Measured electron-impact ionization of Be-like ions: B^{+}, C^{2+}, N^{3+}, and O^{4+}

Electron-impact-ionization cross sections have been measured from threshold to about 20 times threshold for Be-like ions B/sup +/, C/sup 2 +/, N/sup 3 +/, and O/sup 4 +/ with the use of the crossed-beams technique. The ion beams consist of mixtures of ions in the 2s/sup 2/ /sup 1/S ground state and the 2s2p /sup 3/P/sub 0,1,2/ metastable states. For B/sup +/ and C/sup 2 +/ the metastable fractions could be changed, allowing estimates of ground-state cross sections, while for N/sup 3 +/ and O/sup 4 +/ the metastables appear to dominate the beams and the fractions could not be changed. The measured cross sections are compared with a variety of predictions. Distorted-wave calculations compare most favorably, but discrepancies up to 40% are found. The metastable-state--ground-state mixture complicates the comparisons for these beam experiments and is probably an issue for all environments where light (Z< or =15) Be-like ions occur.