metastable

A metastable system with a weakly stable state (1), an unstable transition state (2) and a strongly stable state (3).

Metastability is a general scientific concept which describes states of delicate equilibrium. A system is in a metastable state when it is in equilibrium (not changing with time) but is susceptible to fall into lower-energy states with only slight interaction. It is analogous to being at the bottom of a small valley when there is a deeper valley close by — a local stability of a system at a local (but not global) minimum of a potential.

Almost any system can demonstrate metastability, but it is most prevalent in systems of weakly interacting particles in physics and chemistry. Often, the weak interaction between particles is the only energy barrier that must be overcome for the system to reach a lower-energy state.

In phases of matter

Metastable phases of matter include those which are supercooled or superheated. For example, supercooled water can exist in liquid form at temperatures below freezing, and will remain there until external interaction (vibration or introduction of a seed particle) causes the water to crystallize.

In aggregate matter

Small avalanches demonstrate metastability at Mount Baker Ski Area.

Sandpiles are one system which can exhibit metastability if a steep slope or tunnel is present. Sand grains form a pile due to friction. It is possible for an entire large sand pile to reach a point where it is stable, but the addition of a single grain causes large parts of it to collapse.

The avalanche is a well-known problem with large piles of snow and ice crystals on steep slopes. In dry conditions, snow slopes act similarly to sandpiles. An entire mountainside of snow can suddenly slide due to the presence of a skier, or even a loud noise or vibration.

In electronic circuits

Metastability in electronics is usually seen as a problem. A changing circuit is supposed to "settle" into one of a small number of desired states, but if the circuit is vulnerable to metastability, it can get "stuck" in an undesirable state.

In computational neuroscience

Metastability in the brain is a phenomenon which is being studied in computational neuroscience to elucidate how the human mind recognizes patterns. The term "metastability" here is used rather loosely. There is no "lower energy" state, but there are semi-transient signals in the brain which persist for a while and are different than the usual equilibrium state.

In chemical systems

In chemistry, a system of atoms or molecules involving a change in phase of matter, crystal structure, or chemical bond can be in a metastable state, which lasts for a relatively long period of time. Molecular vibrations due to heat 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).

For example, diamond is a metastable form of carbon at standard temperature and pressure. It can be converted to graphite (plus leftover kinetic energy), but only after overcoming an activation energy - an intervening hill. Martensite is a metastable phase used to control the hardness of most steel. The bonds between the building blocks of polymers such as DNA, RNA and proteins are also metastable.

Being "stuck" in a thermodynamic trough without being at the lowest energy state is known as being "kinetically persistent". The particular "motion" or "kinetics" of the atoms involved has resulted in getting "stuck", despite there being preferable (lower-energy) alternatives.

The stability or metastability of a given molecule depends in part on its environment, including temperature, pressure, and the presence of catalysts or seed crystals (in the case of a solid state system). The presence of a liquid layer can help facilities transitions between crystal states. 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.^[1]

Reaction intermediates are very short-lived, and are actually thermodynamically unstable rather than "metastable". The IUPAC recommends referring to these as "transient" rather than "metastable".^[2]

Non-equilibrium thermodynamics includes the study of chemical systems which are in unstable states.

Metastability is also used to refer to specific situations in mass spectrometry^[3] and spectrochemistry^[4].

In nuclear physics

An atomic nucleus can achieve a number of different energetic states, known as nuclear isomers. Some of these states are metastable isomers, meaning that there can be relatively long-lived nuclei of the same isotope in different energetic states.

In quantum mechanics

Systems which are described by quantum mechanics may possess metastable states. In this case, the metastable state may decay to a global minimum state via quantum mechanical effects, such as tunnelling.

Some atomic energy levels are metastable. Transitions from these 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. Of course, since transitions away from a metastable state are not impossible (merely unlikely), the electron will eventually be able to decay to a less energetic state by spontaneous emission. This property is made use of in lasers.

when light of suitable wavelength falls on atoms their electrons jump to a higher energy state.When the incoming radiations are removed the excited electron goes back to its original level within a duration of 10^-8 seconds. However when an electron goes to a metastable state it remains there for a relatively longer duration of 10^-3 seconds. This phenomenon leads to accumulation of electrons in the metastable state since the rate of addition of electrons to the metastable state is higher than the rate of there de-excitation.This leads to the phenomenon called population inversion, which forms the basis of lasing action of lasers.

See also

• Hysteresis

References