A review of electron impact excitation data for the beryllium isoelectronic sequence (Z = 4 to 28)

Each beryllium-like ion is considered in turn, from Z = 4 to 28 (Be I to Ni XXV). References are given for the main electron impact excitation calculations performed (for example, Coulomb-Born, distorted-wave, and R-matrix calculations), in chronological order, noting whether tabular data (for example, collision strengths, rates, fits) are available. Comparisons of the thermally averaged effective collision strengths are made for three Δn = 0 and one Δn = 1 sample transitions for the even-charged ions. The most accurate data come from large R-matrix calculations which explicitly include configuration interactions, resonances, channel coupling associated with n ≤ 3 states, and, at higher Z, relativistic effects. Only a few ions have been treated in such a sophisticated way; further work will therefore be necessary if reliable data are required across the isoelectronic sequence. Recommendations are made for the best currently available for each ion. 50 refs., 12 figs