Quantum-State-Controlled Collisions of Ultracold Polyatomic Molecules

Collisions between ultracold calcium monohydroxide (CaOH) molecules are realized and studied. Inelastic collision rate constants are measured for CaOH prepared in ground and excited vibrational states, and the electric field dependence of these rates is measured for molecules in single quantum states of the parity-doubled bending mode. Theoretical calculations of collision rate coefficients are performed and found to agree with measured values. The lowest collisional loss rates are for states with repulsive long-range potentials that shield ultracold molecules from loss channels at short distance. These results unveil the collisional behavior of parity doublet molecules in the ultracold regime, and lay the foundation for future experiments to evaporatively cool polyatomic molecules to quantum degeneracy.