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arXiv:2406.16017 · Journal of Physics B Atomic Molecular and Optical Physics

Competing excitation quenching and charge exchange in ultracold Li-Ba+ collisions

Xiaodong Xing, Pascal Weckesser, Fabian Thielemann, Tibor Jónás, Romain Véxiau, Nadia Bouloufa-Maafa, Eliane Luc-Koenig, Kirk W. Madison, Andrea Orbán, Ting Xie, Tobias Schaetz, Olivier Dulieu

Published Oct 22, 2024• 4 citations
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Abstract

Abstract Hybrid atom-ion systems are a rich and powerful platform for studying chemical reactions, as they feature both excellent control over the electronic state preparation and readout as well as a versatile tunability over the scattering energy, ranging from the few-partial wave regime to the quantum regime. In this work, we make use of these excellent control knobs, and present a joint experimental and theoretical study of the collisions of a single 138Ba+ ion prepared in the 5 d 2 D 3 / 2 , 5 / 2 metastable states with a ground state 6Li gas near quantum degeneracy. We show that in contrast to previously reported atom-ion mixtures, several non-radiative processes, including charge exchange, excitation exchange and quenching, compete with each other due to the inherent complexity of the ion-atom molecular structure. We present a full quantum model based on high-level electronic structure calculations involving spin-orbit couplings. Results are in excellent agreement with observations, highlighting the strong coupling between the internal angular momenta and the mechanical rotation of the colliding pair, which is relevant in any other hybrid system composed of an alkali-metal atom and an alkaline-earth ion.

