Physicists observe gigantic molecules predicted in 1970

The triatomic molecules fit inside one another like an infinitely large set of Russian nesting dolls. (Credit: iStockphoto)

Scientists have experimentally observed for the first time a phenomenon in ultracold, three-atom molecules predicted by Russian theoretical physicist Vitaly Efimov in 1970.

In this quantum phenomenon, called geometric scaling, the triatomic molecules fit inside one another like an infinitely large set of Russian nesting dolls.

This illustration shows the sizes of triatomic molecules following the geometrical scaling predicted by Vitaly Efimov in 1970.
This illustration shows the sizes of triatomic molecules following the geometrical scaling predicted by Vitaly Efimov in 1970. (Credit: Shih-Kuang Tung/Cheng Chin)

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

Chin and four members of his research group published their findings in Physical Review Letters.

“Quantum theory makes the existence of these gigantic molecules inevitiable, provided proper—and quite challenging—conditions are created,” says Efimov, now at the University of Washington.

The Chicago team observed three molecules in the series, consisting of one lithim 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).

Extend to infinity

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

This remarkable idea stems from the exotic nature of quantum mechanics, which conforms confirms to different laws of physics than those that govern the universe on a macroscopic scale.

“These are certainly exotic molecules,” says Shih-Kuang Tung, the postdoctoral scholar, now at Northwestern University, who led the project.

Only under strict conditions could Tung and his colleagues see the geometric scaling in their Efimov molecules. It appears that neither two-atom nor four-atom molecules can achieve the Efimov state.

“There’s a special case for three atoms,” Chin says.

Efimov’s reaction

“First, I am amazed by the predictive power of the quantum theory,” Efimov says. “Second, I am amazed by the skill of the experimentalists who managed to create those challenging conditions.”

The finding is important because it shows that Efimov molecules, like other complex phenomena in nature, follow a simple mathematical rule. One other example in nature that displays geometric scaling is snowflakes, rooted in the microscopic physics of their hexagonal crystal structure.

A team at the University of Innsbruck in Austria, which included Chin, experimentally observed the first Efimov molecular state in 2006 in molecules consisting of three cesium atoms. In this Efimov state, three cesium atoms become entangled at temperatures slightly above absolute zero.

They form a Borromean ring of three interlocking circles. Any two of them, however, will not interlock.

Chin switched his interest to lithium-cesium molecules in 2010 because observing geometrical scaling in the cesium system presented severe experimental difficulties.

Scaling factor

“The difficulty is that based on what we understand of Efimov’s theory, the scaling factor is predicted to be 22.7 for the cesium system, which is a very large number,” explains Chin, who also is a member of Chicago’s James Franck and Enrico Fermi institutes. Scaling at such a large value demands an extremely low temperature, challenging to reach experimentally.

But the scaling factor of the lithium-cesium triatomic molecule was predicted to be more managable of 4.8. Indeed, after setting up their experiment, “We were able to see three of them at a more accessible temperature of 200 nano-Kelvin,” Chin says. “Their sizes are measured to be 17, 86, and 415 nano-meters, respectively. They closely follow a geometric progression with the predicted scaling factor.”

Dangling a monkey and an elephant

But even the lithium-cesium system presented a difficulty: the significantly differing masses of the two elements, which was critical for observing multiple Efimov states.

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Lithium is one of the lightest elements on the periodic table, while cesium is quite heavy. “One is really massive compared to the other,” Tung explains.

He compared working both elements into an ultracold experiment to dangling a monkey and an elephant from springs. They would hang at different levels, but still need to interact.

In the experiment, the physicists lowered the temperatures of the lithium and ceisum atoms separately, then brought them together to form the triatomic, Efimov molecules.

“It’s a very complicated experiment,” Tung notes, one requiring an ultracold experimental tool called Feshbach resonance. Carried out in a magnetic field, Feshbach resonance allowed researchers to bind and control the interactions between the cesium and lithium atoms.

Cold atoms are subject to manipulation via Feshbach resonance, which allows the observation of geometric scaling.

“Feshbach resonance is a really important tool for us,” Tung says. He and his associates learned how to wield the tool effectively in the past three years.

“We needed to tune the Feshbach resonances very carefully in order to generate these Efimov molecules,” Tung explains.

The National Science Foundation and the Army Research Office funded the work.

Source: University of Chicago