IQST Researchers publish in Science

State-to-state chemistry in the ultracold: Physicists study quantum-mechanical processes in the formation of molecules. A collaboration between German and American scientists successfully resolved elemental steps in the formation of molecules in one of the simplest chemical reactions, the three-body recombination. For the first time, the scientists are able to quantitatively determine the quantum state distribution of the final products immediately after the three-particle collision in an ultracold gas.

Graphic chart of the three-body recombination with possible products.

Photo of the Paul trap, which is used to trap and count the state-selective ionized molecules. Photos: Institute of Quantum Matter, Ulm University

Chemistry is often associated with steaming liquids and explosions. The characterization of the resulting products is only possible after numerous state transitions, the so-called relaxation processes. The instantaneous determination of molecular states of the formed products affords a deeper understanding of chemical processes and facilitates the targeted regulation and control of reactions.
With the advent of ultracold quantum gases and the quantum optics tools a new, paradigm-changing approach can now be taken. In the scientific article that is published in the renowned journal Science the physicists describe state-to-state chemistry in the ultracold regime with an unprecedented resolution down to the hyperfine level, thus resolving essentially all quantum levels of the molecular product.

A laser beam cools the gas of rubidium atoms to a temperature of a millionth degrees Kelvin within an ultra-high vacuum apparatus, where it is confined in a trap and forms a small cloud of roughly four million atoms with a diameter of less than one tenth of a millimeter. In this quantum mechanically precisely defined cloud three atoms often come so close to each other that two of them react to form a molecule and the third carries away a portion of the released energy. The newly formed molecule can take on a great many quantum mechanical states, which the physicists were now able to identify for the first time. The signals were interpreted using spectroscopic input from the University of Hannover. In parallel, theorists from the Joint Quantum Institute (JQI) and JILA developed a mathematical model to simulate what the experimentalists of IQST fellow Johannes Hecker Denschlag from Ulm University were observing. 

From the observed population of quantum states of the molecular products several propensity (i.e. selection) rules for the chemical reaction have been inferred. An important and surprising insight that the researchers have gained is that spins flips of electrons and nuclei during the reaction are suppressed. This, however, is contrasted by a generally sizeable change of rotational angular momentum of several quanta during the reaction. Furthermore, contrary to early expectations, many deeply bound states are populated considerably.

The results of the experimental and theoretical works of the research team are game-changing for the investigation of other ultracold chemical processes. The required experimental infrastructure is present in many laboratories around the world so that the ground for further experiments in this innovative research field is prepared. The experimental results, in turn, challenge the theorists to continuously refine their models and theories. This combination provides a deep understanding of increasingly complex chemical reactions, which can lead to a precisely defined sequence of reactions in the future.

Contact:    Prof. Dr. Johannes Hecker Denschlag
                 +49 731 502 6100
                  johannes.denschlag(at)uni-ulm.de

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Joschka Wolf, Markus Deiß, Artjom Krükow, Eberhard Tiemann, Brandon P.Ruzic, Yujun Wang, José P. D’Incao, Paul S. Julienne, and Johannes Hecker Denschlag:
State-to-state chemistry for three-body recombination in an ultracold rubidium gas
Science 17 Nov 2017: Vol. 358, Issue 6365, pp. 921-924

DOI: 10.1126/science.aan8721 http://science.sciencemag.org/content/358/6365/921

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