Metastable State

Metastable state, in physics, is the particularly excited energy level or higher energy level of an atom, nucleus, or other systems that have a longer lifetime than the ordinary excited states (or the energy levels) and that generally has a shorter lifetime than the lowest, often highly stable, energy state is known as the ground state. 

A metastable state may thus be referred to as a kind of temporary energy level or a somewhat stable intermediate stage of a system the energy of which may be lost in discrete amounts. In quantum mechanical terms, transitions from metastable states are forbidden and are much less probable than the allowed transitions from other excited states or excited energy levels.

Metastable

Now, what do we mean by metastable? What is the actual metastable meaning? The word metastable is often used in atomic physics while studying the light amplification processes. The metastable state plays an important role in the light amplification process in LASERs. Let us have detailed information about this as follows.

Metastable Meaning

We know that for the light amplification process we need a large density of atoms in the excited state, in other words for the light amplification process the population inversion has to be achieved. The population inversion is achieved by optical pumping, i.e., with the help of a continuous supply of energy to the atoms in the lower energy levels and hence they will get excited to the higher energy levels. But, the atoms can not stay in the higher energy level for a longer time, they will eventually get de-excited to the lower energy levels. Due to this de-excitation process, the population inversion can not be achieved. Hence we need an intermediate energy level or the state where the atoms can stay a bit longer or atoms can have a longer lifetime. 

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There are many illustrations or examples of metastable states in atomic and nuclear systems. Analysis of atomic spectra often reveals metastable states as relatively final energy states to which electrons have cascaded from higher energy levels in the act of generating light. Light energy struck for a time in metastable mercury atoms accounts for the many photochemical reactions of this element. Metastable states of an atomic nucleus give rise to nuclear isomers that differ in energy content and mode of radioactive decay from other nuclei of the same element.

Metastable atoms often release or lose their stored energy by collision with other atoms before they can radiate it, but in the rarefied upper atmosphere of Earth, in which atoms travel a long time before the collision, radiation from metastable oxygen atoms seems to account for the characteristic green color of the aurora borealis and aurora australis. Metastable nuclei lose their energy by radioactive decay, usually by gamma radiation.

We know that for the light amplification process we need a large density of atoms in the excited state, in other words for the light amplification process the population inversion has to be achieved. The population inversion is achieved by optical pumping, i.e., with the help of a continuous supply of energy to the atoms in the lower energy levels and hence they will get excited to the higher energy levels. But, the atoms can not stay in the higher energy level for a longer time, they will eventually get de-excited to the lower energy levels. Due to this de-excitation process, the population inversion can not be achieved. Hence we need an intermediate energy level or the state where the atoms can stay a bit longer or atoms can have a longer lifetime.

Metastable State in Lasers 

Metastable states also play some important and key roles in gas lasers:

• In a number of gas lasers, helium atoms are excited into metastable states by an electrical discharge. During collisions with other atoms (e.g. neon in a helium-neon laser), they can then transfer or exchange the excitation energy to those atoms. Efficient resonant energy exchange requires that the excitation energies of the two species are quite similar i.e., metastable equilibrium. That condition is absent in the case of Penning ionization because that leads to a free electron that can move away a variable amount of energy. That happens in helium–cadmium lasers, for example.

• It also occurs that after the laser transition atoms are trapped in a metastable state (metastable ions). That is the case in helium-neon lasers, for example. Here, it is a good solution to use a laser bore tube with a considerably small diameter, so that the metastable atoms can dissipate (release) their energy in collisions with the tube wall. Only thereafter, they can again participate in the lasing action.

Lasers without Metastable States

Generally, laser gain media do not have to exhibit metastable levels, it is a short-lived level that can still be used as the higher (or the excited) laser level provided that the emission cross-sections are large enough. (For the threshold pump power, the σ−τ product is the essential quantity.) However, long metastable level lifetimes are very important for Q-switched lasers, as they permit significant energy storage. They also have a strong impact on laser dynamics, including spiking phenomena. Finally, three-level laser transitions are hardly possible without metastable levels, since a substantial upper-state population as needed for positive net gain would be difficult to achieve.

In laser modelling of doped-insulator solid-state lasers, one usually considers population only of metastable states and the ground state, because only a vanishingly small proportion of the laser-active ions can be in other (short-lived or metastable phase) states. This can substantially simplify laser models.

Did You Know?

• The lasing transition, during this laser, is thanks to the decay of the atom from this first excited metastable state to the bottom state. If the amount of atoms within the state exceeds the number of atoms that are pumped into the excited state, then there's a high likelihood that the lasing photon is going to be absorbed and that we won't get sustained laser light.

• The fact that the lower lasting transition is that the state makes it rather difficult to realize efficient population inversion. During a ruby laser, this task is accomplished by providing the ruby crystal with a really strong pulsating light, called a flash.

• The flash produces a really strong pulse of sunshine that's designed to excite the atoms from their state into any short-lived upper level. during this way, the bottom state is depopulated and population inversion is achieved until a pulse of laser light is emitted.

• In the ruby laser, the flashlight lasts for about 1/1000 of a second (1 ms) and may be repeated about every second. The duration of the laser light is shorter than this, typically it will be around 0.1 ms. In some pulsed lasers, the heartbeat duration is often tailored using special methods to be much shorter than this, right down to about 10 fs (where 1 fs = 10-15 s or one-thousandth of a millionth of a second).

• So, the output of a three-level laser isn't continuous but consists of pulses of laser light. To realize an endless beam of laser light a four-level laser is required.

Metastability of Water

There is no reason to assume that only the matters that are present in solid form can be in the metastable state. The metastability of water can also be clarified with a short example. If you put a bottle of water inside the deep fridge, the water inside it will gradually start to get converted into ice. 

Though it will take some time, the water inside the bottle will not remain super cool for a longer period of time, as due to the atmospheric change and temperature range the water will get condensed and it will take the form of ice. The water will not be in an equilibrium state and the in-between states can be considered as the metastable states.

Descriptions of Metastable Materials

A particular compound or matter that is made of certain particles that change their shape and volume at certain temperature ranges and take the form of something else are considered metastable materials. For example, there are some minerals available in nature that get crystallized at a particular temperature range and they are able to retain their initial structure at a certain temperature range.

Similarly, chocolates have a melting point and if they are placed at a temperature range that is above the melting point, then they will start melting. They can only retain the shape of the solidified chocolates at a certain temperature range. Both these minerals and chocolates can be considered metastable materials because the molecules and particles present in them change their structures under certain temperature ranges. To know more you can take the help of the course materials available on the website of the Vedantu.