Photon-mediated charge exchange reactions between K atoms and Ca ions in a hybrid trap.

We present experimental evidence of charge exchange between laser-cooled potassium K atoms and calcium Ca ions in a hybrid atom-ion trap and give quantitative theoretical explanations for the observations. The K atoms and Ca ions are held in a magneto-optical (MOT) and a linear Paul trap, respectively. Fluorescence detection and high resolution time of flight mass spectra for both species are used to determine the remaining number of Ca ions, the increasing number of K ions, and K number density as functions of time. Simultaneous trap operation is guaranteed by alternating periods of MOT and Ca cooling lights, thus avoiding direct ionization of K by the Ca cooling light. We show that the K-Ca charge-exchange rate coefficient increases linearly from zero with K number density and the fraction of Ca ions in the 4p P electronically-excited state. Combined with our theoretical analysis, we conclude that these data can only be explained by a process that starts with a potassium atom in its electronic ground state and a calcium ion in its excited 4p P state producing ground-state K ions and metastable, neutral Ca (3d4p P) atoms, releasing only 150 cm equivalent relative kinetic energy. Charge-exchange between either ground- or excited-state K and ground-state Ca is negligibly small as no energetically-favorable product states are available. Our experimental and theoretical rate coefficients are in agreement given the uncertainty budgets.