For the first time, physicists have observed a molecular bond forming between a positive ion and a Rydberg atom, which is a special excited state where electrons occupy orbitals that are much further away from the nucleus than usual. A Rydberg atom can be 1,000 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 forming with this atom, as reported in the journal Nature, is something never seen before.
There are three main types of molecular bonds. Ionic bonds see two ions of opposite charge join together. One example of this is table salt. A covalent bond has two or more neutral atoms sharing electrons with each other. Water is a classic example of this. The third is metallic bonding, where the electrons are delocalized over 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 else altogether. The ion creates a dipole in the Rydberg atom, the electric charge accumulates on one side of it – but the ion can make it flip. At close distance, the Rydberg atom and the ion would repel each other, as the side facing the ion would become positive. Further out, they attract each other, with the negative side towards the positive ion. The distance where it flips is the length of the molecule.
The bond was observed in a cloud of rubidium cooled down to just a fraction of a degree above absolute zero. The ultra-cold temperature is necessary to allow the delicate bond to form. The team first used a laser to ionize some of the rubidium atoms by kicking out their electrons. Then additional laser beams excited other atoms into the Rydberg state.
At that point, the ion and the Rydberg atom begin their dance of a flipping electric field, oscillating around an equilibrium point. Thus, a molecule is formed. Thanks to a special ion microscope that measures electric fields, the researchers could image the molecule.
“We could image the free floating molecule and its constituents with this microscope and directly observe and study the alignment of this molecule in our experiment,” lead author Nicolas Zuber, graduate researcher at the University of Stuttgart, said in a statement.
The next step for the team is to study the motion of these molecules such as rotations and vibration, which are much slower than regular molecules due to their impressive size.
[h/t: Physics World]