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The process of causing strong deviations from thermal equilibrium populations of selected quantized states of different energy in atomic or molecular systems by the use of optical radiation (that is, light of wavelengths in or near the visible spectrum), called the pumping radiation.
Optical pumping is vital for light amplification by stimulated emission in an important class of lasers. For example, the action of the ruby laser involves the fluorescent emission of red light by a transition from an excited level E2 to the ground level E1. In this case E2 is relatively high above El and the equilibrium population of E2 is practically zero. Amplification of the red light by laser action requires that number of atoms N2 exceed N1 (population inversion). The inversion is accomplished by intense green and violet light from an external source which excites the chromium ion in the ruby to a band of levels, E3 above E2. From E3 the ion rapidly drops without radiation to E2, in which its lifetime is relatively long for an excited state. Sufficiently intense pumping forces more luminescent ions into E2 by way of the E3 levels than remain in the ground state E1, and amplification of the red emission of the ruby by stimulated emission can then occur. See Laser
the excitation of the microparticles, such as atoms and molecules, that make up matter from a lower energy level to a higher level by the use of light.