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arXiv:2407.14824 · Physical Review Research

Ultracold charged atom-dimer collisions: State-selective charge exchange and three-body recombination

Amrendra Pandey, Romain Véxiau, Luís Gustavo Marcassa, Olivier Dulieu, Nadia Bouloufa-Maafa

Published Oct 4, 2024• 3 citations
arXivHTMLPDF

Abstract

Based on an accurate determination of the potential energy surfaces of Rb 3 + correlated to its first asymptotic limit Rb + + Rb ( 5 s ) + Rb ( 5 s ) , we identify the presence of intersections of a pair of singlet and triplet surfaces over all interparticle distances, leading to Jahn-Teller (JT) couplings. We elaborate scenarios for charge exchange between ultracold charged atom-dimer complex ( Rb + Rb 2 + or Rb + + Rb 2 ), predicting a strong selectivity on the preparation of the initial state of the dimer. We also demonstrate that the JT couplings must drive the three-body recombination (TBR) of Rb + , Rb, and Rb at ultracold energies. Using the current analysis, we provide a consistent picture of the TBR experiments performed in ion-atom hybrid Rb samples. We also demonstrate the presence of JT coupling as a general phenomenon in the singly charged homonuclear alkali triatomic systems. Published by the American Physical Society 2024

DOI
10.1103/physrevresearch.6.043010

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Based on an accurate determination of the potential energy surfaces of Rb 3 + correlated to its first asymptotic limit Rb + + Rb ( 5 s ) + Rb ( 5 s ) , we identify the presence of intersections of a pair of singlet and triplet surfaces over all interparticle distances, leading to Jahn-Teller (JT) couplings. We elaborate scenarios for charge exchange between ultracold charged atom-dimer complex ( Rb + Rb 2 + or Rb + + Rb 2 ), predicting a strong selectivity on the preparation of the initial state of the dimer. We also demonstrate that the JT couplings must drive the three-body recombination (TBR) of Rb + , Rb, and Rb at ultracold energies. Using the current analysis, we provide a consistent picture of the TBR experiments performed in ion-atom hybrid Rb samples. We also demonstrate the presence of JT coupling as a general phenomenon in the singly charged homonuclear alkali triatomic systems. Published by the American Physical Society 2024
The ultracold state-to-state chemistry for three-body recombination (TBR) in realistic systems has recently been experimentally investigated with full quantum state resolution. However, many detected phenomena remain challenging to be explored and explained from the theoretical viewpoints because this generally requires computational powers beyond state of the art. Here, the product-state distributions after TBR of 3He2–alkaline-earth-metal systems, i.e., after the processes of 3He + 3He + X → 3HeX + 3He with X being 9Be, 24Mg, 40Ca, 88Sr, or 138Ba, in the zero-collision-energy limit are theoretically studied. Two propensity rules for the distribution of the products found in current experiments have been checked, and the mechanism underlying these product-state distributions is explored. Particularly, two main intriguing transition pathways are identified, which may be responsible for the nonlinear distribution of the products vs their respective rotational quantum number. In addition, the TBR rates of these systems are also influenced by details of the interaction potential and relevant nonadiabatic couplings.
The atom-atom-anion three-body recombination (TBR) and collision induced dissociation (CID) processes of the 3He–3He–T− system at ultracold temperatures are investigated by solving the Schrödinger equation in the adiabatic hyperspherical representation. The variations of the TBR and CID rates with the collision energies in the ultracold temperatures are obtained. It is found that the JΠ = 1− symmetry dominates the TBR and CID processes in most of the considered collision energy range. The rate of TBR (CID) into (from) the l = 1 anion is larger than those for the l = 0 and l = 2 anions, with the l representing the rotational quantum number of 3HeT−. This can be understood via the nonadiabatic couplings among the different channels.
In this thesis, I report on experiments on the detection and the production of molecules in an ultracold atomic gas. These experiments can be divided into two main parts. The first part is about state-to-state experiments where chemical reaction products are detected in a quantum state resolved way. The second part is about Rydberg molecules. In the first part, I investigate three-body recombination of neutral Rb, which is a chemical reaction leading to the formation of Rb2 dimers. The quantum states of the three-body recombination product molecules are investigated for different initial atomic samples. This includes different atomic hyperfine states and different atomic magnetic sub states that are prepared. The molecular reaction product is state-selectively ionized and detected afterwards. The detection scheme resolves total electron spin, the vibrational, the rotational, the hyperfine and even the magnetic sub state of the product molecules. We determine the population distribution of the product molecules covering the majority of all produced molecules. The insights gained enable us to formulate various propensity rules for the three-body recombination of 87Rb. The second part is furthermore divided into two parts: the investigations of charged and uncharged Rydberg molecules. The investigations of charged Rydberg molecules are the first step towards one of our goals, the photoassociation of BaRb+ or Rb2+ Rydberg molecules. For this work, we have developed a novel method to measure Stark spectra within an ion trap. In experiments with neutral Rb2 Rydberg molecules we observe a complex spin structure of the molecular states. The substructure emerges partially due to a novel, recently predicted spin-orbit interaction of the Rydberg electron and the perturber atom, which we prove for the first time.
