THREEATOM MOLECULE
This illustration shows the sizes of triatomic molecules that follow the geometrical scaling predicted by Vitaly Efimov in 1970. University of Chicago physicists have reported evidence of this geometric scaling in three-atom, lithium-ceisum Efimov molecules.Cheng Chin group, University of Chicago

University of Chicago scientists have observed for the first time a phenomenon in exotic three-atom molecules where the molecules fit inside one another like Russian nesting dolls.

They watched three such molecules in the series, consisting of one lithium atom and two cesium atoms in a vacuum chamber at the ultracold temperature of approximately 200 nanokelvin, a tiny fraction of a degree above absolute zero (minus 459.6 degrees Fahrenheit).

Given an infinitely large universe, the number of increasingly larger molecules in this cesium-lithium system also would be infinite.

The three atom molecule is called an Efimov molecule after the Russian physicist who first predicted the ultracold state in 1970.

The finding, reported in Physical Review Letters, is important because it shows that Efimov molecules follow a simple mathematical rule.

"This is a new rule in chemistry that molecular sizes can follow a geometric series, like 1, 2, 4, 8…," said Cheng Chin, professor in physics at UChicago. "In our case, we find three molecular states in this sequence where one molecular state is about five times larger than the previous one."

The geometric scaling is unique to three-atom molecules and cannot be seen in two or four combinations.

Given the vast differences in the mass of lithium and cesium, the scientists had to use various tricks to 'get the monkey and elephant hanging on the same spring' to interact.

They lowered the temperatures of the lithium and cesium atoms separately, then brought them together to form the triatomic, Efimov molecules.

This required a tool called Feshbach resonance, carried out in a magnetic field, which allowed the researchers to bind and control the interactions between the cesium and lithium atoms.

The first Efimov state was experimentally observed in 2006 in molecules consisting of three cesium atoms which get entangled at temperatures slightly above absolute zero.

Efimov said the latest results made him feel like the parent of a successful child. "The parent is proud of the child's achievement, and he is also proud that in a sense he is part of the child's success."