Data-based predictions of line strengths in alkali-metal-like isoelectronic sequences

The empirically observed isoelectronic linearization of alkali-metal-like line strengths ${\mathit{Z}}^2$S=A+B/(Z-C) is examined for the Li, Na, Cu, and Ag sequences. It is found that small deviations from this linearity observed at high Z can be removed by use of a correction factor obtained from the relativistic formulation of the hydrogenlike line strength. With this correction, low-Z data can be linearly interpolated to the nonrelativistic hydrogenic value in the high-Z limit. The linearization is tested in the data-rich Li, Na, and Cu sequences and is utilized to make precise predictions for the Ag sequence on the basis of a small number of accurate measurements.