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Muonium particles are exotic atoms made up of an antimuon and an electron, and are 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).
Due to the mass difference between the antimuon and the electron, muonium is more similar to atomic hydrogen than positronium. Its Bohr radius and ionization energy are within 0.5% of hydrogen, deuterium, and tritium.
Physical chemists consider muonium to be an isotope of hydrogen and, though it is short-lived, 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 is called muon spin resonance or μSR.) There are variants of "muon spin resonance", 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. (C.J. Rhodes, Perkin Transactions 2, 2002)
What is called "True muonium" , matter made of a muon and an anti-muon, is a theoretical exotic atom which has never been observed. It may have been generated in the collision of electron and positron beams but not searched for in the particle debris. 
- ^ International Union of Pure and Applied Chemistry. "muonium". Compendium of Chemical Terminology Internet edition.
- ^ Names for muonium and hydrogen atoms and their ions iupac.org (PDF)
- ^ Stanley J. Brodsky and Richard F. Lebed. Production of the Smallest QED Atom: True Muonium (µ µ-). Physical Review Letters, 2009; DOI: doi:10.1103/PhysRevLett.102.213401
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