Adiabatic channel capture theory applied to cold atom–molecule reactions: Li + CaH $\to$ LiH + Ca at 1K

We use quantum and classical adiabatic capture theories to study the chemical reaction Li + CaH $\to$ LiH + Ca. Using a recently developed ab initio potential energy surface, which provides an accurate representation of long-range interactions in the entrance reaction channel, we calculate the adiabatic channel potentials by diagonalizing the Li-CaH Hamiltonian as a function of the atom-molecule separation. The resulting adiabatic channel potentials are used to calculate both the classical and quantum capture probabilities as a function of collision energy, as well as the temperature dependencies of the partial and total reaction rates. The calculated reaction rate agrees well with the measured value at 1 K (V Singh et al 2012 Phys. Rev. Lett. 108 203201), suggesting that the title reaction proceeds without an activation barrier. The calculated classical adiabatic capture rate agrees well with the quantum result in the multiple-partial-wave regime of relevance to the experiment. Significant differences are found only in the ultracold limit ($T<1$ mK), demonstrating that adiabatic capture theories can predict the reaction rates with nearly quantitative accuracy in the multiple-partial-wave regime. (paper)