ELECTRON-IMPACT-EXCITATION CROSS-SECTIONS OF LITHIUM-LIKE IONS

We present an easy to use expression for cross sections of electron-im pact-induced 1s(2)nl-->1s(2)n'l' excitation transitions with 2 less th an or equal to n less than or equal to n'less than or equal to 4 in mu ltiply charged ions of lithium isoelectronic sequence. This expression is based on our computations by convergent close-coupling (CCC) and C oulomb-Born with exchange and normalization (CBE) methods. We show sca ling of the CCC and CBE cross sections with atomic number Z and use th is scaling for presentation of the cross-section data. For 6 less than or equal to Z less than or equal to 30 the scaling is accurate to bet ter than +/-20% at any energy except in the vicinity of resonances. Co ntributions from indirect excitation channels do not scale with Z; how ever, for calculation of excitation rates it is enough to average loca lly these contributions over energy and to take them into account in a frame of a general scaling-based expression for the cross sections. F or 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 excita tion rates, computed using our scaling-based expression for the cross sections, with the excitation rates, computed using high-resolution (C CC and R matrix) cross sections and experimental data. [S1050-2947(97) 06211-2].