Triplet-state transitions in beryllium: Accurate energies and oscillator strengths

We present highly accurate variational calculations of triplet excited states of the beryllium atom using all-electron explicitly correlated Gaussian basis sets. The employed approach that treats the electrons and the nucleus on the same footing enables precise determination of the energy spectrum, wave functions, and transition properties of low-lying ${}^{3}{S}^{e}$ and ${}^{3}{P}^{o}$ states of the naturally occurring $^{9}\mathrm{Be}$ isotope and the model ${}^{\ensuremath{\infty}}\mathrm{Be}$ system. Benchmark-quality total and transition energies, as well as oscillator strengths determined in both the length and velocity gauges, are reported. The results are compared with available theoretical and experimental data. The comparison shows excellent agreement of the present results with the experiment. They also provide refinement for the values where discrepancies had existed before.