Aggregated systems of
subatomic particles described by
quantum mechanics (
quarks inside
nucleons, nucleons inside
atomic nuclei,
electrons inside
atoms,
molecules, or
atomic clusters) are found to have many distinguishable states. Of these, one (or a small
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. Of all these other states, the
metastable states are the ones having
lifetimes lasting at least 102 to 103 times longer than the shortest lived states of the set. A
metastable state is then long-lived (locally
stable with respect to configurations of 'neighbouring' energies) but not eternal (as the global
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
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 isomers of the same
isotope), e.g.
technetium-99m. The isotope
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 years, over 3 million times the current
age of the universe.
Atomic and molecular physics Some atomic energy levels are metastable.
Rydberg atoms are an example of metastable excited atomic states. Transitions from metastable excited levels are typically those forbidden by electric dipole
selection rules. 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 10−8 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 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
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
polymorph can result in failure of a drug while in storage between manufacture and administration. 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 intermediates are relatively short-lived, and are usually thermodynamically unstable rather than metastable. The
IUPAC recommends referring to these as
transient rather than metastable. Metastability is also used to refer to specific situations in mass spectrometry and spectrochemistry. ==Electronic circuits==