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arXiv:2102.05707 · Physical Review Letters

Quantum Spin State Selectivity and Magnetic Tuning of Ultracold Chemical Reactions of Triplet Alkali-Metal Dimers with Alkali-Metal Atoms

Rebekah Hermsmeier, Jacek A. Kłos, Svetlana Kotochigova, Timur V. Tscherbul

Published Aug 31, 2021• 25 citations
arXivHTMLPDF

Abstract

We demonstrate that it is possible to efficiently control ultracold chemical reactions of alkali-metal atoms colliding with open-shell alkali-metal dimers in their metastable triplet states by choosing the internal hyperfine and rovibrational states of the reactants as well as by inducing magnetic Feshbach resonances with an external magnetic field. We base these conclusions on coupled-channel statistical calculations that include the effects of hyperfine contact and magnetic-field-induced Zeeman interactions on ultracold chemical reactions of hyperfine-resolved ground-state Na and the triplet NaLi(a^{3}Σ^{+}) producing singlet Na_{2}(^{1}Σ_{g}^{+}) and a Li atom. We find that the reaction rates are sensitive to the initial hyperfine states of the reactants. The chemical reaction of fully spin-polarized, high-spin states of rotationless NaLi(a^{3}Σ^{+},v=0,N=0) molecules with fully spin-polarized Na is suppressed by a factor of 10-100 compared to that of unpolarized reactants. We interpret these findings within the adiabatic state model, which treats the reaction as a sequence of nonadiabatic transitions between the initial nonreactive high-spin state and the final low-spin states of the reaction complex. In addition, we show that magnetic Feshbach resonances can similarly change reaction rate coefficients by several orders of magnitude. Some of these resonances are due to resonant trimer bound states dissociating to the N=2 rotational state of NaLi(a^{3}Σ^{+},v=0) and would thus exist in systems without hyperfine interactions.

