Abstract
While attaining external field control of bimolecular chemical reactions has long been a coveted goal of physics and chemistry, the role of hyperfine interactions and dc magnetic fields in achieving such control has remained elusive. We develop an extended coupled-channel statistical theory of barrierless atom-diatom chemical reactions, and apply it to elucidate the effects of magnetic fields and hyperfine interactions on the ultracold chemical reaction Li($^2\text{S}_{1/2}$) + CaH($^2\Sigma^+$) $\to$ LiH($^1\Sigma^+$) + Ca($^1\text{S}_{0}$) on a newly developed set of ab initio potential energy surfaces. We observe large field effects on the reaction cross sections, opening up the possibility of controlling ultracold barrierless chemical reactions by tuning selected hyperfine states of the reactants with an external magnetic field.
Highlights
Using external electromagnetic fields to control chemical reactivity is a central goal of chemical physics [1,2], which stimulated the development of new research avenues ranging from mode-selective chemistry [1] and coherent control [2] to the study of stereodynamics and vector correlations in molecular collisions [3,4,5] and ultracold controlled chemistry [6,7]
Molecular chemical reactions are most readily controlled at ultralow temperatures, where the reactants can be prepared in single internal and motional quantum states [8], which maximizes the effects of external electromagnetic fields [9] and allows for the manifestation of quantum phenomena, which would otherwise be obscured by thermal averaging, such as threshold and resonance scattering [7,8,10], tunneling [7,11], and interference [12,13]
The matrix elements of the Hamiltonian (1) are evaluated as described in our previous work [59] with the following essential modifications: (1) both the singlet and triplet potential energy surfaces (PESs) of Li-CaH are included in CCS calculations; (2) the singlet PES is modified at R = Rm to account for its reactive nature (the results of the calculations are largely insensitive to Rm as shown in Appendix B; (3) the hyperfine degrees of freedom of the reactants are explicitly included, as are their interactions with an external magnetic field
Summary
Using external electromagnetic fields to control chemical reactivity is a central goal of chemical physics [1,2], which stimulated the development of new research avenues ranging from mode-selective chemistry [1] and coherent control [2] to the study of stereodynamics and vector correlations in molecular collisions [3,4,5] and ultracold controlled chemistry [6,7]. Recent experimental advances in laser cooling and trapping [24,25] have led to the production of dense, trapped ensembles of molecular radicals (i.e., molecules with nonzero electron spins) such as CaF(2 +) [26,27,28,29], SrF(2 +) [30], YbF(2 +) [31], and SrOH(2 +) [32] Cotrapping of these molecules with ultracold alkali-metal atoms [30,31] would open up the fascinating prospect of studying spin-selective ultracold controlled chemistry [6,33,34]. We find that the reaction can be efficiently suppressed by tuning the hyperfine states of the reactants with an external magnetic field, opening up the possibility for controlling ultracold spin-dependent chemical reactions. Our results show that sympathetic cooling of chemically reactive 2 radicals could be facilitated by applying external magnetic fields
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