Ba oscillator strengths from a laser-excited vapor.

Abstract

We describe a method for evaluating the consistency of oscillator strengths in the neutral and ionized species of an atom. The method is based on the constant number density of a dense vapor. In our procedure the vapor is excited and then ionized within less than 1 \ensuremath{\mu}s by resonant laser irradiation. The column densities ${N}_{i}$${l}_{i}$ of the levels that are appreciably occupied during the excitation and ionization are sampled by absorption, emission, and/or anomalous-dispersion measurements in a set of suitable spectral lines. Given correct oscillator strengths ${f}_{i}$ for these lines, the column density ${\mathcal{J}}_{i}$ ${N}_{i}$${l}_{i}$ (which is actually determined from the measured quantities ${N}_{i}$${f}_{i}$${l}_{i}$) will remain constant while excitation and ionization take place. In barium---with diagnostics based on accepted f values in the literature---the column density appeared reduced by a factor of 2 during the initial portion of the excitation, i.e., while only the low-lying levels of neutral barium were predominantly excited. Upon complete ionization, the column density took again the value derived before the laser irradiation, when all atoms had been in the ground state. From this behavior we concluded that the literature oscillator strengths for absorption lines involving the neutral ground state and the ground state and low-lying levels of ${\mathrm{Ba}}^{+}$ were consistent on a relative scale, but that the f values for absorption transitions from the low-lying metastable levels and from the resonance level of neutral barium were too large by a factor of 2. We recommend a set of correction factors to be applied to the existing data in the literature. Our conclusions are supported by the results of a critical analysis of recent oscillator strength and lifetime measurements for neutral barium.