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 ${}^6\mathrm{Li}-{\mathrm{Yb}}^{+}$ 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 $\sim 1–120\mathrm{mK}$. 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 $30\times {10}^3{\mathrm{cm}}^{-1}$, and use these to give qualitative explanations of the observed rates.

• Received 7 July 2017

DOI:https://doi.org/10.1103/PhysRevA.96.030703