We study three-body recombination of Ba^{+}+Rb+Rb in the mK regime where a single ^{138}Ba^{+} ion in a Paul trap is immersed into a cloud of ultracold ^{87}Rb atoms. We measure the energy dependence of the three-body rate coefficient k_{3} and compare the results to the theoretical prediction, k_{3}∝E_{col}^{-3/4}, where E_{col} is the collision energy. We find agreement if we assume that the nonthermal ion energy distribution is determined by at least two different micromotion induced energy scales. Furthermore, using classical trajectory calculations we predict how the median binding energy of the formed molecules scales with the collision energy. Our studies give new insights into the kinetics of an ion immersed in an ultracold atom cloud and yield important prospects for atom-ion experiments targeting the s-wave regime.
Over the recent years a new research field emerged that combines technologies for cooling and trapping of neutral atoms and charged ions in one experimental apparatus. These atom-ion hybrid traps enable the investigation of atom-ion interactions at unprecedentedly low collision energies targeting the ultracold s-wave regime. In this thesis I report on experiments performed in an hybrid atom-ion apparatus combining single Ba^+ or Rb^+ ions with ultracold neutral Rb atoms. The main focus of this work is studying the emerging three-body reaction dynamics in a cold charged gas. First, I will present measurements investigating cold reactions in the hetero-nuclear Ba^+ - Rb system. So far, most theoretical and experimental studies have been dealing with reactions between two particles. However, we find that three-body reactions already dominate over two-body ones at atomic densities down to 10^{−11} cm^{−3} once collision energies are in the low mK regime. We then investigate the energy scaling of inelastic ternary collisions between an ion and two atoms. A power-law scaling of the three-body rate coefficient is predicted by theory. We find agreement with the experiment if we include the ion’s kinetic energy distribution that we access numerically. The next study elaborates on the very first observation of ternary reactions in a hybrid cold atom-ion setup. For this we immerse a single Rb^+ ion into ultracold Rb atoms with sufficiently reduced excess micromotion. Occurring reactions are signaled by the creation of highly energetic Rb^+ ions. I will also showcase novel techniques to investigate the atom and the ion system by using the respective other one which were developed by us over the course of the last years. In order to reduce the total ion kinetic energy in a Paul trap one has to compensate the ion’s excess micromotion. This is routinely being done by analyzing the ion’s fluorescence. By observing collision energy dependent elastic atom-ion collisions in an ultracold atom cloud we directly infer the ion’s micromotion energy. This way we can perform atom-ion experiments with optically dark Rb^+ ions and good excess micromotion. Without the need of visible laser light for micromotion compensation we are able to investigate the evolution of quasi-static stray electric fields in a Paul trap over a period of several months. The last study I will present is targeted at neutral Rb only. There, we spectroscopically access the molecular state distribution of Rb_2 molecules formed in neutral three-body recombination by state-selective ionization of these molecules. Trapping and subsequent detection of these ions is being done with our Paul trap, which has the role of a very sensitive single ion detector in these measurements.
The control of the ultracold collisions between neutral atoms is an extensive and successful field of study. The tools developed allow for ultracold chemical reactions to be managed using magnetic fields, light fields and spin-state manipulation of the colliding particles among other methods. The control of chemical reactions in ultracold atom-ion collisions is a young and growing field of research. Recently, the collision energy and the ion electronic state were used to control atom-ion interactions. Here, we demonstrate spin-controlled atom-ion inelastic processes. In our experiment, both spin-exchange and charge-exchange reactions are controlled in an ultracold Rb-Sr+^++ mixture by the atomic spin state. We prepare a cloud of atoms in a single hyperfine spin-state. Spin-exchange collisions between atoms and ion subsequently polarize the ion spin. Electron transfer is only allowed for (RbSr)+^++ colliding in the singlet manifold. Initializing the atoms in various spin states affects the overlap of the collision wavefunction with the singlet molecular manifold and therefore also the reaction rate. We experimentally show that by preparing the atoms in different spin states one can vary the charge-exchange rate in agreement with theoretical predictions.