Vibronic coupling: the limit of using higher electronic states for optical cycling transitionsHaowen Zhou, Paweł Wójcik, Taras Khovost, Eric Hudson, Anastassia Alexandrova, Anna Krylov, Justin Caram

Optical cycling center (OCC) functionalization has been proposed to be a promising method of extending laser cooling capabilities to large molecules. However, as the molecules grow larger, the increased density of rovibrational states also promotes significant couplings between them, creating extra branching channels which is unfavorable for successful laser cooling. We have previously observed this effect between vibrational couplings within a single electronic state. In this work, we show that such coupling can occur between different electronic states, creating additional vibronic states. This effect is observed in various CaOPh-X and SrOPh-X species near the C state 0-0 transition, and in some cases near the B state if the nearby vibronic states are energetically favored. Combined with theoretical calculations, we are able to verify the presence of vibronic coupling by changing the relative energy gap, and roughly estimate a coupling strength on the order of ~1 cm-1. Such coupling is ubiquitous in many of the OCC-functionalized molecules. In general, this discourages the use of higher electronic states as optical cycling transitions, especially in larger molecules where many coupling channels to vibrational excited states of the lower electronic states must be considered.