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We report new measurements of branching fractions for 20 UV and blue lines in the spectrum of neutral silicon (Sii) originating in the 3s²3p4s³P^o_1,2,¹P^o₁, and 3s3p³¹D^o_1,2upper levels. Transitions studied include both strong, nearly pure LS multiplets as well as very weak spin-forbidden transitions connected to these upper levels. We also report a new branching fraction measurement of the⁴P_1/2–²P^o_1/2,3/2intercombination lines in the spectrum of singly ionized silicon (Siii). The weak spin-forbidden lines of Siiand Siiiprovide a stringent test on recent theoretical calculations, to which we make comparison. The branching fractions from this study are combined with previously reported radiative lifetimes to yield transition probabilities and log(gf) values for these lines. We apply these new measurements to abundance determinations in five metal-poor stars.

Improved energy levels for singly ionized and neutral hafnium of both even and odd parity are determined from Fourier transform spectrometer data using a least-squares optimization procedure. Data from interferometric spectrometers provide much tighter control of systematic uncertainties of line position measurements than can be achieved using dispersive spectrometers. The strong optical and near-UV lines connecting these levels are most likely to be used in the determination of isotopic abundance patterns. Comparisons of new results to published ones strongly suggest that our energy levels have systematic uncertainties in the mK (1 mK = 0.001 cm⁻¹) range or smaller, and that widely used tables of energy levels for ionized Hf have systematic errors of approximately 70 mK.