Photon-mediated charge-exchange reactions between 39K atoms and 40Ca+ ions in a hybrid trap

Abstract

We present experimental evidence of charge exchange between laser-cooled potassium 39 K atoms and calcium 40 Ca + ions in a hybrid atom-ion trap and give quantitative theoretical explanations for the observations. The 39 K atoms and 40 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 40 Ca + ions, the increasing number of 39 K + ions, and 39 K number density as functions of time. Simultaneous trap operation is guaranteed by alternating periods of MOT and 40 Ca + cooling lights, thus avoiding direct ionization of 39 K by the 40 Ca + cooling light. We show that the K-Ca + charge-exchange rate coefficient increases linearly from zero with 39 K number density and, surprisingly, the fraction of 40 Ca + ions in the 4p 2 P 1/2 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 2 P 1/2 state producing ground-state 39 K + ions and metastable, neutral Ca (3d4p 3 P1) atoms, releasing only 150 cm −1 equivalent relative kinetic energy. Chargeexchange between either ground-or excited-state 39 K and ground-state 40 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.