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

TL;DR

This paper provides detailed theoretical energy levels and transition data for Ce III, crucial for interpreting kilonova spectra, using advanced relativistic atomic calculations to improve accuracy over existing data.

Contribution

It offers the first comprehensive theoretical study of Ce III energy levels and transition probabilities relevant to kilonova spectral analysis, using the GRASP2018 computational package.

Findings

Calculated energy levels with high accuracy compared to NIST data.

Identified Ce III lines that explain kilonova NIR spectral features.

Validated the use of Babushkin gauge for more accurate line strengths.

Abstract

Doubly ionized cerium (Ce$^{2+}$) is one of the most important ions to understand the kilonova spectra. In particular, near-infrared (NIR) transitions of Ce III between the ground (5p$^6$ 4f$^2$) and first excited (5p$^6$ 4f 5d) configurations are responsible for the absorption features around 14,500 A. 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 database, our calculations reach the accuracy with the root-mean-square (rms) of 2732 cm$^{-1}$ 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 provide 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.