DOI
10.1103/physrevlett.127.103402

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We demonstrate that it is possible to efficiently control ultracold chemical reactions of alkali-metal atoms colliding with open-shell alkali-metal dimers in their metastable triplet states by choosing the internal hyperfine and rovibrational states of the reactants as well as by inducing magnetic Feshbach resonances with an external magnetic field. We base these conclusions on coupled-channel statistical calculations that include the effects of hyperfine contact and magnetic-field-induced Zeeman interactions on ultracold chemical reactions of hyperfine-resolved ground-state Na and the triplet NaLi(a^{3}Σ^{+}) producing singlet Na_{2}(^{1}Σ_{g}^{+}) and a Li atom. We find that the reaction rates are sensitive to the initial hyperfine states of the reactants. The chemical reaction of fully spin-polarized, high-spin states of rotationless NaLi(a^{3}Σ^{+},v=0,N=0) molecules with fully spin-polarized Na is suppressed by a factor of 10-100 compared to that of unpolarized reactants. We interpret these findings within the adiabatic state model, which treats the reaction as a sequence of nonadiabatic transitions between the initial nonreactive high-spin state and the final low-spin states of the reaction complex. In addition, we show that magnetic Feshbach resonances can similarly change reaction rate coefficients by several orders of magnitude. Some of these resonances are due to resonant trimer bound states dissociating to the N=2 rotational state of NaLi(a^{3}Σ^{+},v=0) and would thus exist in systems without hyperfine interactions.
Ultracold chemical reactions of weakly bound triplet-state alkali metal dimer molecules have recently attracted much experimental interest. We perform rigorous quantum scattering calculations with a new ab initio potential energy surface to explore the chemical reaction of spin-polarized NaLi( a 3 Σ + ) and Li( 2 S) to form Li 2 ( a 3 Σ u + ) and Na( 2 S). The reaction is exothermic and proceeds readily at ultralow temperatures. Significantly, we observe strong sensitivity of the total reaction rate to small variations of the three-body part of the Li 2 Na interaction at short range, which we attribute to a relatively small number of open Li 2 ( a 3 Σ u + ) product channels populated in the reaction. This provides the first signature of highly non-universal dynamics seen in rigorous quantum reactive scattering calculations of an ultracold exothermic insertion reaction involving a polar alkali dimer molecule, opening up the possibility of probing microscopic interactions in atom+molecule collision complexes via ultracold reactive scattering experiments.
We show that the interaction of polar alkali-metal dimers in the quintet spin state leads to the formation of a deeply bound reaction complex. The reaction complex can decompose adiabatically into homonuclear alkali-metal dimers (for all molecules except KRb) and into alkali-metal trimers (for all molecules). We show that there are no barriers for these chemical reactions. This means that all alkali-metal dimers in the ${a}^{3}{\ensuremath{\Sigma}}^{+}$ state are chemically unstable at ultracold temperature, and the use of an optical lattice to segregate the molecules and suppress losses may be necessary. In addition, we calculate the minimum-energy path for the chemical reactions of alkali-metal hydrides. We find that the reaction of two molecules is accelerated by a strong attraction between the alkali-metal atoms, leading to a barrierless process that produces hydrogen atoms with large kinetic energy. We discuss the unique features of the chemical reactions of ultracold alkali-metal dimers in the ${a}^{3}{\ensuremath{\Sigma}}^{+}$ electronic state.
Collisional resonances of molecules can offer a deeper understanding of interaction potentials and collision complexes, and allow control of chemical reactions.Here, we experimentally map out the spectrum of Feshbach resonances in collisions between ultracold triplet rovibrational ground-state NaLi molecules and Na atoms over a range of 1400 G. Preparation of the spin-stretched state puts the system initially into the nonreactive quartet potential.A total of 25 resonances are observed, in qualitative agreement with quantum-chemistry calculations using a coupled-channels approach.Although the theory cannot predict the positions of resonances, it can account for several experimental findings and provide unprecedented insight into the nature and couplings of ultracold, strongly interacting complexes.Previous work has addressed only weakly bound complexes.We show that the main coupling mechanism results from spin-rotation and spin-spin couplings in combination with the anisotropic atom-molecule interaction, and that the collisional complexes which support the resonances have a size of 30a 0 -40a 0 .This study illustrates the potential of a combined experimental and theoretical approach.
This paper develops a theoretical methodology to describe ultracold atom-diatom chemical reactions without a barrier in the presence of external magnetic fields and hyperfine interactions. The authors apply their method to the chemical reaction Li + CaH -> LiH + Ca and show good agreement with previous experimental work, as well as uncover the possibility of controlling ultracold barrierless chemical reactions by tuning selected hyperfine states of the reactants with an external magnetic field.
This thesis describes two studies with a mixture of ultracold triplet ground state 23Na6Li molecules and 23Na atoms. The first study is to realize the cooling of ultracold NaLi molecules with Na atoms that efficiently exchange kinetic energy when colliding with molecules and bring them to thermal equilibrium with lower temperatures. Such cooling based on inter-particle collisions, which is the so-called collisional cooling, has not been successful in other molecular systems at micro-Kelvin temperatures or below before the work described in this thesis. By harnessing the good-to-bad collision ratio of the NaLi+Na mixture around 200, we increase the phase space density of the molecules by a factor of 20, achieving the temperatures as low as 220 nano-Kelvin. This collisional property arises from the suppression of reactive and inelastic collisions by the polarization of the spin in a chemically non-reactive quartet state, possibly together with the small size and the low mass of the NaLi+Na mixture. The loss rate in the spin-polarized NaLi+Na mixture is more than an order of magnitude lower than the so-called universal rate, which occurs in most molecular collisions as particles are lost or react at short range with unity probability. The second study is about the control of the reactive collisions by magnetically-tunable scattering resonances, so-called Feshbach resonances, between molecules and atoms. By controlling the phase of the scattering wavefunction via a Feshbach resonance at 978 G, we modify the loss rate by more than two orders of magnitude, and such a large dynamic range of tunability in loss is possible only in a molecular system in which the background loss is far below the universal limit, like the spin-polarized NaLi+Na mixture. We explain the observed tunability in loss by a single-channel model based on quantum interference of the scattering wavefunction reflected at long and short ranges, which is analogous to the optical interference in a Fabry-Perot resonator. By fitting our Fabry-Perot model to the precisely measured lineshape of the loss feature at 978 G, we measure short range parameters, such as a background phase shift parameter and a quantum-defect parameter, which indicates that the spin-polarized NaLi+Na has about 4% loss probability at short range. By extending the single-channel model to consider the lifetime of a bound state of a closed channel, we treat a weak Feshbach resonance at 1030 G as a lossy phase shifter. With this extended two-channel model, we measure that the lifetime of a three-body collision complex associated with the resonance at 1030 G is about 60 ns. The work of this thesis shows that the spin-polarization is a simple yet powerful method to suppress reactive collisions in a molecular system and establishes the light triplet molecule as a useful system to study quantum chemistry, molecular scattering resonances, and possibly short range physics.
We consider the magnetically tunable Feshbach resonances that may exist in ultracold mixtures of molecules in ${}^{2}\mathrm{\ensuremath{\Sigma}}$ states and alkali-metal atoms. We focus on Rb+CaF\mathrm{Rb}+\mathrm{CaF}Rb+CaF as a prototype system. There are likely to be Feshbach resonances analogous to those between pairs of alkali-metal atoms. We investigate the patterns of near-threshold states and the resonances that they cause, using coupled-channel calculations of the bound states and low-energy scattering on model interaction potentials. We explore the dependence of the properties on as-yet-unknown potential parameters. There is a high probability that resonances will exist at magnetic fields below 1000 G, and that these will be broad enough to control collisions and form triatomic molecules by magnetoassociation. We consider the effects of CaF rotation and anisotropy of the interaction potential, and conclude that they may produce additional resonances but should not affect the existence of rotation-free resonances.