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Products of reactive collisions between cold Rb atoms and an Ba+ ion

Amir Mohammadi

Published Jan 1, 2020• 0 citations
Publisher

Abstract

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.

DOI
10.18725/oparu-35233

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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.
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 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.
During the last few decades, the two fields of ultracold neutral atoms and of single trapped ions have undergone an amazing development. The incorporation of these systems into a single experiment exploits the complementary physical properties of the subsystems and opens up novel lines of research. I present experimental results obtained in one of the first hybrid atom-ion setups accessing the cold regime. Single ions interact with ultracold samples of 87Rb atoms. The collision energies are set by heating mechanisms in the Paul trap and are on the order of mK·kB. Interactions are generally strong and long-ranged. We observe elastic atom-ion scattering in the semi-classical energy regime in which the cross section depends on the collision energy and favors scattering at small angles. We were able to obtain a general good understanding of atom-ion two-body collisions. We observed sympathetic cooling of the ion over four orders of magnitude in energy through collisions with the ultracold atoms. Further, we utilized atom-ion scattering events to develop a novel method to compensate excess ion micromotion, the primary source of ion heating. Three-body collisions between two atoms and the ion play an important role. We identified these processes through the large amounts of energy that they release. This makes them detectable despite their suppression with respect to two-body collisions. We also showed that atom-ion three-body collisions become more prominent as the collision energy decreases. Finally, we employed our atom-ion apparatus to gain novel insights into three-body recombination between ultracold atoms. In these experiments the ion trap acts as an extremely sensitive ion detector. Through three-body recombination a Rb2 molecule is formed and can undergo ionization by absorbing photons from a strong laser. By scanning the frequency of the ionization laser we obtained a molecular spectrum and identified a relatively broad distribution over final molecular states.
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 describe a hybrid vacuum system in which a single ion or a well-defined small number of trapped ions (in our case Ba(+) or Rb(+)) can be immersed into a cloud of ultracold neutral atoms (in our case Rb). This apparatus allows for the study of collisions and interactions between atoms and ions in the ultracold regime. Our setup is a combination of a Bose-Einstein condensation apparatus and a linear Paul trap. The main design feature of the apparatus is to first separate the production locations for the ion and the ultracold atoms and then to bring the two species together. This scheme has advantages in terms of stability and available access to the region where the atom-ion collision experiments are carried out. The ion and the atoms are brought together using a moving one-dimensional optical lattice transport which vertically lifts the atomic sample over a distance of 30 cm from its production chamber into the center of the Paul trap in another chamber. We present techniques to detect and control the relative position between the ion and the atom cloud.
We analyze reactive collisions of a single Ba+{\mathrm{Ba}}^{+}Ba+ ion in contact with an ultracold gas of Rb atoms at low three-body collision energies of 2.2(9) $\mathrm{mK}\ifmmode\times\else\texttimes\fi{}{k}_{\mathrm{B}}$. Mapping out the Ba+{\mathrm{Ba}}^{+}Ba+ loss rate dependence on the Rb atom density we can discern two-body reactive collisions from three-body ones and determine both rate coefficients, which are ${k}_{2}=3.1(6)(6)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{3}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ and ${k}_{3}=1.04(4)(45)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}24}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{6}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$, respectively (statistical and systematic errors in parentheses). Thus, the measured ternary recombination dominates over binary reactions even at moderate atom densities of $n\ensuremath{\approx}{10}^{12}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. The results for Ba+{\mathrm{Ba}}^{+}Ba+ and Rb are representative for a wide range of cold ion-atom systems and can serve as guidance for the future development of the field of hybrid atom-ion research.