Atomic oscillator-strength data are of great interest to astronomers studying photospheric chemical abundances, stellar interiors and nucleosynthesis. We review the classic methods of measurement and then discuss our approach, which is based on fast-ion-beam laser-fluorescence techniques for both lifetime and branching-fraction measurements. Recently obtained large data sets in singly ionized lanthanides are discussed, with several examples illustrating the major advantages of this method: unambiguous identification of the upper level of a transition and greatly reduced spectral line blending.

The spontaneous-emission branching fractions of 32 levels of Pr II were measured by the fast-ion-beam laser-induced-fluorescence technique. The levels studied had energies from similar to 21 500 to similar to 29 000 cm(-1), and the decay branches detected were in the range from 250 to 850 nm. The experimental uncertainties are within 10%. Using our previously measured radiative lifetimes, we determined the Einstein A coefficients and oscillator strengths for 260 transitions. The results are important for stellar elemental abundance determinations.

We measured the spontaneous-emission branching ratios of 69 levels in Sm II selectively populated via single-frequency laser excitation of a 10 keV ion beam. The levels studied had term energies up to 29 600 cm–1, and decay branches with spontaneous emission in the range 250–850 nm were detected. The experimental accuracy was in the range of 10%. We used these branching ratios along with our previously determined radiative lifetimes to infer transition probabilities and oscillator strengths for 608 transitions in the wavelength range 363–771 nm, which are useful for stellar abundance determinations.PACS Nos.: 32.70.Cs, 95.30.Ky