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arXiv:2503.23690

Effects of the delocalized charge distribution in trapped ion-atom collisions

Shi, Ruiren, Drewsen, Michael, Jesús Pérez‐Ríos

Published Mar 31, 2025• 1 citation
arXivHTMLPDF

Abstract

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.

DOI
10.48550/arxiv.2503.23690

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•Jan 1, 2018•0 citations
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.
This thesis explores the fundamental physical and chemical processes between molecular ions and neutral atoms in the gas phase at very low energies. At temperatures in the mK regime, these are dominated by long-range interactions and only a few partial waves. This study contributes to the understanding of the details of intermolecular interactions and provides valuable data for benchmarking theoretical models and quantum-chemical calculations. Although there is a wide range of previous atomic collision system studies, the investigations of cold collisions with molecular ions are limited. The experimental setup used in this work enabled the measurement of rate coefficients for molecular ion-neutral collisions. The development of a dynamic ion-neutral hybrid trap with improved control over collision energy led to new insights into charge-transfer rate coefficients for molecular ion-neutral atom collisions involving N2+^+_22+​+Rb, O2+^+_22+​+Rb and N2_22​H+^+++Rb. Two different experimental settings enable state-and collision-energy dependent measurements of rate coefficients for all systems revealing an intriguing interplay between long-and short-range effects for the homonuclear diatomics by comparison to theoretical calculations and surprising experimental results for the polyatomic study. A fit of a constant function to the measured CT rate coefficients in the state-dependent experiment of N2_22​H+^+++Rb yielded the similar result as for N2+^+_22+​+Rb(5s5s5s)(2S1/2^2S_{1/2}2S1/2​). In the collision-energy dependent experiment the CT rate coefficient for N2_22​H+^+++Rb(5s5s5s)(2S1/2^2S_{1/2}2S1/2​) and N2_22​H+^+++Rb(5p5p5p)(2P3/2^2P_{3/2}2P3/2​) are both similar to the results for N2+^+_22+​+Rb(5s5s5s)(2S1/2^2S_{1/2}2S1/2​), and also the rate coefficients increase as the collision-energy increases.
The investigation of cold interactions between ions and neutrals has recently emerged as a new scientific frontier at the interface of physics and chemistry. Here, we report a study of charge-transfer (CT) collisions of Rb atoms with N[Formula: see text] and O[Formula: see text] ions in the mK regime using a dynamic ion-neutral hybrid trapping experiment. We observe markedly different CT kinetics and dynamics for the different systems and reaction channels studied. While the kinetics in some channels are consistent with classical capture theory, others show distinct non-universal dynamics. The experimental results are interpreted with the help of classical-capture, quasiclassical-trajectory and quantum-scattering calculations using ab-initio potentials for the highly excited molecular states involved. The theoretical analysis reveals an intricate interplay between short- and long-range effects in the different reaction channels which ultimately determines the CT dynamics and rates. Our results illustrate salient mechanisms that determine the efficiency of cold molecular CT reactions.
Abstract We present a series of recent experimental and numerical results which allow us to propose and explain the mechanisms for collisional cooling of a trapped ion by sequential binary collisions with atoms. Our experiments in hybrid atom-ion traps study the cooling of ions when the reservoir of the coolant atoms is spatially localized at the centre of the ion trap. Under these conditions the widely and long held expectation that for the ion to collisionally cool, the atom must be of lighter mass (i.e. mI > mA ) is overturned. Instead we show ion cooling for mI ⩽ mA in addition to mI > mA , and explain why earlier work establishing mass ratios does not apply to the experiments in hybrid traps. Further, for cooling of an ion A+ by the parent atom A, the mechanism of resonant charge exchange (RCE) allows for extremely efficient cooling of the ion A+. This mechanism is demonstrated by comparing the cooling rate of an ion with a localized ensemble of its parent atoms and with a lighter atomic species, where in the latter case the RCE mechanism does not exist. The difference in measured cooling rates for a given number density of the atoms and the theoretically calculated difference between elastic and RCE cross sections are used to show that the cooling efficiency per collision with RCE is much greater than the cooling by elastic collision. We conclude with some perspectives and prospects of future experiments.
Experiments involving trapped ultracold matter are of great interest to a diverse range of fields, from spectroscopy and quantum computing to ultracold chemistry. Hybrid traps allowing for the simultaneous confinement of charged and uncharged matter extend the scope of these experiments, but have not yet benefited from the miniaturisation of the trapping architectures demonstrated for traps which only confine either ions or neutral particles. This miniaturisation greatly enhances the spatial resolution of the forces with which the trapped particles are manipulated, and this thesis details the design and fabrication of a prototype miniaturised hybrid trap to take advantage of this increased precision. The co-trapping of ions and neutral particles leads to multiple mechanisms by which the energy distributions of the trapped ions may deviate from thermal statistics, which have previously been treated largely empirically. In this thesis, these effects are explored numerically and analytically to provide a theoretical framework for this behaviour through the formalism of superstatistics. The results derived here explain the deviations from thermal statistics observed in precision spectroscopy experiments and resolve outstanding questions about both the mechanism by which ions acquire a non-thermal energy distribution during buffer gas cooling with neutral atoms and the analytical form of this distribution. This significantly improves the ability to correctly interpret the results of experiments, and is applicable not only to the hybrid chip trap developed here, but to hybrid ion-neutral traps in general.