Measurement of a Large Chemical Reaction Rate between Ultracold Closed-Shell ${}^{40}\mathrm{Ca}$ Atoms and Open-Shell ${}^{174}\mathrm{Yb}^{+}$ Ions Held in a Hybrid Atom-Ion Trap

Ultracold ${}^{174}\mathrm{Yb}^{+}$ ions and ${}^{40}\mathrm{Ca}$ atoms are confined in a hybrid trap. The charge exchange chemical reaction rate constant between these two species is measured and found to be 4 orders of magnitude larger than recent measurements in other heteronuclear systems. The structure of the ${\mathrm{CaYb}}^{+}$ molecule is determined and used in a calculation that explains the fast chemical reaction as a consequence of strong radiative charge transfer. A possible explanation is offered for the apparent contradiction between typical theoretical predictions and measurements of the radiative association process in this and other recent experiments.

Figure 1

MOTION trap schematic. ${\mathrm{Yb}}^{+}$ ions and Ca atoms are laser cooled and trapped in the central region. Typical images of the ultracold clouds, shown in the upper left of the figure, are used for a 3D reconstruction that yields the degree of overlap. Ions are also detected with a voltage-gated CEM.

Figure 2

(a) ${\mathrm{Yb}}^{+}$ ion trap lifetime in the presence of the MOT (solid curve) compared to the lifetime without a MOT (dashed curve). Dotted curves are single exponential decay fits. Inset: Images of the ${\mathrm{Yb}}^{+}$ ion cloud in the presence of the MOT at various times. (b) Loss rate constant as a function of the Mathieu stability parameter q and the trap depth $T_D$.

Figure 3

(a) Total experimental (points) and theoretical (black line) charge exchange rate constants as a function of ion kinetic energy. The horizontal location of the data point is the value of the secular kinetic energy as measured by the Doppler fluorescence technique. The horizontal “error bars” represent the contribution of the micromotion to the overall kinetic energy. Also shown are the calculated rate constants for the three charge exchange pathways. (b) The $X{}^2\Sigma ^{+}$ and $A{}^2\Sigma ^{+}$ potential energy curves and (c) the corresponding transition dipole moment of the ${\mathrm{CaYb}}^{+}$ molecular ion as a function of the internuclear separation $R$. The dotted curves are the diabatized potentials constructed from these potentials. The dashed curve gives the coupling constant between the diabatic potentials.