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arXiv:2505.03939 · Physical Review A

Trap-induced atom-ion complexes: A time-independent approach

Zhongqi Liang, R. S. Shi, Jesús Pérez‐Ríos

Published Oct 6, 2025• 3 citations
arXivHTMLPDF

Abstract

A trapped ion immersed in a neutral bath shows long-lived atom-ion complexes that significantly alter its chemical properties and thus the ion stability. In this work, we present a general study of trapped-ion--atom scattering with the ion modeled as a charge distribution defined by the spatial extent of its ground-state wave function. After mapping the time-dependent problem onto a time-independent framework, we investigate the conditions under which chaotic scattering arises and develops using classical trajectory simulations. In particular, we characterize the role of the trap, the atomic species, the atom-ion interaction, and the collision energy in shaping the chaotic dynamics of the system.

DOI
10.1103/mbzh-s8jm

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A trapped ion immersed in a neutral bath shows long-lived atom-ion complexes that significantly alter its chemical properties and thus the ion stability. In this work, we present a general study of trapped-ion--atom scattering with the ion modeled as a charge distribution defined by the spatial extent of its ground-state wave function. After mapping the time-dependent problem onto a time-independent framework, we investigate the conditions under which chaotic scattering arises and develops using classical trajectory simulations. In particular, we characterize the role of the trap, the atomic species, the atom-ion interaction, and the collision energy in shaping the chaotic dynamics of the system.
The study of chemical reactions between ions and neutral species at very low energies reveals precise informations about the dynamics of collisions and fine details of intermolecular interactions. Here, we report progress towards the development of next- generation experiments for the investigation of cold ion-neutral reactions. First, we present a new ''dynamic" hybrid ion-atom trap which enables the study of collisions with a superior energy resolution accessing a regime in which quantum scattering resonances may become observable. Second, we discuss and numerically characterize the concept and properties of a hybrid trap for cold neutral molecules and molecular ions which paves the way for the study of ion-molecule reactions in the millikelvin regime.
We study the classical dynamics of a Paul-trapped ion in a low-density bath of atoms above 1 μKμ\textrm{K}μK. We find that lower energy collisions with more massive atoms, especially at energies less than the initial micromotion heating, are more likely to form atom-ion complexes. These complexes evolve in a fractal structure for every scattering observable, showing non-hyperbolic chaotic dynamics. To explore the chaotic dynamics, we use a GPU-accelerated methodology allowing us to run over 3×1083\times 10^{8}3×108 trajectories of a 174^{174}174Yb+^++ and different atoms. As a result, after analyzing the dynamics as a function of the atom species, collision energy, trap parameters, and ion-atom potential depth, we find a link between heating and the onset of chaos in the first atom-ion interaction that occurs when a low-density atomic bath is merged with a trapped ion.
In the study of ion-atom interactions, the ion often remain trapped during the experiments. However, the effects of the trapping potential of the ion on ion-neutral interactions remain largely unexplored. Although trap-assisted ion-neutral complex formation has been experimentally studied and described by applying semiclassical theories where the ion is treated as a point charge particle, the potential effect of a delocalized charge distribution of a confined ion due to its quantum mechanical wavefunction has not been considered. To remedy this, in the present theoretical work we substitute the point charge of the ion with a delocalized charged distribution according to its motional ground state in the trap. Our results show that the trapping frequency and hence the spatial extension of the ion's ground-state wavefunction drastically affects the elastic and transport cross sections in interactions with neutral atoms. Stimulated by these results, we propose experimental procedures to verify the effects of the delocalize charge distribution in ion-atom interactions via measuring the heating rate of the ion due to the energy transfer in atomic collisions. Our novel approach brings new possibilities for investigating ion-neutral systems and, through them, new perspectives on ionic polarons and potentially a better understanding of trap-induced losses in ion-neutral experiments.
Abstract Experiments in which ultra-cold neutral atoms and charged ions are overlapped, constitute a new field in atomic and molecular physics, with applications ranging from studying out-of-equilibrium dynamics to simulating quantum many-body systems. The holy grail of ion-neutral systems is reaching the quantum low-energy scattering regime, known as the s-wave scattering. However, in most atom–ion systems, there is a fundamental limit that prohibits reaching this regime. This limit arises from the time-dependent trapping potential of the ion, the Paul trap, which sets a lower collision energy limit which is higher than the s-wave energy. In this work, we studied both theoretically and experimentally, the way the Paul trap parameters affect the energy distribution of an ion that is immersed in a bath of ultra-cold atoms. Heating rates and energy distributions of the ion are calculated for various trap parameters by a molecular dynamics (MD) simulation that takes into account the attractive atom–ion potential. The deviation of the energy distribution from a thermal one is discussed. Using the MD simulation, the heating dynamics for different atom–ion combinations is also investigated. In addition, we performed measurements of the heating rates of a ground-state cooled 88Sr+ ion that is immersed in an ultra-cold cloud of 87Rb atoms, over a wide range of trap parameters, and compare our results to the MD simulation. Both the simulation and the experiment reveal no significant change in the heating for different parameters of the trap. However, in the experiment a slightly higher global heating is observed, relative to the simulation.