Cold Heteronuclear Atom-Ion Collisions

We study cold heteronuclear atom-ion collisions by immersing a trapped single ion into an ultracold atomic cloud. Using ultracold atoms as reaction targets, our measurement is sensitive to elastic collisions with extremely small energy transfer. The observed energy-dependent elastic atom-ion scattering rate deviates significantly from the prediction of Langevin but is in full agreement with the quantum mechanical cross section. Additionally, we characterize inelastic collisions leading to chemical reactions at the single particle level and measure the energy-dependent reaction rate constants. The reaction products are identified by in-trap mass spectrometry, revealing the branching ratio between radiative and nonradiative charge exchange processes.

Figure 1

(a) Time-resolved fluorescence of a single ${}^{172}\mathrm{Yb}$ ion. The signal is averaged over 69 repetitions, and the error bars are determined by the photon shot noise. The line is a fit [24] resulting in a mean energy of the ion of $(230\pm 9)\mu \mathrm{eV}$. (b) Measured mean ion energy vs offset electric field used for tuning the excess micromotion. The line is a linear fit to the data.

Figure 2

(a) Atom loss from the magnetic trap vs mean energy of a single ${}^{172}\mathrm{Yb}^{+}$ ion. The experimental data are averaged over $\sim 70$ realizations each, and the standard error is given. The solid line is a numerical simulation based on the full elastic scattering cross section. The dashed line shows the prediction of the Langevin model. (b) Temperature increase of the neutral atom cloud vs mean energy of a single ${}^{172}\mathrm{Yb}^{+}$ ion. The solid line shows the numerical results using the full elastic scattering cross section. The dashed line shows the maximally possible evaporative heating effect. The mean energy of the ion is derived from Fig. 1.

Figure 3

(a) Schematic of the molecular potentials. (b) Measured ion loss probability as a function of offset electric field. The ion’s mean energy is derived from Fig. 1. Each data point is averaged over approximately 100 repetitions of the experiment. The dashed lines show the average of the data. (c) Position dependence of the ion loss probability for ${}^{174}\mathrm{Yb}^{+}$ as the ion is scanned through the cloud. The red line is the theoretical density profile of the thermal cloud. (d) The linear dependence of the ion loss rate of the density signals binary collisions leading to charge exchange reactions. Each data point is averaged over approximately 350 repetitions of the experiment.

Figure 4

(a) Mass spectrometry signal together with dashed lines indicating the expected frequencies. The black trace corresponds to two ${}^{174}\mathrm{Yb}^{+}$ ions. For one ${\mathrm{Rb}}^{+}$ ion and one ${\mathrm{Yb}}^{+}$ ion (red curve) the collective mode frequency is higher. No signals at the resonance for ${\mathrm{Yb}}^{+}$ and $(\mathrm{RbYb}{)}^{+}$ were obtained. (b) Observed distribution of the reaction products for two ${\mathrm{Yb}}^{+}$ ions after 8 s interaction time with the neutral Rb atoms (blue bars, 486 events). The red bars (543 events) show a comparative measurement without neutral atoms.