A heavy Rydberg system consists of a weakly bound positive and negative ion orbiting their common centre of mass.
The peculiar properties of the Rydberg atom come from the large charge separation and the resulting hydrogenic potential.
The extremely large separation between the two components of a heavy Rydberg system results in an almost perfect 1/r hydrogenic potential seen by each ion.
While this classical description is nice for getting a feel for the interactions involved, it is an oversimplification; many other atoms have a greater electron affinity than hydrogen.
The difficulty in the production of heavy Rydberg systems arises in finding an energetic pathway by which a molecule can be excited with just the right energy to form an ion pair, without sufficient internal energy to cause autodissociation (a process analogous to autoionization in atoms) or rapid dissociation due to collisions or local fields.
As well as producing the characteristic hydrogen-like behaviour, this also makes them extremely sensitive to perturbation by external electric and magnetic fields.
Heavy Rydberg systems have a relatively large reduced mass, given by: This leads to a very slow time evolution, which makes them easy to manipulate both spatially and energetically, while their low binding energy makes them relatively simple to detect through field dissociation and detection of the resulting ions, in a process known as threshold ion-pair production spectroscopy.
Kepler's third law states that the period of an orbit is proportional to the cube of the semi-major axis; this can be applied to the Coulomb force: where