An exotic atom is an otherwise normal
atom
Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons.
Every solid, liquid, gas, a ...
in which one or more sub-atomic particles have been replaced by other particles of the same
charge. For example,
electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have n ...
s may be replaced by other negatively charged particles such as
muon
A muon ( ; from the Greek letter mu (μ) used to represent it) is an elementary particle similar to the electron, with an electric charge of −1 '' e'' and a spin of , but with a much greater mass. It is classified as a lepton. As w ...
s (muonic atoms) or
pions (pionic atoms).
[Exotic atoms](_blank)
, AccessScience, McGraw-Hill. accessdate=September 26, 2007. 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.
Muonic atoms
In a ''muonic atom'' (previously called a ''mu-mesic'' atom, now known to be a misnomer as muons are not
mesons), an electron is replaced by a muon, which, like the electron, is a
lepton
In particle physics, a lepton is an elementary particle of half-integer spin (spin ) that does not undergo strong interactions. Two main classes of leptons exist: charged leptons (also known as the electron-like leptons or muons), and neutr ...
. Since
lepton
In particle physics, a lepton is an elementary particle of half-integer spin (spin ) that does not undergo strong interactions. Two main classes of leptons exist: charged leptons (also known as the electron-like leptons or muons), and neutr ...
s are only sensitive to
weak
Weak may refer to:
Songs
* "Weak" (AJR song), 2016
* "Weak" (Melanie C song), 2011
* "Weak" (SWV song), 1993
* "Weak" (Skunk Anansie song), 1995
* "Weak", a song by Seether from '' Seether: 2002-2013''
Television episodes
* "Weak" (''Fear t ...
,
electromagnetic and
gravitational
In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the str ...
forces, muonic atoms are governed to very high precision by the electromagnetic interaction.
Since a muon is more massive than an electron, the
Bohr orbits are closer to the nucleus in a muonic atom than in an ordinary atom, and corrections due to
quantum electrodynamics
In particle physics, quantum electrodynamics (QED) is the relativistic quantum field theory of electrodynamics. In essence, it describes how light and matter interact and is the first theory where full agreement between quantum mechanics and spec ...
are more important. Study of muonic atoms'
energy levels as well as transition rates from
excited state
In quantum mechanics, an excited state of a system (such as an atom, molecule or nucleus) is any quantum state of the system that has a higher energy than the ground state (that is, more energy than the absolute minimum). Excitation refers to ...
s to the
ground state
The ground state of a quantum-mechanical system is its stationary state of lowest energy; the energy of the ground state is known as the zero-point energy of the system. An excited state is any state with energy greater than the ground state. ...
therefore provide experimental tests of quantum electrodynamics.
Muon-catalyzed fusion is a technical application of muonic atoms.
Muonic helium (Hydrogen-4.1)
The symbol
4.1H (Hydrogen-4.1) has been used to describe the exotic atom muonic helium (
4He-μ), which is like
helium-4
Helium-4 () is a stable isotope of the element helium. It is by far the more abundant of the two naturally occurring isotopes of helium, making up about 99.99986% of the helium on Earth. Its nucleus is identical to an alpha particle, and cons ...
in having 2
proton
A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...
s and 2
neutron
The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the atomic nucleus, nuclei of atoms. Since protons and ...
s. However one of its
electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have n ...
s is replaced by a
muon
A muon ( ; from the Greek letter mu (μ) used to represent it) is an elementary particle similar to the electron, with an electric charge of −1 '' e'' and a spin of , but with a much greater mass. It is classified as a lepton. As w ...
, which also has charge –1. Because the muon's orbital radius is less than 1/200th 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 a
nucleus
Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to:
* Atomic nucleus, the very dense central region of an atom
*Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA
Nucl ...
with 2 protons, 2 neutrons and 1 muon, with total nuclear charge +1 (from 2 protons and 1 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 approximately 0.1
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 that of a hydrogen atom and not a noble helium atom. The only radioactive part of the atom is the muon. Therefore, the atom decays with the muon's half-life, 1.52 microseconds (1.52×10
−6 seconds).
Hadronic atoms
A ''hadronic atom'' is an atom in which one or more of the
orbital electrons are replaced by a negatively charged
hadron
In particle physics, a hadron (; grc, ἁδρός, hadrós; "stout, thick") is a composite subatomic particle made of two or more quarks held together by the strong interaction. They are analogous to molecules that are held together by the ...
. Possible hadrons include mesons such as the
pion or
kaon
KAON (Karlsruhe ontology) is an ontology infrastructure developed by the University of Karlsruhe and the Research Center for Information Technologies in Karlsruhe.
Its first incarnation was developed in 2002 and supported an enhanced version of ...
, yielding a ''pionic atom'' or a ''kaonic atom'' (see
Kaonic hydrogen), collectively called ''mesonic atoms'';
antiprotons, yielding an ''antiprotonic atom''; and the particle, yielding a or ''sigmaonic atom''.
[The strange world of the exotic atom](_blank)
Roger Barrett, Daphne Jackson and Habatwa Mweene, ''New Scientist'', August 4, 1990. accessdate=September 26, 2007.
Unlike leptons, hadrons can interact via the
strong force, so the orbitals of hadronic atoms are influenced by
nuclear forces between the
nucleus
Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to:
* Atomic nucleus, the very dense central region of an atom
*Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA
Nucl ...
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.
Hadronic atoms, such as pionic hydrogen and
kaonic hydrogen, thus provide experimental probes of the theory of strong interactions,
quantum chromodynamics
In theoretical physics, quantum chromodynamics (QCD) is the theory of the strong interaction between quarks mediated by gluons. Quarks are fundamental particles that make up composite hadrons such as the proton, neutron and pion. QCD is a type ...
