Lateral interatomic dispersion forces

TL;DR

This paper predicts a novel lateral van der Waals force between an excited atom with angular momentum and a ground-state atom, leading to unique trajectories and expanding understanding of interatomic forces beyond traditional assumptions.

Contribution

It introduces the concept of a lateral van der Waals force between atoms, highlighting a microscopic origin for lateral forces previously associated with Casimir--Polder effects.

Findings

Lateral van der Waals force acts like a planetary gear.

Predicted trajectories differ significantly when lateral forces are considered.

The effect is demonstrated with cesium and rubidium atoms.

Abstract

Van der Waals forces between atoms and molecules are universally assumed to act along the line separating them. Inspired by recent works on effects which can propel atoms parallel to a macroscopic surface via the Casimir--Polder force, we predict a lateral van der Waals force between two atoms, one of which is in an excited state with non-zero angular momentum and the other is isotropic and in its ground state. The resulting force acts in the same way as a planetary gear, in contrast to the rack-and-pinion motion predicted in works on the lateral Casimir--Polder force in the analogous case, for which the force predicted here is the microscopic origin. We illustrate the effect by predicting the trajectories of an excited caesium in the vicinity of ground-state rubidium, finding behaviour qualitatively different to that if lateral forces are ignored.