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Vibronic branching ratios for nearly-closed rapid photon cycling of SrOH

The vibrational branching ratios of SrOH for radiative decay to the ground electronic state, $X^{2}Σ^{+}$, from the first two electronically excited states, $A^{2}Π$ and $B^{2}Σ^{+}$, are determined experimentally at the $\sim10^{-5}$ level. The observed small branching ratios enable the design of a full, practical laser-cooling scheme, including magneto-optical trapping and sub-Doppler laser cooling, with $>10^4$ photon scatters per molecule. Ab initio calculations sensitive to weak vibronic transitions are performed to facilitate the experimental measurement and analysis, and show good agreement with experiment.