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 cl ose-coupling R-matrix (CCR) approach. The 2s(2) S-1 --> 2s2p P-1(0) dipole- allowed. transition from the ground state was investigated for all three io ns and satisfactory agreement between experiment and theory is found. In ad dition, calculations and measurements for the 2s2p P-3(0) --> 2p(2) P-3 all owed transition from the metastable level of C2+ are in excellent agreement . The sum of the cross sections for the 2s(2) S-1 --> 2s2p P-3(0) and 2s2p P-3(0) --> 2s2p P-1(0) spin-forbidden transitions, not completely resolved by the MEIBEL technique, are measured for C2+ and O4+ and compared to CCR c alculations 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 P-3(0) --> 2s2p P-1(0) transition in C2+ that is not predicted by theory. (C) 1999 Elsevier Science B.V.