Scaling of collision strengths for highly-excited states of ions of the H- and He-like sequences

L. Fernández-Menchero (1,2), G. Del Zanna (3), N. R. Badnell (1)

(1) Department of Physics, University of Strathclyde. Glasgow G4 0NG, UK. (2) Department of Physics and Astronomy, Drake University, 2507 University Avenue. Des Moines, IA 50311, USA. (3) Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Cambridge CB3 0WA, UK.

Emission lines from highly-excited states (n ≥ 5) of H- and He-like ions have been detected in astrophysical sources and fusion plasmas. For such excited states, R-matrix or distorted wave calculations for electron-impact excitation are very limited, due to the large size of the atomic basis set needed to describe them. Calculations for n ≥ 6 are also not generally available. We study the behavior of the electron-impact excitation collision strengths and effective collision strengths for the most important transitions  used to model electron collision dominated astrophysical plasmas, solar, for example. We investigate the dependence on the relevant parameters: the principal quantum number n or the nuclear charge Z. We also estimate the importance of coupling to highly-excited states and the continuum by comparing the results of different sized calculations. We provide analytic formulae to calculate the electron-impact excitation collision strengths and effective collision strengths to highly-excited states (n ≥ 8) of H- and He-like ions. These extrapolated effective collision strengths can be used to interpret astrophysical and fusion plasma via collisional-radiative  modelling.

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