Muonium

For atoms where muons have replaced one or more electrons, see muonic atom.

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+
e−
) is more similar to atomic hydrogen (p+
e−
) than positronium (e+
e−
). Its Bohr radius and ionization energy are within 0.5% of hydrogen, deuterium, and tritium^{[citation needed]}.

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 (since muons are typically produced in a spin-polarized state from the decay of pions), 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 to be 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]

What is called "true muonium", a bound state of a muon and an antimuon, is a theoretical exotic atom which has never been observed. It may have been generated in the collision of electron and positron beams but has not been searched for in the particle debris.^[6]

See also

• Exotic atom
• Muon

References

1. ^ IUPAC (1997). "Muonium". In A.D. McNaught, A. Wilkinson. Compendium of Chemical Terminology (2nd ed.). Blackwell Scientific Publications. doi:10.1351/goldbook.M04069. ISBN 0-86542-684-8. http://goldbook.iupac.org/M04069.html. 
2. ^ V.W Hughes, et al. (1960). "Formation of Muonium and Observation of its Larmor Precession". Physical Review Letters 5 (2): 63–65. Bibcode 1960PhRvL...5...63H. doi:10.1103/PhysRevLett.5.63. 
3. ^ W.H. Koppenol (IUPAC) (2001). "Names for muonium and hydrogen atoms and their ions". Pure and Applied Chemistry 73 (2): 377–380. doi:10.1351/pac200173020377. http://www.iupac.org/publications/pac/2001/pdf/7302x0377.pdf. 
4. ^ C.J. Rhodes (2002). "Muonium—the second radioisotope of hydrogen—and its contribution to free radical chemistry". Perkin Transactions 2: 1379. doi:10.1039/b100699l. http://www.rsc.org/delivery/_ArticleLinking/DisplayArticleForFree.cfm?doi=b100699l&JournalCode=P2. 
5. ^ K.P. Jungmann (2004). "Past, Present and Future of Muonium". Proc. of Memorial Symp. in Honor of V. W. Hughes, New Haven, Connecticut, 14–15 Nov 2003. arXiv:nucl-ex/0404013. Bibcode 2004shvw.conf..134J. doi:10.1142/9789812702425_0009. 
6. ^ S.J. Brodsky, R.F. Lebed (2009). "Production of the smallest QED atom: True muonium (µ⁺µ⁻)". Physical Review Letters 102 (21): 213401. Bibcode 2009PhRvL.102u3401B. doi:10.1103/PhysRevLett.102.213401.