2024MNRAS.527.4899Y

Empirical line lists BRYTS for the open shell molecule ⁸⁹Y¹⁶O (yttrium oxide) and its isotopologues are presented. The line lists cover the six lowest electronic states: $X\, {}^{2}\Sigma ^{+}$, $A\, {}^{2}\Pi$, $A^{\prime }\, {}^{2}\Delta$, $B\, {}^{2}\Sigma ^{+}$, $C\, {}^{2}\Pi$, and $D\, {}^{2}\Sigma ^{+}$ up to 60 000 cm^–1 (<0.167 $\mu$m) for rotational excitation up to J = 400.5. An ab initio spectroscopic model consisting of potential energy curves (PECs), spin-orbit, and electronic angular momentum couplings is refined by fitting to experimentally determined energies of YO, derived from published YO experimental transition frequency data. The model is complemented by empirical spin-rotation and Λ-doubling curves and ab initio dipole moment and transition dipole moment curves computed using multireference configuration interaction (MRCI). The ab initio PECs computed using the complete basis set limit extrapolation and the coupled-cluster CCSD(T) method with its higher quality provide an excellent initial approximation for the refinement. Non-adiabatic coupling curves for two pairs of states of the same symmetry A/C and B/D are computed using a state-averaged complete active space self-consistant field theory (CASSCF) and used to build diabatic representations for the $A\, {}^{2}\Pi$, $C\, {}^{2}\Pi$, $B\, {}^{2}\Sigma ^{+}$, and $D\, {}^{2}\Sigma ^{+}$ curves. The experimentally derived energies of ⁸⁹Y¹⁶O utilized in the fit are used to replace the corresponding calculated energy values in the BRYTS line list. Simulated spectra of YO show excellent agreement with the experiment, where it is available. Calculated lifetimes of YO are tuned to agree well with the experiment, where available. The BRYTS YO line lists are available from the ExoMol database (www.exomol.com).