Theoretical investigations on radiative opacity of OsO: Contributions from the multiplet states

We report the first high-precision opacity calculations for osmium monoxide (OsO) over a wide temperature range (300-5000 K). Using a high-level relativistic electronic structure method, we generated a comprehensive line list from the 12 low-lying electronic states. The strong spin-orbit coupling effect in OsO leads to a near-degeneracy between the ground \rm X^1\Sigma^+ state and the first excited \rm a^3\Pi state. This results in a complex composite spectrum where triplet transitions contribute predominantly in the near ultraviolet region. At high temperatures, the high-density quintet transitions blend to form a distinctive quasi-continuous absorption background in the ultraviolet region. This study fills a critical gap for heavy transition-metal oxides in spectroscopic databases, and is helpful for detecting and diagnosing r-process elements in astronomical spectra. The datasets presented in this paper, including the complete line lists and absorption cross-sections for OsO at temperatures from 300 K to 5000 K, are openly available at https://www.doi.org/10.57760/sciencedb.j00113.00282.