Deperturbation Analysis of Low-Lying Electronic States in YbO

Open-shell Yb-containing molecules like YbF and YbOH have played an important role in searches for parity non-conservation in molecules (Phys. Rev. A 106, 062804 (2022), New J. Phys. 25, 073014 (2023)). Comparisons with closed-shell counterparts such as YbO can provide further insight, but closely overlapping low-lying states challenge theoretical analysis. A combination difference analysis of new Fourier transform microwave data across three Yb-isotopologues (¹⁷²YbO, ¹⁷⁴YbO, and ¹⁷⁶YbO) and earlier chemiluminescence data (J. Mol. Spec. 218, 235 (2003)) has yielded improved rotational constants. Previous experimental studies reported a perturbation in the X state (v = 4-3) vibrational spacing (J. Mol. Spec. 101, 332 (1983)), likely due to nearly degenerate (Ω = 1) B and C state vibrational energy levels. Building upon prior A state YbF deperturbation analysis (J. Mol. Spec. 386, 111625 (2022)), a three-state Morse potential deperturbation analysis was performed to recover the unperturbed energy levels. This analysis allows the underlying electronic structure—equilibrium bond lengths, electronic energy offsets, vibrational frequencies, and first-order anharmonicities—as well as electronic coupling parameters responsible for the observed perturbations to be directly characterized.