Structural and electronic trends of optical cycling centers in polyatomic molecules revealed by microwave spectroscopy of MgCCH, CaCCH, and SrCCH

Laser-cooled molecules have enabled ground-breaking advances in quantum science. Transferring sophisticated optical cycling-based cooling techniques developed for simple diatomic molecules to complex polyatomic molecules remains a challenge, but will enable experiments of greater sensitivity and versatility. Achieving this goal requires a comprehensive understanding of the chemical properties that influence optical cycling efficiency. Using microwave rotational spectroscopy, we have derived precise structures and electronic properties for a family of metal-ligand radicals. We identify subtle but significant changes in bond lengths, electron distribution, and orbital hybridization to be quantitative measures of the ionic metal-ligand bonding character critical to efficient optical cycling. This work provides a foundation for elucidating transferable design principles for assessing how chemical modifications affect laser-cooling performance in larger molecules.