Light-Assisted Ion-Neutral Reactive Processes in the Cold Regime: Radiative Molecule Formation versus Charge Exchange

We present a combined experimental and theoretical study of cold reactive collisions between laser-cooled ${\mathrm{Ca}}^{+}$ ions and Rb atoms in an ion-atom hybrid trap. We observe rich chemical dynamics which are interpreted in terms of nonadiabatic and radiative charge exchange as well as radiative molecule formation using high-level electronic structure calculations. We study the role of light-assisted processes and show that the efficiency of the dominant chemical pathways is considerably enhanced in excited reaction channels. Our results illustrate the importance of radiative and nonradiative processes for the cold chemistry occurring in ion-atom hybrid traps.

Figure 1

(a) Schematic of the experimental setup. (b) Laser-cooling schemes for ${}^{40}\mathrm{Ca}^{+}$ and ${}^{87}\mathrm{Rb}$. (c) Superposed fluorescence images of a ${\mathrm{Ca}}^{+}$ ion Coulomb crystal (blue) and a cloud of ultracold Rb atoms in the hybrid trap.

Figure 2

(a) Series of ${\mathrm{Ca}}^{+}$ ion laser-cooling fluorescence images as a function of the time of reaction with ultracold Rb atoms. (b) Pseudo-first-order analysis of the reaction kinetics of the experiment shown in (a). The uncertainties are smaller than the size of the symbols in the plot.

Figure 3

(a) Variation of reaction rate constants $k$ with ${\mathrm{Ca}}^{+}$ level populations. The bars indicate the relative level populations obtained by varying the detunings of the cooling-laser beams, their intercepts with the $y$ axis give the values of the corresponding rate constants. Each data point corresponds to an average of three consecutive measurements (average statistical uncertainty ($2\sigma$) $\Delta k=3\times {10}^{-12}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$). (b) Rate constant as a function of the average collision energy $⟨E_{\mathrm{coll}}⟩/k_{\mathrm{B}}$. The error bars denote the statistical uncertainty ($2\sigma$) of the measurements. (c) Resonant-excitation mass spectra of Coulomb crystals (i) before reaction, (ii) after reaction. The dashed vertical lines indicate the theoretical single-ion motional frequencies for the species indicated.

Figure 4

Computed $\mathrm{Rb}{\mathrm{Ca}}^{+}$ potential energy curves (PECs) in the regions of (a) $\mathrm{Rb}(5s)+{\mathrm{Ca}}^{+}(4s)$, (b) $\mathrm{Rb}(5s)+{\mathrm{Ca}}^{+}(3d)$, (c) $\mathrm{Rb}(5s)+{\mathrm{Ca}}^{+}(4p)$. The PECs of the relevant entrance channels of the reaction are highlighted with thick lines. PECs for ${}^1\Delta$ and ${}^3\Delta$ symmetries are omitted for clarity. Downward arrows suggest possible pathways for formation of ground-state $\mathrm{Rb}{\mathrm{Ca}}^{+}$ ions by radiative association. In (c), only PECs of ${}^1\Sigma ^{+}$ symmetry are displayed for clarity.