population in version

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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American Heritage
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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