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

Timur V Tscherbul,Alexei A Buchachenko

doi:10.1088/1367-2630/17/3/035010

Abstract

We use quantum and classical adiabatic capture theories to study the chemical reaction Li + CaH 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 ( mK), demonstrating that adiabatic capture theories can predict the reaction rates with nearly quantitative accuracy in the multiple-partial-wave regime.

Highlights

We use quantum and classical adiabatic capture theories to study the chemical reaction Li + CaH → LiH + Ca
The computational cost of these calculations grows rapidly with increasing density of rovibrational states of the reaction complex, making it next to impossible to carry out converged calculations on asymmetric reactions involving heavy atoms, especially if the goal is to study the effects of hyperfine interactions and external electromagnetic fields on low-temperature reaction rates [21]
Our calculated quantum and classical adiabatic capture rates are K = 7.1 and 7.2 × 10−10 cm3/s, respectively, in good agreement with the experimental value (3.6 × 10−10 cm3/s, within a factor of 2 uncertainty [35]). These results suggest that the Li + CaH chemical reaction proceeds without an activation barrier and its low-temperature dynamics is accurately described by a classical adiabatic channel (AC) capture theory, providing the first validation of the theory for neutral atom-molecule chemical reactions at low temperatures beyond the single-partialwave regime

Summary

INTRODUCTION

The high level of control over molecular degrees of freedom achieved in experiments with cold molecular ensembles is projected to make a profound impact on chemical physics [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17, 17], in the areas of chemical reaction dynamics [1,2,3, 5, 17], molecular spectroscopy [4], and energy transfer in molecular aggregates [8, 9]. Our calculated quantum and classical adiabatic capture rates are K = 7.1 and 7.2 × 10−10 cm3/s, respectively, in good agreement with the experimental value (3.6 × 10−10 cm3/s, within a factor of 2 uncertainty [35]) These results suggest that the Li + CaH chemical reaction proceeds without an activation barrier and its low-temperature dynamics is accurately described by a classical AC capture theory, providing the first validation of the theory for neutral atom-molecule chemical reactions at low temperatures beyond the single-partialwave regime.

Potential energy surface and adiabatic channels

Classical capture theory

Quantum capture theory

RESULTS

SUMMARY AND CONCLUSIONS