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

TL;DR

This paper investigates the behavior of electron-impact excitation collision strengths for highly-excited states of H- and He-like ions, providing analytic formulas to extend modeling capabilities in astrophysical and fusion plasma research.

Contribution

It introduces new analytic formulas for collision strengths of highly-excited states (n >= 8) of H- and He-like ions, addressing limitations in existing calculations.

Findings

Collision strengths increase with principal quantum number n.

Analytic formulas accurately extrapolate collision strengths for n >= 8.

Coupling to highly-excited states significantly affects excitation rates.

Abstract

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 behaviour 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.