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arXiv:2101.03486 · Physical Review A

Coherent-control mechanisms of Penning and associative ionization in cold He*(2 3S)−He*(2 3S) reactive scattering

Juan J. Omiste, Timur V. Tscherbul, Paul W. Brumer

Published May 28, 2021• 0 citations
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Abstract

We explore coherent control of Penning and associative ionization in cold collisions of metastable ${\mathrm{He}}^{*}(2{\phantom{\rule{4pt}{0ex}}}^{3}S)$ atoms via the quantum interference between different states of the ${\mathrm{He}}_{2}^{*}$ collision complex. By tuning the preparation coefficients of the initial atomic spin states, we can benefit from the quantum interference between molecular channels to maximize or minimize the cross sections for Penning and associative ionization. In particular, we find that we can enhance the ionization ratio by 30% in the cold regime. This work is significant for the coherent control of chemical reactions in the cold and ultracold regimes.

DOI
10.1103/physreva.103.052809

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Coherent control techniques are computationally applied to cold (1  mK<T<1  K1\text{ }\text{ }\mathrm{mK}<T<1\text{ }\text{ }\mathrm{K}1  mK<T<1  K) and ultracold ($T<1\text{ }\text{ }\ensuremath{\mu}\mathrm{K}$) Ne∗(3s,3P2)+Ar(1S0){\mathrm{Ne}}^{*}(3s,^{3}P_{2})+\mathrm{Ar}(^{1}S_{0})Ne∗(3s,3P2​)+Ar(1S0​) collisions. We show that by using various initial superpositions of the Ne∗(3s,3P2){\mathrm{Ne}}^{*}(3s,^{3}P_{2})Ne∗(3s,3P2​) $M={\ensuremath{-}2,\ensuremath{-}1,0,1,2}$ Zeeman sublevels it is possible to reduce the Penning ionization and associative ionization cross sections by as much as 4 orders of magnitude. It is also possible to drastically change the ratio of these two processes. The results are based on combining, within the ``rotating atom approximation'', empirical and ab initio ionization widths.
Coherent control theory is applied to the control of Ne*(3s,(3)P2)+Ar((1)S0) collisions and computations are shown that display extensive control over these processes. Indeed we demonstrate that it is possible to essentially turn on and off the cross sections for both the Penning and associative ionization processes. This facility arises from the interference between matter waves induced by creating a linear superposition of the degenerate M={-2,-1,0,1,2} Zeeman sublevels of the Ne*(3s,(3)P2) target atom. The computations, conducted at collision energies in the 1-8 kcal/mole range, are based on combining, within the "rotating atom approximation," empirically derived and ab initio ionization widths.
We show that quantum interference-based coherent control is a highly efficient tool for tuning ultracold molecular collision dynamics that is free from the limitations of commonly used methods that rely on external electromagnetic fields. By varying the relative populations and phases of initial coherent superpositions of degenerate molecular states, we demonstrate complete coherent control over integral scattering cross sections in the ultracold s-wave regime of both the initial and final collision channels. The proposed control methodology is applied to ultracold O_{2}+O_{2} collisions, showing extensive control over s-wave spin-exchange cross sections and product branching ratios over many orders of magnitude.
In an ultracold, optically trapped mixture of 87Rb^{87}\mathrm{Rb}87Rb and metastable triplet 4He^{4}\mathrm{He}4He atoms we have studied trap loss for different spin-state combinations, for which interspecies Penning ionization is the main two-body loss process. We observe long trapping lifetimes for the purely quartet spin-state combination, indicating strong suppression of Penning-ionization loss by at least two orders of magnitude. For the other spin mixtures we observe short lifetimes that depend linearly on the doublet character of the entrance channel. We compare the extracted loss rate coefficient with recent predictions of multichannel quantum-defect theory for reactive collisions involving a strong exothermic loss channel and find near-universal loss for doublet scattering. Our work demonstrates control of Penning-ionization reactive collisions by internal atomic state preparation.
Debbie Jin and Jun Ye, in “Polar molecules in the quantum regime” (PHYSICS TODAY, May 2011, page 27), provide an enlightening overview of their recent work on cold molecular gases; they note a significant opportunity to study and manipulate chemical reactions in a regime in which quantum effects are important. In the case of their potassium–rubidium system, they regard the observed reaction KRb + KRb → K2 + Rb2 as being of great interest but also as being a significant impediment to the goal of preparing a quantum gas of oriented KRb molecules.The cold gas that Jin and Ye have created does indeed offer a unique environment to study chemical reactions that are strongly affected by quantum mechanics. One such opportunity, not noted in their article, is the ability to manipulate reaction cross sections by “coherent control.”11. M. Shapiro, P. Brumer, Principles of the Quantum Control of Molecular Processes, Wiley, New York (2003); Quantum Control of Molecular Processes, Wiley-VCH, Weinheim, Germany, in press. In that approach one creates an initial superposition of scattering states that allows control over reaction cross sections through quantum interference.Two coherent control scenarios are worth examining toward the goal of controlling or reducing the KRb + KRb reactive cross section. The first was designed to control cross sections in the scattering of identical diatomic molecules, and the second is a method capable of suppressing reactive scattering by suitable preparation of the initial scattering state. Both are discussed in reference 11. M. Shapiro, P. Brumer, Principles of the Quantum Control of Molecular Processes, Wiley, New York (2003); Quantum Control of Molecular Processes, Wiley-VCH, Weinheim, Germany, in press. .Demonstrating coherent control of collisional processes in the cold KRb gas would contribute greatly to understanding and manipulating chemical reactions on a fundamental quantum level. The use of such control to suppress the KRb + KRb reaction, if successful, would be an added technological benefit on the way to producing the desired quantum gas of oriented molecules.REFERENCESSection:ChooseTop of pageREFERENCES <<CITING ARTICLES1. M. Shapiro, P. Brumer, Principles of the Quantum Control of Molecular Processes, Wiley, New York (2003); Google ScholarQuantum Control of Molecular Processes, Wiley-VCH, Weinheim, Germany, in press. Google Scholar© 2011 American Institute of Physics.