Magnetic tuning of ultracold barrierless chemical reactions

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.Comment: 4.2 pages plus Supplemental Materia

Quantum Theory of Molecular Collisions in a Magnetic Field: Efficient Calculations Based on the Total Angular Momentum Representation

An efficient method is presented for rigorous quantum calculations of atom-molecule and molecule-molecule collisions in a magnetic field. The method is based on the expansion of the wavefunction of the collision complex in basis functions with well-defined total angular momentum in the body-fixed coordinate frame. We outline the general theory of the method for collisions of diatomic molecules in the $^{2}\Sigma$ and $^{3}\Sigma$ electronic states with structureless atoms and with unlike $^{2}\Sigma$ and $^{3}\Sigma$ molecules. The cross sections for elastic scattering and Zeeman relaxation in low-temperature collisions of CaH($^{2}\Sigma^{+}$) and NH($^{3}\Sigma^{-}$) molecules with $^{3}$He atoms converge quickly with respect to the number of total angular momentum states included in the basis set, leading to a dramatic >10-fold enhancement in computational efficiency compared to the previously used methods [A. Volpi and J. L. Bohn, Phys. Rev. A 65, 052712 (2002); R. V. Krems and A. Dalgarno, J. Chem. Phys. 120, 2296 (2004)]. Our approach is thus well suited for theoretical studies of strongly anisotropic molecular collisions in the presence of external electromagnetic fields.Astronom