Absolute cross sections for near-threshold electron-impact excitation of Be-like C 2+, N 3+, and O 4+ ions

Abstract

Absolute total cross sections for electron-impact excitation of the Be-like C 2+, N 3+, and O 4+ 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 2 s 2 1 S → 2 s2 p 1 P 0 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 2 s2 p 3 P 0 → 2 p 2 3 P allowed transition from the metastable level of C 2+ are in excellent agreement. The sum of the cross sections for the 2 s 2 1 S → 2 s2 p 3 P 0 and 2 s2 p 3 P 0 → 2 s2 p 1 P 0 spin-forbidden transitions, not completely resolved by the MEIBEL technique, are measured for C 2+ and O 4+ 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 2 s2 p 3 P 0 → 2 s2 p 1 P 0 transition in C 2+ that is not predicted by theory.