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

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 2s^{2 1}S → 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 → 2p^{2 3}P allowed transition from the metastable level of C²⁺ are in excellent agreement. The sum of the cross sections for the 2s^{2 1}S → 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.