Van der Waals forces between atoms measured

Van der Waals forces are everywhere. And while we think of them as weak, they define many properties of molecules and how they interact with each other, from gas clouds condensing to geckoes climbing walls. But while the macroscopic forces resulting from van der Waals interactions can be easily seen, measuring the force between two neutral atoms is much harder because of the atoms’ small size.

Lucas Béguin and co-workers at the French National Centre for Scientific Research (CNRS) in Palaiseau, France, however, have developed a new way of doing just that. The team used two heavy rubidium atoms and irradiated them to create what’s known as Rydberg atoms, neutral atoms that have one electron in a highly excited state. This amplifies the van der Waals effects so that as the two atoms are moved, using optical tweezers, the force can be measured as as a function of the distance between the two atoms. Just as theory suggests, the van der Waals force between the two atoms shows an 1/R⁶ dependence, where R is the distance between the two atoms. The technique may now be used for more complicated systems.