Electron-impact-excitation cross sections of lithiumlike ions

Abstract

We present an easy to use expression for cross sections of electron-impact-induced ${1s}^{2}\mathrm{nl}\ensuremath{\rightarrow}{1s}^{2}{n}^{\ensuremath{'}}{l}^{\ensuremath{'}}$ excitation transitions with $2<~n<~{n}^{\ensuremath{'}}<~4$ in multiply charged ions of lithium isoelectronic sequence. This expression is based on our computations by convergent close-coupling (CCC) and Coulomb-Born with exchange and normalization (CBE) methods. We show scaling of the CCC and CBE cross sections with atomic number $Z$ and use this scaling for presentation of the cross-section data. For $6<~Z<~30$ the scaling is accurate to better than $\ifmmode\pm\else\textpm\fi{}20%$ at any energy except in the vicinity of resonances. Contributions from indirect excitation channels do not scale with $Z;$ however, for calculation of excitation rates it is enough to average locally these contributions over energy and to take them into account in a frame of a general scaling-based expression for the cross sections. For excitation rates, total inaccuracy caused by all simplifications in the cross-section presentation is likely to be less than $\ifmmode\pm\else\textpm\fi{}30%$ even for most risky cases. This assessment is based on comparison of excitation rates, computed using our scaling-based expression for the cross sections, with the excitation rates, computed using high-resolution (CCC and $R$ matrix) cross sections and experimental data.