DOI
10.1088/1361-6455/ad8a35

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Abstract Hybrid atom-ion systems are a rich and powerful platform for studying chemical reactions, as they feature both excellent control over the electronic state preparation and readout as well as a versatile tunability over the scattering energy, ranging from the few-partial wave regime to the quantum regime. In this work, we make use of these excellent control knobs, and present a joint experimental and theoretical study of the collisions of a single 138Ba+ ion prepared in the 5 d 2 D 3 / 2 , 5 / 2 metastable states with a ground state 6Li gas near quantum degeneracy. We show that in contrast to previously reported atom-ion mixtures, several non-radiative processes, including charge exchange, excitation exchange and quenching, compete with each other due to the inherent complexity of the ion-atom molecular structure. We present a full quantum model based on high-level electronic structure calculations involving spin-orbit couplings. Results are in excellent agreement with observations, highlighting the strong coupling between the internal angular momenta and the mechanical rotation of the colliding pair, which is relevant in any other hybrid system composed of an alkali-metal atom and an alkaline-earth ion.
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.
In a recent experiment [P. Weckesser et al., Nature (London) 600, 429 (2021)], the quantum sss-wave regime was attained for an alkali-metal and alkaline-earth atom-ion combination (LiBa+)({\mathrm{LiBa}}^{+})(LiBa+). We investigate possible outcomes from the interaction of this ion-atom pair at quantum regimes from a theoretical point of view. For this purpose, Born-Oppenheimer potential energy surfaces are constructed for the three lowest dissociation channels of the ${(\mathrm{Ba}\text{\ensuremath{-}}\mathrm{Li})}^{+}$ molecular system using a multireference configuration interaction electronic-structure calculation. We present elastic, spin-exchange (SE), and diffusion cross sections in different energy regimes. The collisional properties of this system are calculated in terms of the scattering phase shifts and scattering cross sections, and the semiclassical behavior at a relatively high energy limit is also examined. For SE collisions, phase locking is obtained towards lower partial waves.
Atom-ion interactions occur through the electric dipole which is induced by the ion on the neutral atom. In a Langevin collision, in which the atom and ion overcome the centrifugal barrier and reach a short internuclear distance, their internal electronic states deform due to their interaction and can eventually alter. Here we explore the outcome products and the energy released from a single Langevin collision between a single cold 88^{88}88Sr+^{+}+ ion initialized in the metastable 4d2D5/2,3/24d^2D_{5/2,3/2}4d2D5/2,3/2​ states, and a cold 87^{87}87Rb atom in the 5s2S1/25s^2S_{1/2}5s2S1/2​ ground state. We found that the long-lived D5/2D_{5/2}D5/2​ and D3/2D_{3/2}D3/2​ states quench after roughly three Langevin collisions, transforming the excitation energy into kinetic energy. We identify two types of collisional quenching. One is an Electronic Excitation-Exchange process, during which the ion relaxes to the SSS state and the atom is excited to the PPP state, followed by energy release of ∼\sim∼ 3000 K⋅\cdot⋅kB_BB​. The second is Spin-Orbit Change where the ion relaxes from the higher fine-structure D5/2D_{5/2}D5/2​ level to the lower D3/2D_{3/2}D3/2​ level releasing ∼\sim∼ 400 K⋅\cdot⋅kB_BB​ into kinetic motion. These processes are theoretically understood to occur through Landau-Zener avoided crossings between the different molecular potential curves. We also found that these relaxation rates are insensitive to the mutual spin orientation of the ion and atoms. This is explained by the strong inertial Coriolis coupling present in ultracold atom-ion collisions due to the high partial wave involved, which strongly mixes different angular momentum states. This inertial coupling explains the loss of the total electronic angular-momentum which is transferred to the external rotation of nuclei. Our results provide deeper understanding of ultracold atom-ion inelastic collisions and offer additional quantum control tools for the cold chemistry field.
The quantum dynamics of ultracold collisions between rubidium atoms and excited metastable strontium ions is treated in the laboratory frame, enlightening the importance of the coupling between internal angular momenta of the particles and their mutual rotation. The study reveals a subtle competition between electronic excitation exchange and fine structure quenching, with no charge exchange, which is found to be very sensitive to the details of ion-atom interactions. The rate constant for electronic excitation exchange is found in agreement with the experimental results of Ben-Shlomi \textit{et al.} (Phys. Rev. A \textbf{102}, 031301(R) (2020)), while the rate for fine structure quenching is predicted to strongly depend on the initial polarization of the reactants.
We theoretically characterize interactions, energetics, and chemical reaction paths in ionic two-body and three-body systems of alkali-metal and alkaline-earth-metal atoms in the context of modern experiments with cold hybrid ion-atom mixtures. Using ab initio techniques of quantum chemistry such as the coupled-cluster method, we calculate ground-state electronic properties of all diatomic AB+A{B}^{+}AB+ and most of the triatomic A2B+{A}_{2}{B}^{+}A2​B+ molecular ions consisting of Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, and Yb atoms. Different geometries and wave-function symmetries of the ground state are found for different classes of molecular ions. We analyze intermolecular interactions in the investigated systems including additive two-body and nonadditive three-body ones. As an example we provide two-dimensional interaction potential-energy surfaces for KRb++K{\mathrm{KRb}}^{+}+\mathrm{K}KRb++K and Rb++Sr2{\mathrm{Rb}}^{+}+{\mathrm{Sr}}_{2}Rb++Sr2​ mixtures. We identify possible channels of chemical reactions based on the energetics of the reactants. The present results may be useful for investigating controlled chemical reactions and other applications of molecular ions formed in cold hybrid ion-atom systems.
Atomic clocks based on an Al+{\mathrm{Al}}^{+}Al+ ion sympathetically cooled by a laser-cooled alkaline-earth-metal ion have achieved unprecedented accuracy. Here, we investigate theoretically interactions and charge-transfer dynamics of an Al+{\mathrm{Al}}^{+}Al+ ion immersed in an ultracold gas of Rb and Sr atoms. We calculate potential energy curves and transition electric dipole moments for the (Al+Rb)+{(\mathrm{Al}+\mathrm{Rb})}^{+}(Al+Rb)+ and (Al+Sr)+{(\mathrm{Al}+\mathrm{Sr})}^{+}(Al+Sr)+ ion-atom systems using coupled cluster and multireference configuration interaction methods with scalar relativistic effects included within the small-core energy-consistent pseudopotentials in Rb and Sr atoms. The long-range interaction coefficients are also reported. We use the electronic structure data to investigate cold collisions and charge-transfer dynamics. Scattering of an Al+{\mathrm{Al}}^{+}Al+ ion with alkali-metal or alkaline-earth-metal atom is governed by one potential energy curve whereas charge transfer can lead to several electronic states mixed by the relativistic spin-orbit coupling. We examine the branching ratios resulting from the interplay of the short- and long-range effects, as well as the prospects for the laser-field control and formation of molecular ions. We propose to employ the atomic clock transition in an Al+{\mathrm{Al}}^{+}Al+ ion to monitor ion-atom scattering dynamics via quantum logic spectroscopy. The presented results pave the way for the application of atomic ions other than alkali-metal and alkaline-earth-metal ones in the field of cold hybrid ion-atom experiments.