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arXiv:1004.3317 · Physical Review A

Product-state control of bi-alkali-metal chemical reactions

Edmund Meyer, John L. Bohn

Published Oct 14, 2010• 33 citations
arXivHTMLPDF

Abstract

We consider ultracold, chemically reactive scattering collisions of the diatomic molecules KRb. When two such molecules collide in an ultracold gas, we find that they are energetically forbidden from reacting to form the trimer species ${\mathrm{K}}_{2}\mathrm{Rb}$ or ${\mathrm{Rb}}_{2}\mathrm{K}$, and hence can only react via the bond-swapping reaction $2\mathrm{KRb}\ensuremath{\rightarrow}{\mathrm{K}}_{2}+{\mathrm{Rb}}_{2}$. Moreover, the tiny energy released in this reaction can in principle be set to zero by applying electric or microwave fields, implying a means of controlling the available reaction channels in a chemical reaction.

DOI
10.1103/physreva.82.042707

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We consider ultracold, chemically reactive scattering collisions of the diatomic molecules KRb. When two such molecules collide in an ultracold gas, we find that they are energetically forbidden from reacting to form the trimer species K2Rb{\mathrm{K}}_{2}\mathrm{Rb}K2​Rb or Rb2K{\mathrm{Rb}}_{2}\mathrm{K}Rb2​K, and hence can only react via the bond-swapping reaction 2KRb\ensuremath→K2+Rb22\mathrm{KRb}\ensuremath{\rightarrow}{\mathrm{K}}_{2}+{\mathrm{Rb}}_{2}2KRb\ensuremath→K2​+Rb2​. Moreover, the tiny energy released in this reaction can in principle be set to zero by applying electric or microwave fields, implying a means of controlling the available reaction channels in a chemical reaction.
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We present a theoretical investigation of the chemical reaction SrF +++ SrF $\ensuremath{\rightarrow}$ products, focusing on reactions at ultralow temperatures. We find that bond swapping SrF +++ SrF $\ensuremath{\rightarrow}$ Sr2{}_{2}2​ +++ F2{\mathrm{F}}_{2}F2​ is energetically forbidden at these temperatures. Rather, the only energetically allowed reaction is SrF +++ SrF $\ensuremath{\rightarrow}$ SrF2{}_{2}2​ +++ Sr, and even then only singlet states of the SrF2{}_{2}2​ trimer can form. A calculation along a reduced reaction path demonstrates that this abstraction reaction is barrierless and proceeds by one SrF molecule ``handing off'' a fluorine atom to the other molecule.
How does a chemical reaction proceed at ultralow temperatures? Can simple quantum mechanical rules such as quantum statistics, single partial-wave scattering, and quantum threshold laws provide a clear understanding of the molecular reactivity under a vanishing collision energy? Starting with an optically trapped near-quantum-degenerate gas of polar 40K87Rb molecules prepared in their absolute ground state, we report experimental evidence for exothermic atom-exchange chemical reactions. When these fermionic molecules were prepared in a single quantum state at a temperature of a few hundred nanokelvin, we observed p-wave-dominated quantum threshold collisions arising from tunneling through an angular momentum barrier followed by a short-range chemical reaction with a probability near unity. When these molecules were prepared in two different internal states or when molecules and atoms were brought together, the reaction rates were enhanced by a factor of 10 to 100 as a result of s-wave scattering, which does not have a centrifugal barrier. The measured rates agree with predicted universal loss rates related to the two-body van der Waals length.
Glimpsing an exchange of partners When two diatomic molecules collide, they can sometimes swap partners. For instance, two potassium-rubidium (KRb) molecules can produce K 2 and Rb 2 . The four-atom intermediate formed upon collision is typically too scarce and short-lived to spot, even using ultrafast techniques. Hu et al. circumvented this problem by studying the reaction at temperatures approaching 0 kelvin. Using a combination of mass spectrometry and velocity-map imaging, the authors directly characterized the ionized K 2 Rb 2 complex as well as the reactant and product populations. Science , this issue p. 1111
We demonstrate a method for controlling the outcome of an ultracold chemical few-body reaction by redirecting a tunable fraction of reaction flux from one selected product channel to another one. In the reaction, three ultracold atoms collide to form a diatomic molecule. This product molecule can be produced in various internal states, characterizing the different product channels of the reaction. Our scheme relies on the coupling between two such product channels at an avoided molecular energy level crossing in the presence of an external magnetic field. The degree of coupling can be set by the magnetic field strength and allows for a widely tunable flux control between the two channels. This scheme is quite general and also holds great promise for a large variety of chemical processes with diverse species, since molecular energy level crossings are ubiquitous in molecular systems and are often easily accessible by standard laboratory equipment.