Low-lying electronic states of rhodium monocarbide and iridium monocarbide

This article presents a theoretical exploration of the electronic spectra of two cobalt group monocarbide molecules, RhC and IrC, utilizing the complete active space second-order perturbation theory (CASPT2) method. It encompasses all the low-lying \varLambda-S and \varOmega states below 4 eV (approximately 32000 cm^-1). The theoretical findings corroborate the previously established ground states: the \rm X ^2\Sigma^+_1/2 state for RhC and the \rm X_1 ^2\Delta_5/2 state for IrC, alongside the closely lying first excited state \rm X_2 ^2\Sigma^+_1/2. Additionally, a couple of excited states observed experimentally have been reasonably assigned based on these theoretical results, and the derived spectroscopic constants and radiative lifetimes (for RhC only) are generally consistent with available experimental data. These theoretical insights not only deepen our understanding of the electronic structures of RhC and IrC but also lay the groundwork for subsequent spectroscopic research on these two molecules.