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

¹ Department of PhysicsUniversity of Strathclyde, Glasgow G4 0NG, UK
e-mail: luis.fernandez-menchero@strath.ac.uk
² Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Cambridge CB3 0WA, UK

Received: 21 March 2014
Accepted: 15 May 2014

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 fine-structure levels in both the configuration interaction target and close-coupling collision expansions. These arise from the configurations 1s² 2 {s, p} nl with n = 3−7, and l = 0−4 for n ≤ 5 and l = 0−2 for n = 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 Zn²⁶⁺. 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.