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Anexotic atom is an otherwise normalatom in which one or more sub-atomic particles have been replaced by other particles. For example,electrons may be replaced by other negatively charged particles such asmuons (muonic atoms) orpions (pionic atoms).[1][2] Because these substitute particles are usually unstable, exotic atoms typically have very short lifetimes and no exotic atom observed so far can persist under normal conditions.
In amuonic atom (previously called amu-mesic atom, now known to be a misnomer as muons are notmesons),[3] an electron is replaced by a muon, which, like the electron, is alepton. Sinceleptons are only sensitive toweak,electromagnetic andgravitational forces, muonic atoms are governed to very high precision by the electromagnetic interaction.
Since a muon is more massive than an electron, theBohr orbits are closer to the nucleus in a muonic atom than in an ordinary atom, and corrections due toquantum electrodynamics are more important. Study of muonic atoms'energy levels as well as transition rates fromexcited states to theground state therefore provide experimental tests of quantum electrodynamics.
Other muonic atoms can be formed when negative muons interact with ordinary matter.[4] The muon in muonic atoms can either decay or get captured by a proton. Muon capture is very important in heavier muonic atoms, but shortens the muon's lifetime from 2.2 μs to only 0.08 μs.[4]
Muonic hydrogen is like normal hydrogen with the electron replaced by a negative muon, which is to say, a proton orbited by a muon. It is important in addressing theproton radius puzzle.
Muonic hydrogen atoms can form muonic hydrogen molecules. The spacing between the nuclei in such a molecule is hundreds of times smaller than in a normal hydrogen molecule –so close that the nuclei can spontaneously fuse together. This is known asmuon-catalyzed fusion, and was first observed between hydrogen-1 and deuterium nuclei in 1957.[5] Muon-catalyzed fusion has been proposed as a means of generating energy using fusion reactions in a room-temperature reactor.
The symbol4.1H (Hydrogen-4.1) has been used to describe the exotic atom muonic helium (4He-μ), which is likehelium-4 in having twoprotons and twoneutrons.[6] However one of itselectrons is replaced by amuon, which also has charge –1. Because the muon's orbital radius is less than1/200th the electron's orbital radius (due to the mass ratio), the muon can be considered as a part of the nucleus. The atom then has anucleus with two protons, two neutrons and one muon, with total nuclear charge +1 (from two protons and one muon) and only one electron outside, so that it is effectively an isotope of hydrogen instead of an isotope of helium. A muon's weight is approximately0.1 Da so the isotopic mass is 4.1. Since there is only one electron outside the nucleus, the hydrogen-4.1 atom can react with other atoms. Its chemical behavior is more like a hydrogen atom than an inert helium atom.[6][7][8]
Ahadronic atom is an atom in which one or more of theorbital electrons are replaced by a negatively chargedhadron.[9] Possible hadrons include mesons such as thepion orkaon, yielding apionic atom[10] or akaonic atom (seeKaonic hydrogen), collectively calledmesonic atoms;antiprotons, yielding anantiprotonic atom; and theΣ−
particle, yielding aΣ−
orsigmaonic atom.[11][12][13]
Unlike leptons, hadrons can interact via thestrong force, so the orbitals of hadronic atoms are influenced bynuclear forces between thenucleus and the hadron. Since the strong force is a short-range interaction, these effects are strongest if the atomic orbital involved is close to the nucleus, when the energy levels involved may broaden or disappear because of the absorption of the hadron by the nucleus.[2][12] Hadronic atoms, such as pionic hydrogen andkaonic hydrogen, thus provide experimental probes of the theory of strong interactions,quantum chromodynamics.[14]
Anonium (plural:onia) is the bound state of a particle and its antiparticle. The classic onium ispositronium, which consists of an electron and a positron bound together as ametastable state, with a relatively long lifetime of 142 ns in the triplet state.[15] Positronium has been studied since the 1950s to understand bound states in quantum field theory. A recent development callednon-relativistic quantum electrodynamics (NRQED) used this system as a proving ground.
Pionium, a bound state of two oppositely chargedpions, is useful for exploring thestrong interaction. This should also be true ofprotonium, which is a proton–antiproton bound state. Understanding bound states of pionium and protonium is important in order to clarify notions related toexotic hadrons such asmesonic molecules andpentaquark states.Kaonium, which is a bound state of two oppositely charged kaons, has not been observed experimentally yet.
The true analogs of positronium in the theory of strong interactions, however, are not exotic atoms but certainmesons, thequarkonium states, which are made of a heavy quark such as thecharm orbottom quark and its antiquark. (Top quarks are so heavy that they decay through theweak force before they can form bound states.) Exploration of these states through non-relativistic quantum chromodynamics (NRQCD) andlattice QCD are increasingly important tests ofquantum chromodynamics.
Muonium, despite its name, isnot an onium state containing a muon and an antimuon, because IUPAC assigned that name to the system of an antimuon bound with an electron. However, the production of a muon–antimuon bound state, whichis an onium (calledtrue muonium), has been theorized.[16] The same applies to theditauonium (or "true tauonium") exotic QED atom.[17]
Atoms may be composed of electrons orbiting ahypernucleus that includesstrange particles calledhyperons. Suchhypernuclear atoms are generally studied for their nuclear behaviour, falling into the realm ofnuclear physics rather thanatomic physics.
Incondensed matter systems, specifically in somesemiconductors, there are states calledexcitons, which are bound states of an electron and anelectron hole.
An exotic molecule contains one or more exotic atoms.
"Exotic molecule" can also refer to a molecule having some other uncommon property such aspyramidal hexamethylbenzene and aRydberg atom.
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