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Physical Review A

Hyperfine state-to-state ultracold atom-dimer reaction and universality of deep dimer product

Jing-Lun Li, Shu‐Lin Cong

Published Feb 19, 2019• 6 citations
Publisher

Abstract

We present a multichannel finite-range three-body scattering theoretical model used for investigating the magnetically tuned atom-dimer collision reaction. We take the $^{6}\mathrm{Li}\text{\ensuremath{-}}^{6}\mathrm{Li}_{2}$ system as an example to calculate the atom-dimer scattering length and dimer relaxation rate steered by a magnetic field. When the reactants are prepared in three shallow atom-dimer hyperfine channels, our calculation results agree with two experimental measurements [S. Nakajima et al., Phys. Rev. Lett. 105, 023201 (2010); T. Lompe et al., Phys. Rev. Lett. 105, 103201 (2010)]. We find that the two-body $p$-wave interaction has a significant influence on the atom-dimer collision reaction in the three hyperfine channels. By including the hyperfine and Zeeman interactions, we calculate the hyperfine state-to-state transition probability from the reactant channel to the deep atom-dimer hyperfine channel. We find a universality in the atom-dimer collision reaction, i.e., the universality of deep dimer product. The universality of deep dimer product in the $^{6}\mathrm{Li}\text{\ensuremath{-}}^{6}\mathrm{Li}_{2}$ collision reaction is more robust at magnetic field $B>620$ G than that at $B<620$ G.

DOI
10.1103/physreva.99.022708

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We present a multichannel finite-range three-body scattering theoretical model used for investigating the magnetically tuned atom-dimer collision reaction. We take the 6Li\ensuremath-6Li2^{6}\mathrm{Li}\text{\ensuremath{-}}^{6}\mathrm{Li}_{2}6Li\ensuremath-6Li2​ system as an example to calculate the atom-dimer scattering length and dimer relaxation rate steered by a magnetic field. When the reactants are prepared in three shallow atom-dimer hyperfine channels, our calculation results agree with two experimental measurements [S. Nakajima et al., Phys. Rev. Lett. 105, 023201 (2010); T. Lompe et al., Phys. Rev. Lett. 105, 103201 (2010)]. We find that the two-body ppp-wave interaction has a significant influence on the atom-dimer collision reaction in the three hyperfine channels. By including the hyperfine and Zeeman interactions, we calculate the hyperfine state-to-state transition probability from the reactant channel to the deep atom-dimer hyperfine channel. We find a universality in the atom-dimer collision reaction, i.e., the universality of deep dimer product. The universality of deep dimer product in the 6Li\ensuremath-6Li2^{6}\mathrm{Li}\text{\ensuremath{-}}^{6}\mathrm{Li}_{2}6Li\ensuremath-6Li2​ collision reaction is more robust at magnetic field B>620B>620B>620 G than that at B<620B<620B<620 G.
We study the atom-dimer collision reaction for a three bosonic 7Li^{7}\mathrm{Li}7Li system using a multichannel model. We calculate the atom-dimer scattering length and dimer relaxation rate for all three 7Li^{7}\mathrm{Li}7Li atoms in $|f=1,{m}_{f}=1\ensuremath{\rangle}$ (case I) or $|f=1,{m}_{f}=0\ensuremath{\rangle}$ (case II) in an external magnetic field. The dimer is considered as the Feshbach dimer in both cases. In case I there are two atom-dimer resonances, whereas in case II only one atom-dimer resonance induced by the second excited trimer state is observed. It is found that ddd-wave two-body interaction plays an appreciable role in the entire considered magnetic field regime in case I but only when B>B>B> 890 G in case II. We calculate the state-to-state transition probability and normalized transition probability from the incoming atom-dimer channel to the deep atom-dimer hyperfine channel. A propensity that the second-deepest hyperfine channels of atom-dimer collision reaction are dominant in all product channels is revealed for both sss- and ddd-wave dimers.
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.
Abstract Studying chemical reactions on a state-to-state level tests and improves our fundamental understanding of chemical processes. For such investigations it is convenient to make use of ultracold atomic and molecular reactants as they can be prepared in well defined internal and external quantum states. Here, we investigate a single-channel reaction of two Li2-Feshbach molecules where one of the molecules dissociates into two atoms 2AB ⇒ AB + A + B. The process is a prototype for a class of four-body collisions where two reactants produce three product particles. We measure the collisional dissociation rate constant of this process as a function of collision energy/temperature and scattering length. We confirm an Arrhenius-law dependence on the collision energy, an a4 power-law dependence on the scattering length a and determine a universal four body reaction constant.
Fully understanding a chemical reaction on the quantum level is a long-standing goal in physics and chemistry. Experimental investigation of such state-to-state chemistry requires both the preparation of the reactants and the detection of the products in a quantum state resolved way, which has been a long term challenge. Using the high level control in the ultracold domain, we prepare a few-body quantum state of reactants and demonstrate state-to-state chemistry with unprecedented resolution. We present measurements and accompanying theoretical analysis for the recombination of three spin-polarized ultracold Rb atoms forming a weakly bound Rb2_22​ dimer. Detailed insights of the reaction process are obtained that suggest propensity rules for the distribution of reaction products. The scheme can readily be adapted to other species and opens a door to detailed investigations of inelastic or reactive processes in domains never before accessible.
A new narrow three-atom loss resonance associated with an Efimov trimer crossing the three-atom threshold has recently been discovered in a many-body system of ultracold 6Li^{6}\mathrm{Li}6Li atoms in the three lowest hyperfine spin states at a magnetic field near 895 G. O'Hara and coworkers have used measurements of the three-body recombination rate in this region to determine the complex three-body parameter associated with Efimov physics. Using this parameter as the input, we calculate the universal predictions for the spectrum of Efimov states and for the three-body recombination rate in the universal region above 600 G where all three scattering lengths are large. We predict an atom-dimer loss resonance at $672\ifmmode\pm\else\textpm\fi{}2$ G associated with an Efimov trimer disappearing through an atom-dimer threshold. We also predict an interference minimum in the three-body recombination rate at $759\ifmmode\pm\else\textpm\fi{}1$ G where the three-spin mixture may be sufficiently stable to allow experimental study of the many-body system.