The role of metastable levels in Be-like ionization cross sections

We present recent experimental measurements and theoretical calculations for the electron impact ionization cross sections of C$^{2+}$, N$^{3+}$ and O$^{4+}$, with a view to studying the role of metastable levels. The theoretical calculations consist of R-matrix with pseudostates results for both the ground 1s$^2$ 2s$^2$ $^1S$ and the metastable 1s$^2$ 2s2p $^3P$ terms. We compare this with distorted-wave calculations, to determine the point along the iso-electronic sequence at which non-perturbative methods are appropriate. The experimental measurements were taken at the Oak Ridge National Laboratory, using a crossed-beam technique. A gas attenuation method was used to experimentally measure the metastable fraction. We compare these measurements with the metastable fractions inferred from our theoretical calculations.