Charge Exchange Dynamics in Cold Collisions of ⁴⁰ CaH ⁺ and ³⁹ K

We report the observation of charge-exchange collisions between trapped calcium monohydride molecular ions (40CaH+) and ultracold potassium atoms (39K) in a hybrid ion-atom trap. The measured charge-exchange rate coefficient is significantly suppressed relative to the Langevin rate constant for the system. We use ab initio quantum-chemical calculations to model the (CaH-K)+ complex in the ground and excited electronic states and to identify possible charge-exchange mechanisms. Our calculations rule out a direct nonradiative charge-exchange reaction and instead point to a radiative mechanism, but do not quantitatively reproduce the measured rate, highlighting the need for a full-dimensional quantum dynamics treatment that includes vibrational motion and intermediate complex formation. Our work demonstrates that cold hybrid ion-atom platforms with molecular ions enable access to richer chemical complexity and collisional dynamics inaccessible in purely atomic systems.