The Rydberg atom is a highly excited atom with one or more electrons in orbits with a very high principle quantum number n. These atoms are therefore quite well described by the Bohr hydrogen atom model except that their size is n2 greater than that of this atom.
When the probability distribution of an electron in these orbits is calculated, it is found that they have a trajectory in a torus around the nucleus. The de-excitation time of such an atom is quite long, but under normal density conditions there are collisions between atoms that greatly reduce the lifetime of such a state.
This is not the case in interstellar clouds where the population of Rydberg atoms can be large and lead to a high intensity emission. Transitions at 2.4 GHz between levels n=109 and n=108 are known in hydrogen atoms in space.
Rydberg atoms of rubidium are currently much used in experiments on quantum decoherence using optical cavities. This type of atom is quite difficult to obtain and requires a laser source. Furthermore, they are very fragile and low pressures must be used to avoid collisions between the atoms.