Complexes formed in collisions between ultracold alkali-metal diatomic molecules and atoms

TL;DR

This paper investigates the formation and properties of three-atom complexes involving ultracold alkali-metal diatomic molecules and atoms, analyzing their vibrational states, spin interactions, and implications for inelastic collisions and loss rates.

Contribution

It provides the first detailed analysis of vibrational state densities and spin interactions in alkali-metal diatomic molecule-atom complexes at ultracold temperatures.

Findings

Vibrational state densities range from 2.2 to 350 K$^{-1}$.

Spin exchange interactions can cause inelastic collisions and resonances.

Background loss rates are influenced by overlapping resonances and spin dynamics.

Abstract

We explore the properties of 3-atom complexes of alkali-metal diatomic molecules with alkali-metal atoms, which may be formed in ultracold collisions. We estimate the densities of vibrational states at the energy of atom-diatom collisions, and find values ranging from 2.2 to 350~K$^{-1}$. However, this density does not account for electronic near-degeneracy or electron and nuclear spins. We consider the fine and hyperfine structure expected for such complexes. The Fermi contact interaction between electron and nuclear spins can cause spin exchange between atomic and molecular spins. It can drive inelastic collisions, with resonances of three distinct types, each with a characteristic width and peak height in the inelastic rate coefficient. Some of these resonances are broad enough to overlap and produce a background loss rate that is approximately proportional to the number of outgoing inelastic channels. Spin exchange can increase the density of states from which laser-induced loss may occur.