Unraveling vibronic interactions in molecules functionalized with optical cycling centers

We report detailed characterization of the vibronic interactions between the first two electronically excited states, à and B̃, in SrOPh (Ph = phenyl, -C6H5) and its deuterated counterpart, SrOPh-d5 (-C6D5). The vibronic interactions, which arise due to non-adiabatic coupling between the two electronic states, mix the B̃,ν0 state with the energetically close vibronic level, Ã,ν21ν33, resulting in extra transition probability into the latter state. This state mixing is more prominent in the deuterated molecule because of the smaller energy gap between the interacting states. We model the mixing of the à and B̃ states using the Köppel-Domcke-Cederbaum (KDC) Hamiltonian parameterized in the diabatic framework of Ichino, Gauss, and Stanton on the basis of equation-of-motion coupled-cluster calculations. The simulation attributes the observed mixing to a second-order effect mediated by linear quasi-diabatic couplings between the Ã-C̃ and B̃-C̃ states. Based on the measured spectra, we deduce an effective coupling strength of ∼0.5 cm-1. Non-adiabatic couplings between different electronic states are an important factor that should be considered in the design of laser-cooling protocols for complex molecules.