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Muonium is an exotic atom made up of an antimuon and an electron,[1] which was discovered in 1960[2] and is given the chemical symbol Mu. During the muon's 2 µs lifetime, muonium can enter into compounds such as muonium chloride (MuCl) or sodium muonide (NaMu).[3] Due to the mass difference between the antimuon and the electron, muonium (μ+
) is more similar to atomic hydrogen (p+
) than positronium (e+
). Its Bohr radius and ionization energy are within 0.5% of hydrogen, deuterium, and tritium.

Although muonium is short-lived, physical chemists use it in a modified form of electron spin resonance spectroscopy for the analysis of chemical transformations and the structure of compounds with novel or potentially valuable electronic properties. (This form of electron spin resonance (eSR) is called muon spin resonance (μSR).) There are variants of μSR, e.g. muon spin rotation, which is affected by the presence of a magnetic field applied transverse to the muon beam direction, and Avoided Level Crossing (ALC), which is also called Level Crossing Resonance (LCR). The latter employs a magnetic field applied longitudinally to the beam direction, and monitors the relaxation of muon spins caused by magnetic oscillations with another magnetic nucleus. One author has considered "muonium" as the second radioisotope of hydrogen, after tritium.[4]

Because the muon is a lepton, the atomic energy levels of muonium can be calculated with great precision from quantum electrodynamics (QED), unlike the case of hydrogen, where the precision is limited by uncertainies related to the internal structure of the proton. For this reason, muonium is an ideal system for studying bound-state QED and also for searching for physics beyond the standard model.[5]

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