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

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 ( < T 1 mK), demonstrating that adiabatic capture theories can predict the reaction rates with nearly quantitative accuracy in the multiple-partial-wave regime.

Cold collisions of heavy Σ2 molecules with alkali-metal atoms in a magnetic field: Ab initio analysis and prospects for sympathetic cooling of SrOH(Morita1, Kłos2, Buchachenko3 et al. 2017Phys. Rev. A252441View full textAdd to dashboardCite

“…A combination of these two effects makes the classical capture approximation quite reasonable down to the temperatures as low as 300 µK. Overall, the magnitude of the Li + SrOH reaction rate and its temperature dependence are very similar to those calculated previously for the Li + CaH → LiH + Ca chemical reaction [64]. However, the larger reduced mass of the Li-SrOH collision complex and its stronger long-range dispersion forces make the crossover effect more pronounced and the classical approach more reliable for the J > 0 partial waves.…”

“…To estimate this rate, we applied here a quantum version of the statistical adiabatic channel model [63] implemented as described in Ref. [64].…”

“…[63,64] on a grid of collision energies extending from 10 −11 to 1000 cm −1 . Inner capture boundary conditions are applied at R = R 0 within the short-range region.…”

“…We used 6 values of R 0 ∈ [5.7, 13.2]a 0 to obtain the average capture probability at each collision energy. The classical capture probabilities are determined from the height of the centrifugal barrier for each J ≤ 40 [64].…”

Perspective: Ultracold molecules and the dawn of cold controlled chemistry

“…The performance of the capture model in the multiple-partial-wave regime (∼1 K) was recently assessed for the Li + CaH→ LiH + Ca chemical reaction and good agreement was found between the measured 96 and calculated values. 261 An adiabatic capture theory for cold atom-molecule collisions with strong anisotropic interaction was recently developed by Pawlak et al 262 The method was successfully applied to Penning ionization reaction in He( 3 S) + HD collisions reported by Lavert-Ofir et al 55 We expect these methods to gain more popularity in the coming years.…”