Optical cycling functionalization of aromatic molecules, towards laser cooling

Laser cooling relies on photon cycling, which can be enabled in polyatomic molecules when an “optical cycling center” (OCC) is attached to an electronegative ligand. It was proposed that molecules with a (metal) alkaline-earth(I)-oxide-radical structure, would have good OCC properties and, thus, would be amenable to laser cooling [1]. More recent theoretical work has indicated that the alkaline-earth(I)-oxide unit attached to a benzene ring would offer a good OCC that would be tunable by substituting the ring hydrogen atoms with more electronegative species [2]. Theory has also indicated that larger rings (such as naphthalene, pyrene, and coronene) can also provide good optical cycling properties [3]. We report the results of dispersed fluorescence measurements on CaO-Ph-X (Ph : phenyl, X = F, CH3, CF3) in a cryogenic buffer gas at 9 K. We find that the vibrational branching ratio (VBR) to the ground vibrational state is >90% for all species and 99% for CaO-Ph-3,4,5F [4]. We also study the napthol-based molecules CaO-Nap and SrO-Nap, and find that the 2-naphthyl positional variant of CaO-Nap also has a highly diagonal VBR, 96% [5]. These results demonstrate that the same principles that have led to laser cooling of di-, tri- and poly-atomic molecules (e.g. SrF, CaOH and CaOCH3) can likely be extended to phenolic and aromatic molecules. 

[1] Kozyryev, et al, ChemPhysChem 2016, 17, 3641

[2] Dickerson, et al, PRL 2021, 126, 123002

[3] Dickerson, et al, JPCL 2021, 12, 3989–3995

[4] Zhu, et al, 2022, manuscript in preparation

[5] Mitra, et al, 2022, arXiv:2202.01685