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arXiv:2306.09580

Time-Resolved Rubidium-Assisted Electron Capture by Barium (II) Cation

Axel Molle, Jan Philipp Drennhaus, Viktoria Noel, Nikola L. Kolev, Annika Bande

Published Jun 16, 2023• 1 citation
arXivHTMLPDF

Abstract

Non-local energy transfer between bound electronic states close to the ionisation threshold is employed for efficient state preparation in dilute atom systems from technological foundations to quantum computing. The generalisation to electronic transitions into and out of the continuum is lacking quantum simulations necessary to motivate such potential experiments. Here, we present the first development of a electron-dynamical model simulating fully three-dimensional atomic systems for this purpose. We investigate the viability of this model for the prototypical case of recombination of ultracold barium(II) by environment-assisted electron capture thanks to a rubidium atom in its vicinity. Both atomic sites are modelled as effective one-electron systems using the Multi Configuration Time Dependent Hartree (MCTDH) algorithm and can transfer energy by dipole-dipole interaction. We find that the simulations are robust enough to realise assisted capture over a dilute interatomic distance which we are able to quantify by comparing to simulations without interatomic energy exchange. For our current parameters not yet optimised for reaction likelihood, an environment-ionising assisted capture has a probability of $1.9\times10^{-5}~\%$ over the first $15~\mathrm{fs}$ of the simulation. The environment-exciting assisted-capture path to $[\text{Ba}^{+*}\text{Rb}^{*}]$ appears as a stable long-lived intermediate state with a probability of $8.2\times10^{-4}~\%$ for at least $20~\mathrm{fs}$ after the capture has been completed. This model shows potential to predict optimised parameters as well as to accommodate the conditions present in experimental systems as closely as possible. We put the presented setup forward as a suitable first step to experimentally realise environment-assisted electron capture with current existing technologies.

