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arXiv:1707.01729 · Physical Review A

Observation of collisions between cold Li atoms and Yb+ ions

J. Joger, Henning A. Fürst, Norman Vincenz Ewald, Thomas Feldker, Michał Tomza, R. Gerritsma

Published Sep 25, 2017• 65 citations
arXivHTMLPDF

Abstract

We report on the observation of cold collisions between $^{6}\mathrm{Li}$ atoms and ${\mathrm{Yb}}^{+}$ ions. This combination of species has recently been proposed as the most suitable for reaching the quantum limit in hybrid atom-ion systems, due to its large mass ratio. For atoms and ions prepared in the $^{2}S_{1/2}$ ground state, the charge-transfer and association rate is found to be at least ${10}^{3}$ times smaller than the Langevin collision rate. These results confirm the excellent prospects of $^{6}\mathrm{Li}\ensuremath{-}{\mathrm{Yb}}^{+}$ for sympathetic cooling and quantum information applications. For ions prepared in the excited electronic states $^{2}P_{1/2}, ^{2}D_{3/2}$, and $^{2}F_{7/2}$, we find that the reaction rate is dominated by charge transfer and does not depend on the ionic isotope or the collision energy in the range $\ensuremath{\sim}1--120\phantom{\rule{0.16em}{0ex}}\mathrm{mK}$. The low charge-transfer rate for ground-state collisions is corroborated by theory, but the $4f$ shell in the ${\mathrm{Yb}}^{+}$ ion prevents an accurate prediction for the charge-transfer rate of the $^{2}P_{1/2}, ^{2}D_{3/2}$, and $^{2}F_{7/2}$ states. Using ab initio methods of quantum chemistry we calculate the atom-ion interaction potentials up to energies of $30\ifmmode\times\else\texttimes\fi{}{10}^{3}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, and use these to give qualitative explanations of the observed rates.

DOI
10.1103/physreva.96.030703

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Charge-transfer cold Yb+{}^{+}++++ Rb collision dynamics is investigated theoretically using high-level ab initio potential energy curves, dipole moment functions, and nonadiabatic coupling matrix elements. Within the scalar-relativistic approximation, the radiative transitions from the entrance $A{\phantom{\rule{0.16em}{0ex}}}^{1}{\ensuremath{\Sigma}}^{+}$ to the ground $X{\phantom{\rule{0.16em}{0ex}}}^{1}{\ensuremath{\Sigma}}^{+}$ state are found to be the only efficient charge-transfer pathway. The spin-orbit coupling does not open other efficient pathways, but alters the potential energy curves and the transition dipole moment for the $A\ensuremath{-}X$ pair of states. The radiative, as well as the nonradiative, charge-transfer cross sections calculated within the ${10}^{\ensuremath{-}3}--10$ cm${}^{\ensuremath{-}1}$ collision energy range exhibit all features of the Langevin ion-atom collision regime, including a rich structure associated with centrifugal barrier tunneling (orbiting) resonances. Theoretical rate coefficients for two Yb isotopes agree well with those measured by immersing Yb+{}^{+}+ ions in an ultracold Rb ensemble in a hybrid trap. Possible origins of discrepancy in the product distributions and relations to previously studied similar processes are discussed.
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