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

Electron-impact-ionization cross sections have been measured from threshold to about 20 times threshold for Be-like ions ${\mathrm{B}}^{+}$, ${\mathrm{C}}^{2+}$, ${\mathrm{N}}^{3+}$, and ${\mathrm{O}}^{4+}$ with the use of the crossed-beams technique. The ion beams consist of mixtures of ions in the $2s^2{}_{}{}^1{}_{}{}^{}S$ ground state and the $2s2p{}_{}{}^3{}_{}{}^{}P_{0,1,2}^{}{}_{}{}^{}$ metastable states. For ${\mathrm{B}}^{+}$ and ${\mathrm{C}}^{2+}$ the metastable fractions could be changed, allowing estimates of ground-state cross sections, while for ${\mathrm{N}}^{3+}$ and ${\mathrm{O}}^{4+}$ 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 ($Z\le 15$) Be-like ions occur.