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Journal of Physics Conference Series

Quantum-State Resolved Study of the Ultracold K+KRb Reaction

James F. E. Croft, Constantinos Makrides, Mingzhe Li, Alexander N. Petrov, Svetlana Kotochigova, Brian K. Kendrick, N. Balakrishnan

Published Jul 1, 2017• 0 citations
Publisher

Abstract

We report a numerically exact quantum mechanical treatment of the K+KRb reaction and discuss both universal and statistical aspects of its reaction dynamics. It is shown that the total rate is universal in character - validating the use of universal models in other similar reactions. However, the rotational distribution is shown to be statistical in character - which we argue is due to the chaotic nature of the reaction complex.

DOI
10.1088/1742-6596/875/9/082004

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We report a numerically exact quantum mechanical treatment of the K+KRb reaction and discuss both universal and statistical aspects of its reaction dynamics. It is shown that the total rate is universal in character - validating the use of universal models in other similar reactions. However, the rotational distribution is shown to be statistical in character - which we argue is due to the chaotic nature of the reaction complex.
Abstract A fundamental question in the study of chemical reactions is how reactions proceed at a collision energy close to absolute zero. This question is no longer hypothetical: quantum degenerate gases of atoms and molecules can now be created at temperatures lower than a few tens of nanokelvin. Here we consider the benchmark ultracold reaction between, the most-celebrated ultracold molecule, KRb and K. We map out an accurate ab initio ground-state potential energy surface of the K 2 Rb complex in full dimensionality and report numerically-exact quantum-mechanical reaction dynamics. The distribution of rotationally resolved rates is shown to be Poissonian. An analysis of the hyperspherical adiabatic potential curves explains this statistical character revealing a chaotic distribution for the short-range collision complex that plays a key role in governing the reaction outcome.
A fundamental question in the study of chemical reactions is how reactions proceed at a collision energy close to absolute zero. This question is no longer hypothetical: quantum degenerate gases of atoms and molecules can now be created at temperatures lower than a few tens of nanoKelvin. In this work we consider the benchmark ultracold reaction between, the most-celebrated ultracold molecule, KRb and K. For the first time we map out an accurate ab initio ground state potential energy surface of the KRbK complex in full dimensionality and report numerically exact quantum-mechanical reaction dynamics. The distribution of rotationally resolved rates is shown to be Poissonian. An analysis of the hyperspherical adiabatic potential curves explains this statistical character revealing a chaotic distribution for the short-range collision complex that plays a key role in governing the reaction outcome. We compare this with a lighter system with a smaller density of states (here the LiYbLi trimer) which displays random, and not chaotic, behavior.
A general and rigorous quantum method is proposed for studying capture dynamics between two diatomic molecules in full dimensionality. By solving the time-independent Schrödinger equation with proper boundary conditions, this method is ideally suited for studying quantum dynamics of cold and ultracold reactions. To illustrate its applicability, the capture dynamics between ultracold KRb molecules is characterized in full six dimensions for the first time using a first-principles based long-range interaction potential. The calculated capture rates for collisions involving distinguishable and indistinguishable 40K87Rb molecules are in good agreement with the experiment and exhibit clear Wigner threshold behaviors. Predictions for ultracold collisions between internally excited 40K87Rb suggest minor changes in the loss rate, consistent with experimental observations in similar systems.
A general and rigorous quantum method is proposed for studying capture dynamics between two diatomic molecules in full dimensionality. By solving the time-independent Schrodinger equation with proper boundary conditions, this method is ideally suited for studying quantum dynamics of cold and ultracold reactions. To illustrate its applicability, the capture dynamics between ultracold KRb molecules is characterized in full six dimensions for the first time using a first-principles based long-range interaction potential. The calculated capture rates for collisions involving distinguishable and indistinguishable 40K87Rb molecules are in good agreement with the experiment and exhibit clear Wigner threshold behaviors. Predictions for ultracold collisions between internally excited 40K87Rb suggest minor changes in the loss rate, consistent with experimental observations in similar systems.
Studying ultracold chemical reactions with quantum state resolution reveals details of the reaction processes, especially with single quantum state preparation of the reactants and state-selective detection of reaction products. In this thesis, we investigate two main processes: KRb + KRb → K2 + Rb2 reactions, and Rb and KRb atom-molecule collisions. In the KRb bimolecular reaction, we demonstrate the preservation of quantum coherence in bimolecular reactions for the first time, which is surprising in a reaction considered to be largely chaotic. Additionally, we proposed a coherent control scheme to manipulate product yields across different product channels. These results represent a critical step toward probing quantum coherence and entanglement in the chemical reaction. In atom-molecule collisions between Rb and KRb, we discovered an exceptionally long-lived intermediate complex, KRb2, which can be photo-excited by trapping light. The observed complex lifetime was orders of magnitude longer than theoretical predictions, underscoring the limitations of the current theory. We further explored the dependence of complex lifetimes on the initial quantum states and external electric and magnetic fields, providing additional experimental benchmarks to guide future theoretical work. Separately, we found that inelastic collisions between hyperfine-excited Rb atoms and KRb molecules can lead to rotation excitation of KRb post-collision. By probing the product state distribution, the result suggests that mechanical angular momentum is coupled to the spins. Such couplings are too weak in the current theory models to explain the result. Moreover, our result contradicts state-of-the-art coupled-channel calculations. These suggest that some subtle effects, such as molecular vibration and conical intersections, play a critical role in the reaction dynamics. The final piece of the thesis describes the observation of resonant interaction between a Rydberg atom and an ensemble of polar molecules. Such hybrid quantum systems have been proposed for a wide range of applications. Our work provides an experimental demonstration of resonant dipolar interactions between Rydberg atoms and ultracold polar molecules, paving the way for the realization of hybrid systems for quantum computation and simulation.