The dependence of atomic oscillator strengths (f values) on nuclear charge, which leads to systematic trends along isoelectronic sequences, is discussed with particular reference to beam foil spectroscopy. The theoretical background for the nuclear charge dependence is given first, and then several characteristic examples are reviewed in detail. The beam foil technique of measuring lifetimes is one of the most important sources of data for these studies, because it readily permits reliable experimental determinations of f values over several stages of ionization within isoelectronic sequences. It is shown that experimental data are most urgently needed when f values are affected by cancellation in the transition integral or configuration interaction effects, since in these cases most theoretical approaches run into great difficulties.
W. L. Wiese, "Systematic Trends of Atomic Oscillator Strengths in Isoelectronic Sequences," Appl. Opt. 7, 2361-2366 (1968)