Muonium
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A muonium particle is an exotic atom made up of a positive muon and an electron, and is given the symbol Mu or μ+e–. During the muon's 2 microsecond lifetime, muonium can enter into compounds such as muonium chloride (MuCl) or sodium muonide (NaMu). [1]
Due to the mass difference between the muon and the electron, muonium is more similar to atomic hydrogen than positronium. It's Bohr radius and ionization energy are within 0.5% of H, D. and T.
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 (which is not surprisingly called muon spin resonance) spectroscopy for the analysis of chemical transformations. There are varians of "muon spin resonance", e.g. Muon spin rotation, which used 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).