population inversion

noun Physics.
a condition of matter in which more electrons are in a high energy state than in a lower energy state, as is required for the operation of a laser.

Origin:
1960–65

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Science Dictionary
population inversion  
The condition of having enough excited or high-energy states distributed in a material that a chain-reaction of stimulated emission can occur. Lasers, for example, need a constant power source that maintains population inversion in order to generate radiation continuously, since each stimulated emission reduces the population of high-energy states. See also stimulated emission.
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Encyclopedia Britannica
Encyclopedia

population inversion

in physics, the redistribution of atomic energy levels that takes place in a system so that laser action can occur. Normally, a system of atoms is in temperature equilibrium and there are always more atoms in low energy states than in higher ones. Although absorption and emission of energy is a continuous process, the statistical distribution (population) of atoms in the various energy states is constant. When this distribution is disturbed by pumping energy into the system, a population inversion will take place in which more atoms will exist in the higher energy states than in the lower

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
Align the nuclei, tickle them, detect the energy drain incoming or emission when the stimulated population inversion decays.
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