Electron-impact-excitation cross sections of lithiumlike ions

We present an easy to use expression for cross sections of electron-impact-induced 1s²nl→1s²n^′l^′ excitation transitions with 2<~n<~n^′<~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 ±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 ±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.