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three-level laser
three-level laser A burst of energy excites electrons in more than half of the atoms from their ground state to a higher state, creating a population inversion. The electrons then drop into a long-lived state with slightly less energy, where they can be stimulated to quickly shed excess energy as a laser burst, returning the electrons to a stable ground state.

population inversion

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population inversion, in physics, the redistribution of atomic energy levels that takes place in a system so that laser or maser 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.

The Editors of Encyclopaedia BritannicaThis article was most recently revised and updated by Erik Gregersen.