Population Inversion
The number of atoms present in the excited state or higher energy state is greater than the number of atoms present in the ground state or lower energy state is called population inversion.
As the name suggests that this is an inverted phenomenon. In general, the lower energy level is more populated which means it has more number of atoms in the lower energy level as compared to a higher energy level. But by pumping we will obtain a state when the number of atoms present in the higher energy state is greater than the number of atoms present in the lower energy state.
Phenomena
Let us consider a two-level energy system of energies E1 and E2 as shown in the figure.
Further, Let N1 and N2 be the populations that mean several atoms per unit volume of energy levels E1 and E2 respectively.
However, Boltzmann’s distribution gives the population of an energy level E at temperature T as
Where N0 is the population of the lower level or ground state and k is Boltzmann’s constant.
Thus, from the above relation, the population of energy levels E1 and E2 are
At normal conditions N1 >N2 . This implies that the population in the ground state or lower state is always greater than the population in the excited state or higher states. The stage of making, the population of higher energy level greater than the population of lower energy level call as population inversion i.e. N1 < N2.
When the population inversion method is used to enforce more and more atoms to give up photons. This initiates a chain reaction and releases a massive amount of energy.
This results in a rapid build-up of the energy of emitting one particular wavelength traveling coherently in a fixed direction. This process is amplification by stimulated emission using population inversion.
This population inversion situation is essential for laser action. For any stimulated emission, the upper energy level or metastable state must have a long lifetime. That mean the atoms should pause at the metastable state for more time than at the lower level.
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