A geonium atom is a useful "atomic system" created in a magnetic Penning trap, named so because it is bound to the Earth. A Penning trap is a combination of a homogeneous magnetic field and an electrostatic quadrupole potential. A single charged particle can be trapped indefinitely. Hans Dehmelt of the University of Washington created the first geonium atom in 1973, stably trapping an electron.
In the simplest case, the trapped system consists of only one particle or ion. Such a quantum system is determined by quantum states of one particle, like in the hydrogen atom. Hydrogen consists of two particles, the nucleus and electron, but the electron motion relative to the nucleus is equivalent to one particle in an external field, see center-of-mass frame. The properties of geonium are indeed very different from a typical atom. The charge undergoes cyclotron motion around the trap axis and oscillates along the axis. An inhomogeneous magnetic "bottle field" is applied to measure the quantum properties by the "continuous Stern-Gerlach" technique. Energy levels and g-factor of the particle can be measured with very high precision. Van Dyck, Jr et al. explored the magnetic splitting of geonium spectra in 1978 and in 1987 published high-precision measurements of electron and positron g-factors, which constrained the electron radius.
Dehmelt and Wolfgang Paul of Bonn University received the 1989 Nobel Prize for Physics for their work on trapping and measuring individual electrons and ions. Geonium studies of electrons and positrons are one of the best tests of QED and yield precise determinations of the fine-structure constant.
- Brown, L.S.; Gabrielse, G. (1986). "Geonium theory: Physics of a single electron or ion in a Penning trap" (PDF). Reviews of Modern Physics 58: 233. Bibcode:1986RvMP...58..233B. doi:10.1103/RevModPhys.58.233.
- Dehmelt, Hans (1988). "A Single Atomic Particle Forever Floating at Rest in Free Space: New Value for Electron Radius". Physica Scripta T22: 102–110. Bibcode:1988PhST...22..102D. doi:10.1088/0031-8949/1988/T22/016.
- "Geonium: A Fake but Useful Atom (Powerpoint Presentation)". Berkeley. Retrieved 2010-02-22.
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