The special ion microscope used in the discovery. Image credit: Nicolas Zuber
For the first time, physicists have observed a molecular bond between a positive ion and a Rydberg atom, which is a special excited state in which electrons occupy orbitals much further from the nucleus than normal. A Rydberg atom can be 1000 times larger than the regular version of the same atom.
Given the location of their electrons, these atoms have several unexpected properties, especially when it comes to their responses to electric and magnetic fields. This is why a bond forms with this atom, as reported in the journal Natureis something that has never been seen before.
There are three main types of molecular bonds. Ionic bonds see two ions of opposite charge come together. An example of this is table salt. A covalent bond has two or more neutral atoms that share electrons with each other. Water is a classic example of this. The third is metal bonding, where the electrons are delocalized across a lattice of atoms. These give the metals their properties such as their conductivity and luster – they shine.
However, the bond between a Rydberg atom and an ion is something quite different. The ion creates a dipole in the Rydberg atom, the electric charge accumulates on one side of it – but the ion can flip it. At close range, the Rydberg atom and the ion would repel each other because the side to the ion would become positive. Further out, they attract each other, with the negative side facing the positive ion. The distance it turns around is the length of the molecule.
The binding was observed in a cloud of rubidium cooled to only a fraction of a degree above absolute zero. The ultra-cold temperature is needed to create the delicate bond. The team first used a laser to ionize some rubidium atoms by kicking out their electrons. Then additional laser beams drove other atoms into the Rydberg state.
At that point, the ion and the Rydberg atom begin their dance of a spinning electric field, oscillating around an equilibrium point. This is how a molecule is formed. Thanks to a special ion microscope that measures electric fields, the researchers were able to image the molecule.
“We can image the free-floating molecule and its constituents with this microscope and directly observe and study the alignment of this molecule in our experiment,” said lead author Nicolas Zuber, a graduate researcher at the University of Stuttgart, in a statement. pronunciation†
The next step for the team is to study the movement of these molecules, such as rotations and vibrations, which are much slower than regular molecules due to their impressive size.
[h/t: Physics World]