R-matrix electron-impact excitation data for the Be-like iso-electronic sequence

Abstract

Aims. Emission lines from ions in the Be-like isoelectronic sequence can be used for reliable diagnostics of temperature and density of astrophysical and fusion plasmas over a wide range of temperatures. Surprisingly, interpolated data is all that is available for a number of astrophysically important ions. Methods. We have carried out intermediate coupling frame transformation R-matrix calculations which include a total of 238 finestructure levels in both the configuration interaction target and close-coupling collision expansions. These arise from the configurations 1s 2 2{s, p}nl with n = 3−7, and l = 0− 4f orn ≤ 5a ndl = 0− 2f orn = 6, 7. Results. We obtain ordinary collision strengths and Maxwell-averaged effective collision strengths for the electron-impact excitation of all the ions of the Be-like sequence, from B + to Zn 26+ . We compare with previous R-matrix calculations and interpolated values for some benchmark ions. We find good agreement for transitions n = 2−2 with previous R-matrix calculations but some disagreements with interpolated values. We also find good agreement for the most intense transitions n = 2−3 which contribute via cascade to the (n = 2) diagnostic radiating levels.