Collisional losses of ultracold molecules due to intermediate complex formation

TL;DR

This paper investigates the loss mechanisms of ultracold alkali-metal molecules, focusing on intermediate complex formation, and provides theoretical insights into how nuclear couplings influence complex lifetimes, aligning with experimental observations.

Contribution

It offers ab initio calculations of nuclear couplings in bialkali tetramers and links these to increased complex lifetimes, explaining unexpected loss rates in ultracold molecule experiments.

Findings

Nuclear spin--spin and quadrupole couplings are strong enough to couple rotational states.

These couplings increase the density of states and lifetimes of collision complexes.

Results are consistent with recent experimental loss rate observations.

Abstract

Understanding the sources of losses and chemical reactions of ultracold alkali-metal molecules is among the critical elements needed for their application in precision measurements and quantum technologies. Recent experiments with nonreactive systems have reported unexpectedly large loss rates, posing a challenge for theoretical explanation. Here, we examine the dynamics of intermediate four-atom complexes formed in bimolecular collisions. We calculate the nuclear spin--rotation, spin--spin, and quadrupole coupling constants for bialkali tetramers using ab intio quantum-chemical methods. We show that the nuclear spin--spin and quadrupole couplings are strong enough to couple different rotational manifolds to increase the density of states and lifetimes of the collision complexes, which is consistent with experimental results. We propose further experiments to confirm our predictions.