Changing the paradigm in f-containing cold molecules: the impact of spin-orbit coupling and f-d transitions on quasi-bound vibrational states

Abstract

Present-day state-of-the-art ab initio many-body calculations on f-block containing cold molecules heavily focus on perturbative approaches for spin-orbit coupling and exclude a substantial part of the atomic transitions in the $f$- and $d$-shell. Here, we demonstrate the cruciality of a proper relativistic treatment of the $f$- and $d$-shell in Yb-containing diatomics and the inclusion of $f\rightarrow d$ transitions to obtain physically sound elastic scatterings and pre-dissociation lifetimes. We focus on state-of-the-art relativistic many-body calculations for the Yb atom's ground- and excited-state and the YbLi$^+$ potential energy surface. For that purpose, we exploit various quantum many-body methods, namely a spin-free and four-component implementation of the coupled cluster singles and doubles (CCSD) model and its equation of motion extensions, spin-free complete active space self-consistent field, and internally contracted multi-reference (MR) configuration interaction approaches, and spin-free MRCCSD with a perturbative and full triples correction. We oppose scalar relativistic calculations to four-component variants to support the reliability of our EOM-CCSD study and shed new light on the interplay between these systems' spin-orbit coupling and the proper treatment of relativistic effects. Most importantly, we observe a significant shift in the electronic spectra of the $f\rightarrow d$ excitation block. We also provide new reference potential energy surfaces for ground and excited states for which theoretically sound elastic scattering and pre-dissociation lifetimes are calculated.