DOI
10.48550/arxiv.2306.09580

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Non-local energy transfer between bound electronic states close to the ionisation threshold is employed for efficient state preparation in dilute atom systems from technological foundations to quantum computing. The generalisation to electronic transitions into and out of the continuum is lacking quantum simulations necessary to motivate such potential experiments. Here, we present the first development of a electron-dynamical model simulating fully three-dimensional atomic systems for this purpose. We investigate the viability of this model for the prototypical case of recombination of ultracold barium(II) by environment-assisted electron capture thanks to a rubidium atom in its vicinity. Both atomic sites are modelled as effective one-electron systems using the Multi Configuration Time Dependent Hartree (MCTDH) algorithm and can transfer energy by dipole-dipole interaction. We find that the simulations are robust enough to realise assisted capture over a dilute interatomic distance which we are able to quantify by comparing to simulations without interatomic energy exchange. For our current parameters not yet optimised for reaction likelihood, an environment-ionising assisted capture has a probability of 1.9×10−5 %1.9\times10^{-5}~\%1.9×10−5 % over the first 15 fs15~\mathrm{fs}15 fs of the simulation. The environment-exciting assisted-capture path to [Ba+∗Rb∗][\text{Ba}^{+*}\text{Rb}^{*}][Ba+∗Rb∗] appears as a stable long-lived intermediate state with a probability of 8.2×10−4 %8.2\times10^{-4}~\%8.2×10−4 % for at least 20 fs20~\mathrm{fs}20 fs after the capture has been completed. This model shows potential to predict optimised parameters as well as to accommodate the conditions present in experimental systems as closely as possible. We put the presented setup forward as a suitable first step to experimentally realise environment-assisted electron capture with current existing technologies.
Interatomic coulombic electron capture is a non-local process involving the environment-assisted attachment of a free electron with implied consequences for various systems. Starting from the established numerical model of quantum confinements in a nanowire, this dissertation sets out to deduce model-independent hypotheses for future investigations of theoretical or experimental nature and develops a generalised adaptation of the model to test whether the effective-two-electron treatment suffices to successfully capture a free electron in the experimentally motivated system of a barium (II) cation engulfed in a Bose-Einstein condensate of neutral rubidium atoms. Appearing associated to differing electronic states of the confinement region, two subprocesses can contrast in spatial preferences and resonant energies. For the investigated range of parameters, the energy levels of these associated states suggest to provide a starting point for a more comprehensive description beyond the particular parameters of an individual model. A rather simple electric dipole-dipole coupled adaptation of the model is then able to successfully show environment assisted electron attachment to a barium (II) cation aided by a surrounding cloud of ultracold rubidium atoms in typical experimental conditions.
We explore the correlated quantum dynamics of a single atom, regarded as an open system, with a spatio-temporally localized coupling to a finite bosonic environment. The single atom, initially prepared in a coherent state of low energy, oscillates in a one-dimensional harmonic trap and thereby periodically penetrates an interacting ensemble of N A bosons held in a displaced trap. We show that the inter-species energy transfer accelerates with increasing N A and becomes less complete at the same time. System-environment correlations prove to be significant except for times when the excess energy distribution among the subsystems is highly imbalanced. These correlations result in incoherent energy transfer processes, which accelerate the early energy donation of the single atom and stochastically favour certain energy transfer channels, depending on the instantaneous direction of transfer. Concerning the subsystem states, the energy transfer is mediated by non-coherent states of the single atom and manifests itself in singlet and doublet excitations in the finite bosonic environment. These comprehensive insights into the non-equilibrium quantum dynamics of an open system are gained by ab initio simulations of the total system with the recently developed multi-layer multi-configuration time-dependent Hartree method for bosons.
Systems are known to thermalize, or reach equilibrium, with the passing of time. However, when this does not occur, the quantum states of the system may be localized. Localized states carry information of the system’s initial state that is typically lost during the process of thermalization. We aim to study thermalization and localization in systems of rubidium Rydberg atoms, and to further understand these phenomena via simulation while we are developing a more refined experiment. These atoms exchange energy via tuned resonances while contained within a magneto optical trap. This process can be categorized as either a two, three, or four body interaction; localization is only expected to occur in the latter two. As the atoms transfer energy via dipole-dipole interactions, the population of atoms occupying different states shifts. Some atoms are shifted to a measurable state, and each trial of the experiment lasts for only several microseconds as data is collected. The energy transport within the system is simulated on a supercomputer in order to produce a model for the experimental data, while also allowing us to closely analyze parameters that are not accessible through experiment. From the model, we hope to learn whether the system thermalizes or localizes. We present initial simulation results exploring the energy exchange dynamics in a Rubidium many-body system.
Electron capture by isolated atoms and ions proceeds by photorecombination. In this process, a species captures a free electron by emitting a photon which carries away the excess energy. It is shown here that in the presence of an environment a competing non-radiative electron capture process can take place due to long range electron correlation. In this interatomic (intermolecular) process the excess energy is transferred to neighbouring species. The asymptotic expression for the cross section of this process is derived. We demonstrate by explicit examples that under realizable conditions, the cross section of this interatomic process can clearly dominate that of photorecombination.
Collisions of ultracold Ba + ions on a Rb Bose–Einstein condensate have been suggested as a possible benchmark system for ultracold ion-neutral collision experiments. However, a priori knowledge of the possible processes is desirable. For this purpose, we here present high-level four-component coupled cluster and multi-reference configuration interaction calculations of potential energy curves, dipole moments and spectroscopic constants of the experimentally interesting low-lying electronic states of the (RbBa) + molecule. Our results show significant avoided crossings between the Rb + Ba + entrance channels and low-lying charge transfer states of the Rb + and Ba ( 3 D) atomic channels, indicating that a fast non-radiative charge transfer is possible. Population analysis shows that a partially covalent polar bond is formed in the ground state, which thus deviates significantly from a pure Rb + + Ba interaction. This finding is corroborated by the electric dipole moment which is found only to be 4.5 D at the equilibrium bond distance, compared with the 14 D for a pure Rb + + Ba interaction, thereby supporting the view of a partial charge transfer between the two atoms.
Light-assisted reactive collisions between laser-cooled Ba+ ions and Rb atoms were studied in an ion–atom hybrid trap. The reaction rate was found to strongly depend on the electronic state of the reaction partners with the largest rate constant [7(2) × 10−11 cm3 s−1] obtained for the excited Ba+(6s)+Rb(5p) reaction channel. Similar to the previously studied Ca++Rb system, charge transfer and radiative association were found to be the dominant reactive processes. The generation of molecular ions by radiative association could directly be observed by their sympathetic cooling into a Coulomb crystal. Potential energy curves up to the Ba+(6s)+Rb(5p) asymptote and reactive-scattering cross sections for the radiative processes were calculated. The theoretical rate constant obtained for the lowest reaction channel Ba+(6s)+Rb(5s) is compatible with the experimental estimates obtained thus far.