractions of the target ion beams. The results for these unresolved transitions are in reasonable agreement except for a resonance feature measured in the 2s2p 3P0 → 2s2p 1P0 transition in C2+ that is not predicted by theory.

Merged-beam measurements of absolute cross sections for electron-impact excitation of S4+ (3s2 1S→3s3p1P) and S5+(3s2S→3p2P)

Absolute cross sections for electron-impact excitation of the dipole-allowed transitions S4+ (3s(2) S-1 -> 3s3p P-1) and S5+ (3s S-2 -> 3p P-2) were measured near threshold using the merged electron-ion beams energy-loss technique. Although the magnitudes of the measured cross sections are in reasonable agreement with available theoretical data, the experimental data indicate that the contributions of dielectronic resonances in the near-threshold region are underestimated by these calculations.

Recent experiments on near-threshold electron-impact excitation of multiply charged ions

Some recent measurements of excitation of multiply charged ions by electrons studied in beam-beam experiments are highlighted. The emphasis is on absolute total cross sections measured with the merged electron-ion beams energy-loss (MEIBEL) technique, although some results obtained with the crossed-beams fluorescence method are also presented. The MEIBEL technique allows the investigation of optically-allowed and forbidden transitions with sufficient energy resolution, typically about 0.2 eV, to resolve resonance structures in the cross sections. Results from the JILA/ORNL MEIBEL experiment on dipole-allowed transitions in several ions demonstrate the success of various theoretical methods in predicting cross sections in the absence of resonances. Comparisons of R-matrix calculations and measured cross sections for spin-forbidden transitions in Mg-like Si{sup 2+} and Ar{sup 6+}, however, show that further refinements to the theory are needed in order to more accurately predict cross sections involving significant contributions from dielectric resonance`s and interactions between neighboring resonances.

Absolute measurements of cross-sections for near-threshold electron-impact excitation of Na-like and Mg-like multiply-charged ions

Abstract The JILA-ORNL merged electron–ion beam energy-loss apparatus has been used to measure absolute cross-sections for excitation of multiply-charged ions over the past several years. Recent measurements for the Na-like ions Al2+ and Cl6+ and the Mg-like ion Cl5+ are presented and are compared with earlier measurements on the isoelectronic ions and with theoretical results.

Backscattering in electron‐impact excitation of multiply charged ions

For the first time, evidence for strong backscattering of electrons during near threshold excitation of highly charged ions has been observed. By measuring the apparent cross section for the electron‐impact excitation of Ar7+(3s→3p) as a function of center‐of‐mass velocity, over 90% of the inelastically scattered electrons were found to be deflected by more than 90 degrees in the center‐of‐mass frame. This unusual angular scattering has not been previously observed for atoms or ions, but may be typical for multiply charged ions. Our observation of backscattering has been confirmed recently by close‐coupling calculations and distorted‐wave calculations. Absolute total excitation cross sections measured in an energy range extending to 2.2 eV above threshold are also reported and are in agreement with close‐coupling calculations.