Accurate transition probabilities in ions obtained by isoelectronic smoothing of line strengths

Lifetime measurements, based on the method of beam-foil spectroscopy, together with careful corrections for cascade effects, have in recent years yielded the line strength of the 2$s^2$ ${\mathrm{}}^1$S–2s2p ${\mathrm{}}^1$P° transition in Be-like ions with uncertainties ranging from 3.4% to 8.1% in Bei–O. By applying a method of isoelectronic smoothing of line strengths, similar to those earlier used for excitation energies and transition wavelengths, we have reduced the experimental uncertainties by about a factor of 2, to 1.9–3.6 % for these transitions. The fitted results are in excellent agreement with the best theoretical data. The applicability of this method to other isoelectronic sequences is briefly discussed.