Untangling ground state perturbations in YbO

Open shell Yb-containing molecules like YbF and YbOH have provided important testbeds in the search for parity non-conservation in molecules (Phys. Rev. A 106, 062804 (2022), New J. Phys. 25, 073014 (2023)). Comparing properties with closed shell YbO can shed further light, but overlapping low-lying states confound straightforward X (ground) state analysis and challenge theoretical approaches. A combination difference analysis of new Fourier transform microwave (FTMW) spectroscopy measurements of ¹⁷⁴YbO combined with earlier near infrared chemiluminescence data (J. Mol. Spec. 218, 235 (2003)) have resulted in much improved YbO rotational constants. Adding additional new FTMW data for even isotopes ¹⁷²YbO and ¹⁷⁶YbO results in a further improvement and also reveals large Born-Oppenheimer breakdown offsets. A previously noted perturbation in the X state 4-3 vibrational spacing (J. Mol. Spec. 101, 332 (1983)) is likely due to nearly degenerate B state vibrational levels. A deperturbation analysis similar to that performed on the A state of YbF (J. Mol. Spec. 386, 111625 (2022)) is underway.