A Geonium atom is a false "atom" created in a magnetic Penning trap.[1] A Penning trap is a combination of a homogeneous magnetic field and an electrostatic quadrupole potential. A single charged particle can be trapped indefinitely in a Penning trap. A small cloud of charged particles in such a trap is like a many-electron atom, with the difference that the role of the atomic nucleus is played by an adjustable external field in the trap.
In the simplest case, the system consists of only one electron or only one ion in the trap. This is analogous to the hydrogen atom. The properties of this single bound particle, such as the energy levels, g-factor and radius of the electron can be measured and calculated with a very high precision.[2]
Hans Dehmelt of the University of Washington created the first Geonium atom in 1973, by quasi-permanently trapping an electron. In 1989 he and Wolfgang Paul of Bonn University received the Nobel Prize for Physics for their work on isolating individual electrons and ions and making exact measurements of them.[3]