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arXiv:2005.09338 · Physical Review Research

Life and death of a cold BaRb+ molecule inside an ultracold cloud of Rb atoms

Amir Mohammadi, Artjom Krükow, Amir Mahdian, Markus Deiß, Jesús Pérez‐Ríos, Humberto da Silva, Maurice Raoult, Olivier Dulieu, Johannes Hecker Denschlag

Published Feb 26, 2021• 55 citations
arXivHTMLPDF

Abstract

We study the evolution of a cold single BaRb + molecule while it continuously collides with ultracold Rb atoms.The initially weakly bound molecule can undergo a sequence of elastic, inelastic, reactive, and radiative processes.We investigate these processes by developing methods for discriminating between different ion species, electronic states, and kinetic ion energy ranges.By analyzing the experimental data while taking into account theoretical insights, we obtain a consistent description of the typical trajectory through the manifold of available atomic and molecular states.Monte Carlo simulations describe the measured dynamics well.As a further result, we determine rates for collisional and radiative relaxation as well as photodissociation, spin-flip collisions, and chemical reactions.

DOI
10.1103/physrevresearch.3.013196

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arXiv:1510.04938•Physical Review Letters•May 12, 2016•89 citations
We study the evolution of a cold single BaRb + molecule while it continuously collides with ultracold Rb atoms.The initially weakly bound molecule can undergo a sequence of elastic, inelastic, reactive, and radiative processes.We investigate these processes by developing methods for discriminating between different ion species, electronic states, and kinetic ion energy ranges.By analyzing the experimental data while taking into account theoretical insights, we obtain a consistent description of the typical trajectory through the manifold of available atomic and molecular states.Monte Carlo simulations describe the measured dynamics well.As a further result, we determine rates for collisional and radiative relaxation as well as photodissociation, spin-flip collisions, and chemical reactions.
For my dissertation, I have carried out experiments with a single Ba+ ion which is immersed in a cloud of ultracold neutral Rb atoms. For trapping cold ions and ultracold neutral atoms, we use a linear Paul trap and a far-detuned optical trap, respectively. In the first part of my dissertation, I use the elastic and reactive collisions between the atoms and the ion to minimize the ion excess-micromotion in the Paul trap. I show in my thesis that by minimizing losses in the Rb cloud which are due to the elastic and reactive ion-atom collisions one can suppress excess-micromotion quite well by additional radio-frequency fields that are applied to the Paul trap. In the second part, I studied the evolution of a single BaRb+ molecule while it continuously collides with ultracold Rb atoms. The BaRb+ molecule forms within one ms when we immerse the cold Ba+ ion into a dense gas of Rb. The molecule is initially weakly-bound and can undergo a sequence of elastic, inelastic, reactive, and radiative processes. I investigated these processes by developing methods for discriminating between different ion species, electronic states, and kinetic ion energy ranges.
We present a study about vibrational quenching and chemical processes of cold molecular ions immersed in an ultracold atomic gas. In particular, the quasiclassical trajectory method is applied to BaRb+(v)+Rb{\mathrm{BaRb}}^{+}(v)+\text{Rb}BaRb+(v)+Rb collisions at $T\ensuremath{\gtrsim}1\phantom{\rule{0.28em}{0ex}}\mathrm{mK}$, revealing a large vibrational quenching cross section, which follows the Langevin capture model prediction. These results play a key role in the understanding of relaxation phenomena of cold molecular ions in ultracold gases.
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.
We study cold heteronuclear atom-ion collisions by immersing a trapped single ion into an ultracold atomic cloud. Using ultracold atoms as reaction targets, our measurement is sensitive to elastic collisions with extremely small energy transfer. The observed energy-dependent elastic atom-ion scattering rate deviates significantly from the prediction of Langevin but is in full agreement with the quantum mechanical cross section. Additionally, we characterize inelastic collisions leading to chemical reactions at the single particle level and measure the energy-dependent reaction rate constants. The reaction products are identified by in-trap mass spectrometry, revealing the branching ratio between radiative and nonradiative charge exchange processes.
We report on three-body recombination of a single trapped Rb(+) ion and two neutral Rb atoms in an ultracold atom cloud. We observe that the corresponding rate coefficient K(3) depends on collision energy and is about a factor of 1000 larger than for three colliding neutral Rb atoms. In the three-body recombination process large energies up to several 0.1 eV are released leading to an ejection of the ion from the atom cloud. It is sympathetically recooled back into the cloud via elastic binary collisions with cold atoms. Further, we find that the final ionic product of the three-body processes is again an atomic Rb(+) ion suggesting that the ion merely acts as a catalyzer, possibly in the formation of deeply bound Rb(2) molecules.
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.