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John O | March 2018

Researchers create molecules in never-before-seen, complex structures


By Josh Perry, Editor
jperry@coolingzone.com

 

Physicists from the Rice University (Dallas, Texas) Center for Quantum Materials, Harvard University (Cambridge, Mass.) and the Vienna (Austria) University of Technology have worked on a two-year project to create Rydberg polarons from strontium atoms that were one million times colder than deep space.

 


Rice University atomic physicist Joe Whalen works on a laser cooling system for ultracold strontium gas. (Jeff Fitlow/Rice University)

 

According to a report from Rice, the physicists created molecules that were complex and had never before been seen in nature. Researchers added that this work goes beyond the rigid strictures of chemistry, which define how molecules form.

 

The new molecules are only stable at a millionth of a degree above absolute zero where the atoms are still long enough to be “glued together” in new structures.

 

“The team’s efforts centered around making, measuring and predicting the behavior of a specific state of matter called a Rydberg polaron, a combination of two distinct phenomena, Rydberg atoms and polarons,” the report explained.

 

It added, “In Rydberg atoms, one or more electrons are excited with a precise amount of energy so that they orbit far from the atom’s nucleus. They have been studied in laboratories for decades and are believed to exist in cold reaches of deep space. The Rydberg atoms in the PRL study were up to one micron wide, about 1,000 times larger than normal strontium atoms.”

 

Polarons are created when a single particle reacts strongly enough with its environment to cause nearby electrons to form a coating that it carries. Rydberg polarons gather hundreds of atoms into its orbit while moving through a dense cloud.

 

“In the Rice experiments, researchers began by creating a supercooled cloud of several hundred thousand strontium atoms,” the report said. “By coordinating the timing of laser pulses with changes in the electric field, the researchers were able to create and count Rydberg polarons one by one, ultimately forming millions of them for their study.”

 

Using strontium allowed the scientists to better measure the energy in the coated atoms during the microseconds that each polaron existed before a collision with another particle broke them apart.

 

The research was recently published in Physical Review Letters. The abstract read:

 

“We report spectroscopic observation of Rydberg polarons in an atomic Bose gas. Polarons are created by excitation of Rydberg atoms as impurities in a strontium Bose-Einstein condensate. They are distinguished from previously studied polarons by macroscopic occupation of bound molecular states that arise from scattering of the weakly bound Rydberg electron from ground-state atoms.

 

“The absence of a p-wave resonance in the low-energy electron-atom scattering in Sr introduces a universal behavior in the Rydberg spectral line shape and in scaling of the spectral width (narrowing) with the Rydberg principal quantum number, n.

 

“Spectral features are described with a functional determinant approach (FDA) that solves an extended Fröhlich Hamiltonian for a mobile impurity in a Bose gas. Excited states of polyatomic Rydberg molecules (trimers, tetrameters, and pentamers) are experimentally resolved and accurately reproduced with a FDA.”

 

Learn more about the creation of Rydberg polarons in the video below:

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