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arXiv:1403.5091 · Physical Review A

Charge-exchange collisions between ultracold fermionic lithium atoms and calcium ions

Shinsuke Haze, Ryoichi Saito, Munekazu Fujinaga, Takashi Mukaiyama

Published Mar 30, 2015• 50 citations
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Abstract

Charge-exchange collisions between ultracold fermionic $^{6}\mathrm{Li}$ atoms and $^{40}\mathrm{Ca}^{+}$ ions are observed in the millikelvin temperature range. The reaction product of the charge-exchange collision is identified via mass spectrometry during which the motion of the ions is excited parametrically. The cross sections of the charge-exchange collisions between $^{6}\mathrm{Li}$ atoms in the ground state and $^{40}\mathrm{Ca}^{+}$ ions in the ground and metastable excited states are determined. Investigation of the inelastic collision characteristics in the atom-ion mixture is an important step toward ultracold chemistry based on ultracold atoms and ions.

DOI
10.1103/physreva.91.032709

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Charge-exchange collisions between ultracold fermionic 6Li^{6}\mathrm{Li}6Li atoms and 40Ca+^{40}\mathrm{Ca}^{+}40Ca+ ions are observed in the millikelvin temperature range. The reaction product of the charge-exchange collision is identified via mass spectrometry during which the motion of the ions is excited parametrically. The cross sections of the charge-exchange collisions between 6Li^{6}\mathrm{Li}6Li atoms in the ground state and 40Ca+^{40}\mathrm{Ca}^{+}40Ca+ ions in the ground and metastable excited states are determined. Investigation of the inelastic collision characteristics in the atom-ion mixture is an important step toward ultracold chemistry based on ultracold atoms and ions.
We investigate the energy dependence and the internal-state dependence of the charge-exchange collision cross sections in a mixture of 6Li^{6}\mathrm{Li}6Li atoms and 40Ca+{^{40}\mathrm{Ca}}^{+}40Ca+ ions. Deliberately excited ion micromotion is used to control collision energy of atoms and ions. The energy dependence of the charge-exchange collision cross section obeys the Langevin model in the temperature range of the current experiment, and the measured magnitude of the cross section is correlated to the internal state of the 40Ca+{^{40}\mathrm{Ca}}^{+}40Ca+ ions. Revealing the relationship between the charge-exchange collision cross sections and the interaction potentials is an important step toward the realization of the full quantum control of the chemical reactions at an ultralow-temperature regime.
Ultracold atom-ion collisions are an emerging field of research that can ultimately lead to their precise quantum control. In collisions in which the ion is prepared in an excited state, previous studies showed that the dominant reaction pathway was charge exchange. Here, we explored the outcome products and the energy released from a single ultracold collision between a single 88Sr+^{88}\mathrm{Sr}^{+}88Sr+ ion and a single 87Rb^{87}\mathrm{Rb}87Rb atom prepared in excited metastable and ground electronic states, respectively, with control over their relative spins. We found that the ion's long-lived D5/2{D}_{5/2}D5/2​ and D3/2{D}_{3/2}D3/2​ states quench after roughly three collisions, acquiring immense kinetic energy in the process. By performing single-shot thermometry on the ion after the collision, we identified two dominant reaction pathways: electronic excitation exchange and spin-orbit change. In contrast to previous experiments, we observed no charge-exchange events. These processes are theoretically understood to occur through Landau-Zener avoided crossings leading to the observed reaction pathways. We also found that spin orientation has almost no effect on the reaction pathways, due to strong Coriolis-spin mixing. Our results provide a deeper understanding of ultracold atom-ion inelastic collisions and offer additional quantum control tools for the cold chemistry field.
Abstract We report the observation of charge-exchange collisions between trapped calcium monohydride molecular ions (40CaH+) and ultracold potassium atoms (39K) in a hybrid ion-atom trap. The measured charge-exchange rate coefficient is significantly suppressed relative to the Langevin rate constant for the system. We use ab initio quantum-chemical calculations to model the (CaH-K)+ complex in the ground and excited electronic states and to identify possible charge-exchange mechanisms. Our calculations rule out a direct nonradiative charge-exchange reaction and instead point to a radiative mechanism, but do not quantitatively reproduce the measured rate, highlighting the need for a full-dimensional quantum dynamics treatment that includes vibrational motion and intermediate complex formation. Our work demonstrates that cold hybrid ion-atom platforms with molecular ions enable access to richer chemical complexity and collisional dynamics inaccessible in purely atomic systems.
The role of electronic excitation in inelastic collisions between ultracold Ca atoms and Ba(+) ions, confined in a hybrid trap, is studied for the first time. Unlike previous investigations, this system is energetically precluded from undergoing inelastic collisions in its ground state, allowing a relatively simple experimental determination and interpretation of the influence of electronic excitation. It is found that while the electronic state of the ion can critically influence the inelastic collision rate, the polarizability mismatch of the neutral atom electronic states suppresses short-range collisions, and thus inelastic processes, involving electronically excited neutral atoms. As a result of these features, it is experimentally demonstrated that it is possible to mitigate inelastic collision loss mechanisms in these systems, marking an important step toward long-lived hybrid atom-ion devices.