Theoretical investigation of energy levels and transitions for Ce III with applications to kilonova spectra

Doubly ionized cerium (Ce²⁺) is one of the most important ions to understand the kilonova spectra. In particular, near-infrared (NIR) transitions of Ce III between the ground (5p⁶ 4f²) and first excited (5p⁶ 4f 5d) configurations are responsible for the absorption features around 14 500 Å. However, there is no dedicated theoretical studies to provide accurate transition probabilities for these transitions. We present energy levels of the ground and first excited configurations and transition data between them for Ce III. Calculations are performed using the GRASP2018 package, which is based on the multiconfiguration Dirac-Hartree-Fock and relativistic configuration interaction methods. Compared with the energy levels in the NIST data base (Kramida et al. 2024), our calculations reach the accuracy with the root-mean-square (rms) of 2732 or 1404 cm^-1 (excluding one highest level) for ground configuration, and rms of 618 cm^-1 for the first excited configuration. We extensively study the line strengths and find that the Babushkin gauge provides the more accurate values. By using the calculated gf values, we show that the NIR spectral features of kilonova can be explained by the Ce III lines.