High-Sensitivity Measurements of Franck-Condon Factors Enabled by Optical Cycling

Recent experiments have successfully laser cooled a variety of molecules, including diatomic, linear triatomic, and symmetric top species. Laser cooling and trapping can require repeatedly scattering more than 10,000 photons per molecule, so all potential losses above the level of 1 part in 10⁵ must be identified and repumped to mitigate losses. Here, we use optical cycling to measure vibrational branching ratios of polyatomic molecules, achieving relative intensity sensitivities at the 10^-5 level. The apparatus described can be adapted to probe any laser coolable molecule simply by tuning two laser wavelengths. Using CaOH, YbOH, and CaOCH₃ as examples, we discuss how these high-precision branching ratio measurements allow us to infer values for Renner-Teller and (pseudo)-Jahn-Teller parameters in polyatomic molecules.