.
Onium
An ''onium'' (plural: ''onia'') is the bound state of a particle and its antiparticle. The classic onium is
positronium, which consists of an electron and a positron bound together as a
metastable state, with a relatively long lifetime of 142 ns in the triplet state.
Positronium has been studied since the 1950s to understand bound states in quantum field theory. A recent development called
non-relativistic quantum electrodynamics (NRQED) used this system as a proving ground.
Pionium
Pionium is a composite particle consisting of one and one meson. It can be created, for instance, by interaction of a proton beam accelerated by a particle accelerator and a target nucleus. Pionium has a short lifetime, predicted by chiral pertur ...
, a bound state of two oppositely-charged
pions, is useful for exploring the
strong interaction
The strong interaction or strong force is a fundamental interaction that confines quarks into proton, neutron, and other hadron particles. The strong interaction also binds neutrons and protons to create atomic nuclei, where it is called th ...
. This should also be true of
protonium
Protonium (symbol: Pn), also known as antiprotonic hydrogen, is a type of exotic atom in which a proton (symbol: p) and an antiproton (symbol: ) orbit each other. Since protonium is a bound system of a particle and its corresponding antiparti ...
, which is a proton–antiproton bound state. Understanding bound states of pionium and protonium is important in order to clarify notions related to
exotic hadrons such as
mesonic molecules and
pentaquark states.
Kaonium
Kaonium is an exotic atom consisting of a bound state of a positively charged and a negatively charged kaon. Kaonium has not been observed experimentally and is expected to have a short lifetime on the order of 10−18 second
The second (sy ...
, 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 certain
mesons, the ''
quarkonium
In particle physics, quarkonium (from quark and -onium, pl. quarkonia) is a flavorless meson whose constituents are a heavy quark and its own antiquark, making it both a neutral particle and its own antiparticle.
Light quarks
Light quarks ( up ...
states'', which are made of a heavy quark such as the
charm
Charm may refer to:
Social science
* Charisma, a person or thing's pronounced ability to attract others
* Superficial charm, flattery, telling people what they want to hear
Science and technology
* Charm quark, a type of elementary particle
* Ch ...
or
bottom quark and its antiquark. (
Top quark
The top quark, sometimes also referred to as the truth quark, (symbol: t) is the most massive of all observed elementary particles. It derives its mass from its coupling to the Higgs Boson. This coupling y_ is very close to unity; in the Standard ...
s are so heavy that they decay through the
weak force before they can form bound states.) Exploration of these states through non-relativistic quantum chromodynamics (NRQCD) and
lattice QCD are increasingly important tests of
quantum chromodynamics
In theoretical physics, quantum chromodynamics (QCD) is the theory of the strong interaction between quarks mediated by gluons. Quarks are fundamental particles that make up composite hadrons such as the proton, neutron and pion. QCD is a type ...
.
Muonium
Muonium is an exotic atom made up of an antimuon and an electron,
which was discovered in 1960 by Vernon W. Hughes
and is given the chemical symbol Mu. During the muon's lifetime, muonium can undergo chemical reactions. Due to the mass diff ...
, despite its name, is ''not'' an onium 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, which ''is'' an onium (called
true muonium
In particle physics, true muonium is a theoretically predicted exotic atom representing a bound state of an muon and an antimuon (μ+μ−). The existence of true muonium is well established theoretically within the Standard Model. Its propert ...
), has been theorized.
Hypernuclear atoms
Atoms may be composed of electrons orbiting a
hypernucleus
A hypernucleus is similar to a conventional atomic nucleus, but contains at least one hyperon in addition to the normal protons and neutrons. Hyperons are a category of baryon particles that carry non-zero strangeness quantum number, which is c ...
that includes
strange particles called
hyperon
In particle physics, a hyperon is any baryon containing one or more strange quarks, but no charm, bottom, or top quark. This form of matter may exist in a stable form within the core of some neutron stars. Hyperons are sometimes generically rep ...
s. Such
hypernuclear atoms are generally studied for their nuclear behaviour, falling into the realm of
nuclear physics
Nuclear physics is the field of physics that studies atomic nuclei and their constituents and interactions, in addition to the study of other forms of nuclear matter.
Nuclear physics should not be confused with atomic physics, which studies t ...
rather than
atomic physics
Atomic physics is the field of physics that studies atoms as an isolated system of electrons and an atomic nucleus. Atomic physics typically refers to the study of atomic structure and the interaction between atoms. It is primarily concerned wit ...
.
Quasiparticle atoms
In
condensed matter systems, specifically in some
semiconductor
A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way ...
s, there are states called
exciton
An exciton is a bound state of an electron and an electron hole which are attracted to each other by the electrostatic Coulomb force. It is an electrically neutral quasiparticle that exists in insulators, semiconductors and some liquids. ...
s, which are bound states of an electron and an
electron hole
In physics, chemistry, and electronic engineering, an electron hole (often simply called a hole) is a quasiparticle which is the lack of an electron at a position where one could exist in an atom or atomic lattice. Since in a normal atom or ...
.
Exotic molecules
An exotic molecule contains one or more exotic atoms.
*
Di-positronium Di-positronium, or dipositronium, is an exotic molecule consisting of two atoms of positronium. It was predicted to exist in 1946 by John Archibald Wheeler, and subsequently studied theoretically, but was not observed until 2007 in an experiment ...
, two bound positronium atoms
*
Positronium hydride, a positronium atom bound to a hydrogen atom
"Exotic molecule" can also refer to a molecule having some other uncommon property such as a pyramidal
hexamethylbenzene#Dication and a
Rydberg atom.
See also
References
{{Authority control
Quantum chromodynamics