Identifying laser cooling schemes for molecules with non-diagonal Franck-Condon Factors*

Due to their rich internal structure and complex interactions, ultracold neutral and ionic molecules offer exciting prospects to advance quantum chemistry, sensing, simulation, and information. In recent years, ultracold neutral molecules have been prepared via assembly from ultracold atoms (e.g., KRb, NaCs) or via direct laser cooling of molecules with highly diagonal Franck-Condon factors (e.g., SrF, CaF, YO). Much less explored are ultracold ionic molecules (e.g., N₂⁺, CaH⁺, HCO⁺, N₂H⁺). They are typically cooled via sympathetic cooling with atomic ions, but this approach limits research to experiments that are unaffected by the presence of atomic ions in the trap.

Recently, we have theoretically demonstrated the viability of laser cooling in C₂ [1], a molecule with substantial off-diagonal Franck-Condon factors. This insight motivates the investigation of broader classes of molecules. As an example, we present a scheme for direct laser cooling of OH⁺ and evaluate the prospects for its practical implementation. In addition, we are developing a graph-based algorithm for the systematic analysis of molecular spectroscopic data to reveal laser cooling schemes for molecules that have so far been unexplored.

[1] N. Bigagli, D. W. Savin, & S. Will. Laser cooling scheme for the carbon dimer (¹²C₂). Phys. Rev. A 105, L051301 (2022)