Establishing a nearly closed cycling transition in a polyatomic molecule

We study optical cycling in the polar free radical calcium monohydroxide (CaOH) and establish an experimental path towards scattering $\sim$${10}^4$ photons. We report rovibronic branching ratio measurements with precision at the $\sim {10}^{-4}$ level and observe weak symmetry-forbidden decays to bending modes with non-zero vibrational angular momentum. Calculations are in excellent agreement with these measurements and predict additional decay pathways. Additionally, we perform high-resolution spectroscopy of the $\tilde{\text{X}}{}^2{\mathrm{\Sigma }}^{+}({12}^{0}0)$ and $\tilde{\text{X}}{}^2{\mathrm{\Sigma }}^{+}({12}^{2}0)$ hybrid vibrational states of CaOH. These advances establish a path towards radiative slowing, 3D magneto-optical trapping, and sub-Doppler cooling of CaOH.