Reaction kinetics of ultracold molecule-molecule collisions

TL;DR

This study investigates the reaction kinetics of a single-channel ultracold molecule-molecule collision, measuring dissociation rates and confirming theoretical dependencies, thereby advancing understanding of four-body ultracold reactions.

Contribution

It provides the first measurement of the collisional dissociation rate of a specific four-body ultracold reaction and confirms theoretical scaling laws.

Findings

Dissociation rate follows an Arrhenius law with collision energy.

Reaction rate scales as the fourth power of the scattering length.

Determined a universal four-body reaction constant.

Abstract

Studying chemical reactions on a state-to-state level tests and improves our fundamental understanding of chemical processes. For such investigations it is convenient to make use of ultracold atomic and molecular reactants as they can be prepared in well defined internal and external quantum states$^{1-4}$. In general, even cold reactions have many possible final product states$^{5-15}$ and reaction channels are therefore hard to track individually$^{16}$. In special cases, however, only a single reaction channel is essentially participating, as observed e.g. in the recombination of two atoms forming a Feshbach molecule$^{17-19}$ or in atom-Feshbach molecule exchange reactions$^{20,21}$. Here, we investigate a single-channel reaction of two Li$_2$-Feshbach molecules where one of the molecules dissociates into two atoms $2\mathrm{AB}\Rightarrow \mathrm{AB}+\mathrm{A}+\mathrm{B}$. The process is a prototype for a class of four-body collisions where two reactants produce three product particles. We measure the collisional dissociation rate constant of this process as a function of collision energy/ temperature and scattering length. We confirm an Arrhenius-law dependence on the collision energy, an $a^4$ power-law dependence on the scattering length $a$ and determine a universal four body reaction constant.