Rydberg atoms — atoms with a highly excited electron — can form unusual types of molecular bonds. These bonds differ from the well-known ionic and covalent bonds not only by their binding mechanisms, but also by their bond lengths ranging up to several micrometers. Using ion microscope, physicists at the Universität Stuttgart have observed a new type of molecular bond between an ion and a Rydberg atom.
“When single particles like atoms and ions bond, molecules emerge,” said Universität Stuttgart Ph.D. student Nicolas Zuber and colleagues.
“Such bonds between to particles can arise if they have, for example, opposite electrical charges and hence attract each other.”
“The molecule observed by our team exhibits a special feature: it consists of a positive electrically charged ion and a neutral atom in a so-called Rydberg state.”
“As the charge of the ion deforms the Rydberg atom in a very specific way, the bond between the two particles emerges.”
To study their Rydberg-atom-ion molecule, the authors prepared an ultracold rubidium cloud and cooled it down close to absolute zero.
“Only at these low temperatures, the force between the particles is strong enough to form a molecule,” they explained.
“In these ultra-cold atomic ensembles, the ionization of single atoms with laser fields prepares the first building block of the molecule — the ion.”
“Additional laser beams excite a second atom into the Rydberg state. The electric field of the ion deforms this gigantic atom.”
“Interestingly, the deformation can be attractive or repulsive depending on the distance between the two particles, letting the binding partners oscillate around an equilibrium distance and inducing the molecular bond.”
“The distance between the binding partners is unusually large and amounts to about the tenth of the thickness of a human hair.”
Using a high-resolution ion microscope, the physicists measured the vibrational spectrum and spatially resolved the bond length and the alignment of the Rydberg-atom-ion molecule.
“We could image the free-floating molecule and its constituents with this microscope and directly observe and study the alignment of this molecule,” Zuber said.
“Because of its gigantic size and the weak binding of the molecule, the dynamical processes are slower compared to usual molecules.”
“We hope to gain new and more detailed knowledge about the inner structure of the molecule.”
The team’s work was published in the journal Nature.