Physicists Observe Butterfly Rydberg Molecule for First Time

An international team of physicists, led by Prof. Chris Greene from Purdue University and Prof. Herwig Ott from the University of Kaiserslautern, has observed a butterfly Rydberg molecule, a weak pairing of two highly excitable atoms. Their work was published in the Oct. 5 issue of the journal Nature Communications (arXiv.org preprint).

Electron density of a butterfly molecule. Image credit: University of Kaiserslautern.

Rydberg molecules are formed when an electron is kicked far from an atom’s nucleus. In 2002, Prof. Greene and researchers from the University of Colorado and Harvard-Smithsonian Center for Astrophysics theorized that such a molecule could attract and bind to another atom.

“Engineering molecules with a tunable bond length and defined quantum states lies at the heart of quantum chemistry. The unconventional binding mechanism of Rydberg molecules makes them a promising candidate to implement such tunable molecules,” the physicists explained.

“A very peculiar type of Rydberg molecules are the so-called butterfly molecules, which are bound by a shape resonance in the electron-perturber scattering.”

Now, Prof. Greene, Prof. Ott and co-authors have proven the existence of the butterfly Rydberg molecule.

“For all normal atoms, the electrons are always just one or two angstroms away from the nucleus, but in these Rydberg atoms you can get them 100 or 1,000 times farther away,” Prof. Greene explained.

“Following preliminary work in the late 1980s and early 1990s, we saw in 2002 the possibility that this distant Rydberg electron could bind the atom to another atom at a very large distance.”

“This electron is like a sheepdog. Every time it whizzes past another atom, this Rydberg atom adds a little attraction and nudges it toward one spot until it captures and binds the two atoms together.”

“This new binding mechanism, in which an electron can grab and trap an atom, is really new from the point of view of chemistry. It’s a whole new way an atom can be bound by another atom.”

The physicists cooled rubidium gas to a temperature of 100 nano-Kelvin, about one ten-millionth of a degree above absolute zero.

Using a laser, they were able to push an electron from its nucleus, creating a Rydberg atom, and then watch it.

“Whenever another atom happens to be at about the right distance, you can adjust the laser frequency to capture that group of atoms that are at a very clear internuclear separation that is predicted by our theoretical treatment,” Prof. Greene explained.

The team was able to detect the energy of binding between the two atoms based on changes in the frequency of light that the Rydberg molecule absorbed.

“It’s satisfying to know that the predictions made so long ago have been proven,” Prof. Greene said.

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T. Niederprüm et al. 2016. Observation of pendular butterfly Rydberg molecules. Nat. Commun. 7: 12820; doi: 10.1038/ncomms12820