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${}^3$P${}_1$) atoms, releasing only 150 cm${}^{-1}$ equivalent relative kinetic energy. Charge-exchange 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 of $9\times 10^{-10}$ cm${}^3$/s are in agreement given the uncertainty budgets.

13 pages, 16 figures

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