2021ApJS..254....5D

We report new branching fraction measurements for 199 UV and optical transitions of Hf II. These transitions range in wavelength (wavenumber) from 2068 to 6584 Å (48,322-15,183 cm^–1) and originate in 17 odd-parity upper levels ranging in energy from 38,578 to 53,227 cm^–1. The branching fractions are combined with radiative lifetimes reported in an earlier study to produce a set of transition probabilities and log(gf) values with accuracy ranging from 5% to 25%. Comparison is made to transition probabilities from the literature where such data exist. We use these new transition probabilities to derive improved Hf abundances in two metal-poor stars. HD 196944 is enhanced in s-process elements, and we derive log ε (Hf) = -0.72 ± 0.03 (σ = 0.09) from 12 Hf II lines. HD 222925 is enhanced in r-process elements, and we derive log ε (Hf) = 0.32 ± 0.03 (σ = 0.11) from 20 Hf II lines. These measurements greatly expand the number of potentially useful Hf II lines for analysis in UV and optical spectra.