Editing Metastability (section)

==Quantum mechanics==

Aggregated systems of [[subatomic particle]]s described by [[quantum mechanics]] ([[quarks]] inside [[nucleons]], nucleons inside [[atomic nucleus|atomic nuclei]], [[electron]]s inside [[atom]]s, [[molecule]]s, or [[atomic clusters]]) are found to have many distinguishable states. Of these, one (or a small [[Degenerate energy levels|degenerate set]]) is indefinitely stable: the [[ground state]] or [[global minimum]].

All other states besides the ground state (or those degenerate with it) have higher energies.<ref>{{cite book|url=https://books.google.com/books?id=mGduDQAAQBAJ&pg=PA139|title=Tales of the Quantum: Understanding Physics' Most Fundamental Theory|last=Hobson|first=Art|date=2017|publisher=Oxford University Press|isbn=9780190679637|language=en}}</ref> Of all these other states, the '''metastable''' states are the ones having [[Half-life|lifetimes]] lasting at least 10<sup>2</sup> to 10<sup>3</sup> times longer than the shortest lived states of the set.<ref>{{Cite journal |last1=Hodgman |first1=S. S. |last2=Dall |first2=R. G. |last3=Byron |first3=L. J. |last4=Baldwin |first4=K. G. H. |last5=Buckman |first5=S. J. |last6=Truscott |first6=A. G. |date=2009-07-31 |title=Metastable helium: a new determination of the longest atomic excited-state lifetime |url=https://pubmed.ncbi.nlm.nih.gov/19792494/ |journal=Physical Review Letters |volume=103 |issue=5 |pages=053002 |doi=10.1103/PhysRevLett.103.053002 |issn=0031-9007 |pmid=19792494|bibcode=2009PhRvL.103e3002H |hdl=10440/978 |hdl-access=free }}</ref>

A ''metastable state'' is then long-lived (locally [[chemical stability|stable]] with respect to configurations of 'neighbouring' energies) but not eternal (as the global [[maxima and minima|minimum]] is). Being excited – of an energy above the ground state – it will eventually decay to a more stable state, releasing energy. Indeed, above [[absolute zero]], all states of a system have a non-zero probability to decay; that is, to spontaneously fall into another state (usually lower in energy). One mechanism for this to happen is through [[quantum tunnelling|tunnelling]].

===Nuclear physics===
Some energetic states of an [[atomic nucleus]] (having distinct spatial mass, charge, spin, [[isospin]] distributions) are much longer-lived than others ([[nuclear isomer#Metastable isomers|nuclear isomers]] of the same [[isotope]]), e.g. [[technetium-99m]].<ref>{{cite web|url=http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/technetium.html|title=Technetium-99m |publisher=Hyperphysics}}</ref> The isotope [[isotopes of tantalum#Tantalum-180m|tantalum-180m]], although being a metastable excited state, is long-lived enough that it has never been observed to decay, with a half-life calculated to be least {{val|4.5|e=16}} years,<ref>{{cite web|url=https://www.sciencenews.org/article/rarest-nucleus-reluctant-decay|title=Rarest nucleus reluctant to decay|last=Conover|first=Emily|date=2016-10-03 |website=Science News |access-date=2016-10-05}}</ref><ref>{{cite journal|last1=Lehnert|first1=Björn|last2=Hult|first2=Mikael|last3=Lutter|first3=Guillaume|last4=Zuber|first4=Kai|year=2017|title=Search for the decay of nature's rarest isotope <sup>180m</sup>Ta|arxiv=1609.03725|doi=10.1103/PhysRevC.95.044306|volume=95|pages=044306|journal=Physical Review C|issue=4 |bibcode=2017PhRvC..95d4306L|s2cid=118497863 }}</ref> over 3 million times the current [[age of the universe]].

===Atomic and molecular physics===
Some atomic energy levels are metastable. [[Rydberg atom]]s are an example of metastable excited atomic states. Transitions from metastable excited levels are typically those forbidden by electric dipole [[selection rule]]s. This means that any transitions from this level are relatively unlikely to occur. In a sense, an electron that happens to find itself in a metastable configuration is trapped there. Since transitions from a metastable state are not impossible (merely less likely), the electron will eventually decay to a less energetic state, typically by an electric quadrupole transition, or often by non-radiative de-excitation (e.g., collisional de-excitation).

This slow-decay property of a metastable state is apparent in [[phosphorescence]], the kind of [[photoluminescence]] seen in glow-in-the-dark toys that can be charged by first being exposed to bright light. Whereas spontaneous emission in atoms has a typical timescale on the order of 10<sup>−8</sup> seconds, the decay of metastable states can typically take milliseconds to minutes, and so light emitted in phosphorescence is usually both weak and long-lasting.

===Chemistry===
{{See also|Chemical stability|Chemical equilibrium#Metastable mixtures}}

In chemical systems, a system of atoms or molecules involving a change in [[chemical bond]] can be in a metastable state, which lasts for a relatively long period of time. Molecular vibrations and [[temperature|thermal motion]] make chemical species at the energetic equivalent of the top of a round hill very short-lived. Metastable states that persist for many seconds (or years) are found in energetic ''valleys'' which are not the lowest possible valley (point 1 in illustration). A common type of metastability is [[isomerism]].

The stability or metastability of a given chemical system depends on its environment, particularly [[temperature]] and [[pressure]]. The difference between producing a stable vs. metastable entity can have important consequences. For instances, having the wrong crystal [[polymorphism (materials science)|polymorph]] can result in failure of a drug while in storage between manufacture and administration.<ref>Process Chemistry in the Pharmaceutical Industry. Kumar G. Gadamasetti, editor. 1999, pp. 375–378</ref> The map of which state is the most stable as a function of pressure, temperature and/or composition is known as a [[phase diagram]]. In regions where a particular state is not the most stable, it may still be metastable.
[[Reaction intermediate]]s are relatively short-lived, and are usually thermodynamically unstable rather than metastable. The [[International Union of Pure and Applied Chemistry|IUPAC]] recommends referring to these as ''transient'' rather than metastable.<ref>{{cite journal|url=http://goldbook.iupac.org/T06451.html|website=IUPAC Gold Book |title=transient (chemical) species|date=2014 |doi=10.1351/goldbook.T06451|doi-access=free}}</ref>

Metastability is also used to refer to specific situations in mass spectrometry<ref>{{cite journal|url=http://goldbook.iupac.org/M03874.html|website=IUPAC Gold Book |title=metastable ion in mass spectrometry|doi=10.1351/goldbook.M03874|doi-access=free}}</ref> and spectrochemistry.<ref>{{cite journal|url=http://goldbook.iupac.org/M03876.html|website=IUPAC Gold Book |title=metastable state in spectrochemistry|doi=10.1351/goldbook.M03876|doi-access=free}}</ref>